delivered the opinion of the court:
In cause No. 56629, defendant, Raymond Lee Stewart, was convicted of the murder of Kevin Kaiser (Ill. Rev. Stat. 1981, ch. 38, pars. 9 — 1(a)(1), (a)(3)), following a jury trial in the circuit court of Winnebago County. At a separate sentencing hearing, the same jury determined that the necessary aggravating factors existed and that there were no mitigating circumstances sufficient to preclude imposition of the death penalty. The court thereupon sentenced defendant to death and, following a direct appeal to this court (Ill. Const. 1970, art. VI, §4(b); 87 Ill. 2d R. 603), his conviction and sentence were affirmed. (People v. Stewart (1984), 104 Ill. 2d 463 (hereinafter referred to as Stewart I).) This court denied rehearing, and the United States Supreme Court denied defendant’s petition for certiorari. Stewart v. Illinois (1985), 471 U.S. 1120, 86 L. Ed. 2d 267, 105 S. Ct. 2368.
In cause No. 56332, defendant was indicted in the circuit court of Winnebago County for the murders of Willie Fredd and Albert Pearson (Ill. Rev. Stat. 1981, eh. 38, par. 9 — 1(a)(1)). A jury found defendant guilty of both offenses, and following a separate sentencing hearing held before the same jury, he was sentenced to death. This court affirmed his conviction and sentence on direct appeal (People v. Stewart (1984), 105 Ill. 2d 22 (hereinafter referred to as Stewart II)), and denied his petition for rehearing. The United States Supreme Court subsequently denied his petition for certiorari. Stewart v. Illinois (1985), 471 U.S. 1131, 86 L. Ed. 2d 283, 105 S. Ct. 2666.
Defendant then filed a petition in each case under the Post-Conviction Hearing Act (Ill. Rev. Stat. 1983, ch. 38, *96par. 122 — 1 et seq.). Because the petitions raised the same constitutional issues, they were consolidated pursuant to the State’s motion.
Defendant maintained in his petitions that the statutory grant of discretion to prosecutors has resulted in “uneven, arbitrary, and capricious application” of the death penalty statute in violation of his right to due process and his right to freedom from cruel and unusual punishment. (U.S. Const., amends. V, VIII; Ill. Const. 1970, art. I, §2.) Defendant also asserted that the alleged “racially discriminatory application” of the death penalty statute deprived him of his rights to due process and equal protection of the law. (U.S. Const., amends. V, XIV; Ill. Const. 1970, art. I, §2.) Further, defendant claimed that because four of the seven presently sitting Illinois Supreme Court justices at one time allegedly believed that the death penalty statute is unconstitutional, his death sentence violates the supremacy clause of the United States Constitution and is violative of his rights to due process and freedom from cruel and unusual punishment. (U.S. Const., amends. V, XIII; Ill. Const. 1970, art. 1, §2.) Finally, defendant alleged that execution by lethal injection is violative of the prohibition against cruel and unusual punishment because no guidelines have purportedly been established regarding the “method and manner” of such executions, and because the Food and Drug Administration had not approved “the procedures to be followed or the drugs to be administered.” U.S. Const., amend. VIII.
In a motion to dismiss the petitions, the State asserted that the issues raised by defendant were barred by the application of the doctrines of waiver and res judicata, or were insufficient to establish any constitutional violations.
Following a hearing on the petitions, the trial court granted the State’s motion to dismiss the petitions with*97out an evidentiary hearing on the grounds that the issues raised were waived, barred by the application of res judicata, or insufficient to state a constitutional violation. Defendant brings a direct appeal to this court from the denial of his petitions. 107 Ill. 2d R. 651(a).
Review of the petitions for post-conviction relief, in this court, raises three issues: (1) Was defendant’s right to the effective assistance of counsel violated during the post-conviction proceedings? (2) Did the trial court err in dismissing defendant’s petitions without holding an evidentiary hearing on the allegations regarding prosecutorial discretion and the alleged racially discriminatory application of the death penalty statute? and (3) Did the alleged lack of guidelines relating to the “method and manner” of execution by lethal injection violate the eighth amendment to the United States Constitution? Defendant, in this court, has not raised the dismissal of his claim that his death sentence should be vacated because a majority of this court presently sitting, at one time, allegedly believed that the death penalty statute is unconstitutional.
The facts relating to the underlying crimes for which defendant was convicted will not be set forth in their entirety since defendant does not contend that the evidence was insufficient to support his convictions, and does not raise any issue challenging the validity of those convictions. Rather, the pertinent facts will be discussed only to the extent necessary for resolution of the issues. Moreover, “it is well established that a post-conviction proceeding is not one wherein a defendant’s guilt or innocence is determined, but a new proceeding meant to delve into the constitutional phases of the original conviction which have not previously been determined.” People v. Gaines (1984), 105 Ill. 2d 79, 87.
Defendant initially claims that he did not receive adequate representation during the post-conviction proceed*98ings because his counsel, Mr. Pumilia, failed to comply with Supreme Court Rule 651(c), which governs appeals in post-conviction proceedings. Rule 651 provides, in relevant part:
“(c) *** The record filed in [the trial] court shall contain a showing, which may be made by the certificate of petitioner’s attorney, that the attorney has consulted with petitioner either by mail or in person to ascertain his contentions of deprivation of constitutional right, has examined the record of the proceedings at the trial, and has made any amendments to the petitions filed pro se that are necessary for an adequate presentation of petitioner’s contentions.” 107 Ill. 2d R. 651(c).
Mr. Pumilia failed to file with the trial court a certificate of compliance with Rule 651. However, while the case was being briefed in this court, we granted the State leave to supplement the record with Mr. Pumilia’s affidavit attesting to the fact that he conferred with defendant relative to the issues “which should be presented” in the post-conviction petitions. The affidavit also reflected that Ms. Kyle Wesendorf, who represented defendant on direct appeal to this court in Stewart II, conferred with defendant “and [Mr. Pumilia] concerning the Post Conviction Relief Petition and assisted [him] in all phases of the matter except those involving court appearances.”
At the time defendant filed his reply brief, he moved this court to allow him to supplement the record with his own affidavit and the affidavit of Kyle Wesendorf, in support of his claim that Mr. Pumilia violated Rule 651(c). By order dated July 28, 1987, after the case had been fully briefed, this court granted defendant’s motion to supplement the record and for a summary remand directing the trial court to conduct a hearing “on the evidentiary dispute concerning whether [Mr. Pumilia] complied with Rule 651(c).”
*99During the hearing on summary remand, Kyle Wesendorf testified on behalf of the defendant. She stated that she was previously employed by the State Appellate Defender’s office and represented defendant on direct appeal to this court from one of his murder convictions. Ms. Wesendorf further testified that, following this court’s affirmance of defendant’s conviction and death sentence, she contacted Mr. Pumilia by letter to advise him that she would no longer be handling defendant’s case, but that she intended to draft a “bare bones” petition for post-conviction relief on defendant’s behalf which he could file. She stated that she subsequently forwarded to Mr. Pumilia a copy of the petition along with supporting documents. She did not send defendant a copy of the petition, but she spoke with him by telephone and “probably” informed him that a petition would be filed and to confer with Mr. Pumilia regarding the petition.
On cross-examination, Ms. Wesendorf testified that she also had a telephone conversation with Mr. Pumilia during which she described the post-conviction materials that she was forwarding to him. On redirect examination, she stated that the issues raised in the petition she drafted were those which could be raised in any death penalty case, and did not address any matters relating to defendant’s trial proceedings. She also stated, as she did in her affidavit, that she did not discuss with Mr. Pumilia any issues that should be raised in the petition.
Defendant refused to testify at the hearing because the trial court denied his request to appear without shackles or, in the alternative, to have the public removed from the courtroom. However, defense counsel made an offer of proof which the court accepted, relating that defendant would have testified that he never placed a telephone call to Mr. Pumilia between November of 1985» when the petitions were filed, and June of *1001986, when the trial court dismissed the petitions. He would also testify that, according to prison regulations, inmates may only place collect telephone calls. Finally, the offer of proof reflected that defendant was unaware that Mr. Pumilia represented him until he received a letter from him advising defendant that the petition was dismissed and an appeal would be filed.
In defendant’s affidavit, which he submitted as part of the supplemental record, he also stated that he did not consult with Mr. Pumilia regarding the post-conviction petition. However, he did attest that he conferred with Mr. Pumilia regarding a “personal matter” during the pendency of the post-conviction proceedings. He also stated that Ms. Wesendorf “did discuss the procedural aspects of post-conviction proceedings” with him.
Angela Pappas, an employee of the public defender’s office for Winnebago County, testified that she reviewed telephone records reflecting calls made to the public defender’s office between November of 1985 and June of 1986. She stated that the records reflected that no collect calls were made to the office from Pontiac Correctional Center, wherein the defendant was confined during that time.
Mr. Pumilia was called as a witness on behalf of the State. He testified that he was the public defender for Winnebago County and that he had held that position for approximately four through five years. He stated that he was familiar with the trial records in both Stewart I and Stewart II because he represented defendant in those cases and reviewed the transcripts.
Mr. Pumilia further testified that he maintained close contact with Ms. Wesendorf.throughout defendant’s appeal in the case she handled, and received from her the post-conviction petition and supporting documentation to be filed on defendant’s behalf. ,
*101He also stated that during the pendency of the post-conviction proceeding, he had a “brief” telephone conversation with defendant, which the defendant initiated. He testified that during the conversation, he “briefly outlined the issues in the Petition,” and advised defendant that he had no further issues to raise. He asked defendant whether there were any additional allegations which he would like to include in the petitions, and defendant’s response was “negative.” Finally, Mr. Pumilia testified that defendant could have telephoned him prior to November of 1985, but he believed that was “highly unlikely. ”
During cross-examination, Mr. Pumilia testified that the telephone call to which he attested on direct examination was his only contact with defendant during the pendency of the post-conviction proceedings in the trial court. He also stated that he was familiar with the appeals in Stewart I and Stewart II and read the Illinois Supreme Court opinions in those cases. Mr. Pumilia further testified that he realized he was required to confer with his client and review the reports of proceedings in accordance with Rule 651(c). He stated that he “went over the whole batch” of transcripts at some point, and reviewed certain portions following his appointment as defendant’s post-conviction counsel.
Finally, Mr. Pumilia stated that if a defendant wished to claim ineffective assistance of trial counsel, that same counsel should not represent him during the post-conviction proceedings without first obtaining a “waiver” from the client.
The trial court, in determining that defendant’s counsel complied with Rule 651(c), found that a post-conviction petition drafted by a lawyer was filed; that defendant had the benefit of both Mr. Pumilia’s and Ms. Wesendorf’s thoughts concerning the petition; and that Ms. Wesendorf apparently did not consider raising any *102issue regarding ineffective assistance of counsel. In addition, the trial court noted there was conflicting evidence with respect to whether defendant ever telephoned Mr. Pumilia and discussed the petition, but concluded that “Mr. Pumilia is more worthy of belief. I thought Mr. Pumilia was straightforward about what he did here in this case.”
Defendant’s motion for reconsideration of the trial court’s ruling was denied, and he now appeals claiming that, as a matter of law, his counsel failed to comply with Rule 651(c). We disagree.
The evidence adduced at the hearing, as well as other evidence in the record, establishes that two petitions for post-conviction relief, prepared and reviewed by an attorney, were filed on behalf of defendant. Those petitions were later amended by counsel to include supporting documentation. The uncontroverted evidence also reveals that Mr. Pumilia reviewed the transcripts of proceedings from both of defendant’s trials, at which he represented him, and that Ms. Wesendorf reviewed the transcripts from one of defendant’s trials prior to preparing the per tition.
The sole controverted question was whether Mr. Pumilia conferred with defendant concerning the post-conviction petitions. Mr. Pumilia, both at the evidentiary hearing and in his affidavit, testified that he did have such a discussion with defendant. Defendant did not testify at the hearing and claimed only through an unsworn offer of proof that he never conferred with his counsel until after the petition was filed. In his affidavit, on the other hand, he conceded that he and Mr. Pumilia conferred during the relevant time period, but claimed that the discussion concerned an unrelated “personal matter.”
Thus, whether or not defendant and Mr. Pumilia conferred concerning the post-conviction petitions was a *103question of fact for the trial court to resolve. (E.g., People v. Stewart (1984), 105 Ill. 2d 22, 66 (“It is the function of the [fact finder] to determine credibility of the witnesses”), citing People v. Akis (1976), 63 Ill. 2d 296, 298.) Indeed, to resolve the evidentiary dispute was the very purpose for which this court remanded the case to the trial court for a hearing. The trial court determined that Mr. Pumilia was “more worthy of belief,” and “ ‘[i]t is neither the duty nor the privilege of a reviewing court to substitute its judgment as to the weight of disputed evidence or the credibility of witnesses for that of the trier of fact ***.’ ” People v. Johnson (1986), 114 Ill. 2d 170, 190, quoting People v. Novotny (1968), 41 Ill. 2d 401, 412.
Defendant does not specifically assert that we should overturn the trial court’s findings. Rather, he claims that, even if Mr. Pumilia did confer with him, that telephone conversation was insufficient, as a matter of law, to satisfy the requirements of Rule 651(c). No authority has been cited by defendant for this proposition, nor are we aware of any. The rule does not specify a certain number of conversations that must take place between counsel and the defendant, nor does it mandate that counsel initiate the contact. Where, as here, it is determined that counsel advised defendant of the issues raised in the petition, and offered him an opportunity to suggest additional claims, we believe the requirement of Rule 651(c) that counsel confer with defendant concerning the petition has been satisfied.
For the first time, during oral argument before this court, defendant also claimed that he did not receive effective assistance of counsel during the post-conviction proceedings because Mr. Pumilia’s representation of him during tno^e proceedings created a conflict of interest of which he failed to apprise defendant. Apparently, defendant is asserting that he did not receive adequate repre*104sentation during his two trial proceedings, and Mr. Pumilia would have a conflict in asserting this claim in the post-conviction petitions because he was the attorney who represented defendant during his trials.
Defendant’s claim that he received ineffective assistance of counsel is based on the fact that, in both Stewart I and Stewart II, this court held that he waived review of five issues because they were not raised in the trial court. He also notes that the post-conviction petitions raise general constitutional claims but do not allege any errors specific to his trial proceedings. The State argues that defendant is precluded from raising the question of ineffective assistance of counsel because he failed to assert this claim at any time prior to oral argument before this court.
“The purpose of the post-conviction proceeding is to permit inquiry into constitutional issues involved in the original conviction that have not already been adjudicated 'or could have been.” (Emphasis added.) (People v. Silagy (1987), 116 Ill. 2d 357, 365.) Thus, “[t]he judgment of the reviewing court on a previous appeal is res judicata as to all issues actually decided, and any claim that could have been presented to the reviewing court in the direct appeal is, if not presented, thereafter barred under the doctrine of waiver.” Silagy, 116 Ill. 2d at 365, citing People v. Kubat (1986), 114 Ill. 2d 424, 436; People v. Jones (1985), 109 Ill. 2d 19, 24; People v. Gaines (1984), 105 Ill. 2d 79, 87-88.
On defendant’s direct appeal in Stewart I, during which he was represented by counsel other than Mr. Pumilia, he alleged that Mr. Pumilia’s failure to raise certain issues in the trial court, and to tender certain objections, deprived him of the effective assistance of counsel. This court reviewed Mr. Pumilia’s conduct in each of these instances, and concluded that he adequately represented defendant. On his direct appeal in Stewart II, *105where defendant was again represented by counsel other than Mr. Pumilia, he failed to raise any claim that he received ineffective assistance of counsel during the trial proceedings. Thus, unless application of the doctrines of waiver and res judicata would be fundamentally unfair (see People v. Gaines (1984), 105 Ill. 2d 79, 91), defendant is precluded from raising this issue on appeal.
After a careful review of the record, we do not believe that it would be manifestly unjust to preclude review of defendant’s claim. This is not a situation wherein defendant was represented by the same counsel during his trial proceedings and on his direct appeals. Moreover, he had the benefit of two attorneys’ thoughts concerning the post-conviction petitions. We note parenthetically that the attorney who initially drafted one of the petitions, and reviewed the record in Stewart II, did not choose to raise any claim regarding Mr. Pumilia’s alleged incompetency.
More significantly, defendant did not even raise this issue in his briefs filed with this court appealing the denial of his post-conviction petitions. Our Supreme Court Rule 341(e)(7) provides that points not asserted in an appellant’s brief are waived and shall not be raised for the first time, as defendant did here, during oral argument. 107 Ill. 2d R. 341(e)(7); see also People v. Lewis (1984), 105 Ill. 2d 226, 249-50 (holding that defendant waived review of an issue which he raised for the first time during oral argument).
Nevertheless, defendant claims that Mr. Pumilia failed to inform him that he had a conflict of interest in representing him during the post-conviction proceeding, and that his new counsel was unaware of this failure until the hearing on remand, after the briefs were already filed.
This argument presupposes that Mr. Pumilia operated under a conflict of interest of which he had the duty to, *106sua sponte, advise the defendant. However, unless defendant desired to raise an issue relating to the effectiveness of his counsel, there would, of course, be no conflict. There is no evidence in the record that defendant wished to pursue such a claim. Contrary to counsel’s assertion, Mr. Pumilia did not testify that his representation of defendant during the post-conviction proceedings constituted a conflict of interest. He merely opined that, if a defendant wished to claim ineffective assistance of trial counsel, that counsel should obtain a waiver prior to representing the defendant in the post-conviction proceedings. Moreover, defendant does not direct our attention to any authority for his apparent proposition that counsel was required to advise him .that he may have a conflict in the event defendant wished to raise some hypothetical issue.
Even more fundamentally, the alleged conflict of interest is based on the premise that defendant received ineffective assistance of counsel during the trial proceedings because Mr. Pumilia failed to preserve certain issues for review. However, this alleged ground for defendant’s claim of ineffective assistance of counsel was known to his counsel when he appealed the denial of defendant’s post-conviction petitions. Nevertheless, no such claim was made in defendant’s briefs.
Accordingly, for the aforementioned reasons, we conclude that defendant is precluded from now raising the claim that he did not receive the effective assistance of counsel.
Defendant further contends that the trial court erred in dismissing, without an evidentiary hearing, his claim that the death penalty statute “is being applied in a racially discriminatory manner in violation of the United States and Illinois Constitutions.” In support of his claim, defendant proffers a statistical study which purportedly evidences that a defendant accused of murder*107ing a white victim is four times more likely to receive the death penalty than a defendant accused of murdering a black victim. The study also purportedly shows that a black defendant accused of murdering a white victim is more likely to receive the death penalty than a white defendant accused of murdering a white victim. See Gross & Mauro, Patterns of Death: An Analysis of Racial Disparities in Capital Sentencing & Homicide Victimization, 37 Stan. L. Rev. 27 (1984) (the Gross study).
The State argues that dismissal of this claim was proper because defendant failed to raise this issue on his direct appeals. In the alternative, it is claimed that the Gross study is insufficient, as a matter of law, to establish a violation of defendant’s constitutional rights.
We note initially that a published version of the Gross study was not available at the time of defendant’s direct appeals in Stewart I and Stewart II, and thus it would seem fundamentally unfair to hold that he has waived any claim based on that study. In any event, however, it is now clear that defendant cannot make a substantial showing of a violation of his constitutional rights based on the Gross study.
In People v. Davis (1987), 119 Ill. 2d 61, decided after the briefing was completed in this case, this court was confronted with virtually the identical issue as presented here. In Davis, defendant claimed that the fact he was black and the victim was white increased the likelihood that he would receive the death sentence, and that this racial disparity violated his constitutional rights. The defendant in Davis also proffered the Gross study in support of his contention.
This court, relying on the United States Supreme Court’s recent decision in McCleskey v. Kemp (1987), 481 U.S. 279, 95 L. Ed. 2d 262, 107 S. Ct. 1756, rejected defendant’s claim, holding that “whatever the merits of *108the Gross study, the defendant cannot make a substantial showing of a violation of his Federal constitutional rights.” (People v. Davis (1987), 119 Ill. 2d 61, 66.) This court further held that the Gross study could not substantiate a claim under the Illinois Constitution. Davis, 119 Ill. 2d at 68.
In McCleskey v. Kemp (1987), 481 U.S. 279, 95 L. Ed. 2d 262, 107 S. Ct. 1756, the defendant, relying on a complex statistical study (the Baldus study), alleged that the Georgia death penalty statute is administered in a constitutionally impermissible fashion because of the likelihood that racial considerations are a factor in determining whether a particular defendant receives the death penalty. According to the Baldus study, after controlling for nonracial variables, defendants accused of murdering white victims are 4.3 times more likely to receive the death penalty than those accused of murdering blacks.
The Court, while assuming the validity of the Baldus study, concluded that it “does not demonstrate a constitutionally significant risk of racial bias affecting the Georgia capital-sentencing process.” (481 U.S. at 313, 95 L. Ed. 2d at 292, 107 S. Ct. at 1778.) The Court further held that “[defendant] must prove that the decision-makers in his case acted with discriminatory purpose,” and that “racial considerations played a part in his sentence.” (Emphasis in original.) 481 U.S. at 292, 95 L. Ed. 2d at 278, 107 S. Ct. at 1766-67.
As in Davis and McCleskey, defendant made no attempt to prove that he received the death penalty because the decisionmakers in his case were motivated by racial bias. Moreover, in each of defendant’s cases, constitutionally permissible factors clearly existed upon which the death penalty could be imposed. In Stewart I, defendant was convicted of intentional murder and felony murder. In Stewart II, defendant was convicted of *109murdering two victims during the course of an armed robbery.
Accordingly, absent clear proof to the contrary, there is no basis to assume that defendant’s death sentences were the product of racial bias or “wantonly and freakishly imposed.” (McCleskey, 481 U.S. at 308, 95 L. Ed. 2d at 288, 107 S. Ct. at 1775.) The trial court thus properly dismissed defendant’s claim without an evidentiary hearing. See People v. Silagy (1987), 116 Ill. 2d 357, 365 (a defendant is only entitled to an evidentiary hearing upon a substantial showing that his constitutional rights were violated).
As another basis for post-conviction relief, defendant alleges that the death penalty statute is unconstitutional because it leads to arbitrary and capricious application of the death penalty insofar as it allegedly delegates to prosecutors, without sufficient guidelines, the discretion to determine in which cases the death penalty will be sought. In support of his contention, defendant directs our attention to a survey and accompanying article published in the July 1985 issue of Chicago Lawyer (an independent monthly newspaper) relating to the practices of Illinois prosecutors in seeking imposition of the death penalty. Defendant has also submitted an affidavit summarizing the findings of the survey, and the written responses submitted by various prosecutors in connection with that survey.
The article and survey were based upon information received from a majority of the Illinois State’s Attorneys, and purports to show that some prosecutors always seek the death penalty if a statutory aggravating factor exists, some prosecutors have never sought that penalty, and others decide whether to seek the death sentence on a case-by-case basis. The prosecutors in the latter category have not promulgated written guidelines *110pertaining to the determination of whether to seek the death penalty in a given case.
The State argues that this issue is res judicata because this court already rejected defendant’s claim that the statutory grant of discretion to the prosecutor is improper. (See People v. Stewart (1984), 104 Ill. 2d 463, 498-99; People v. Stewart (1984), 105 Ill. 2d 22, 75.) In the alternative, the State asserts that the survey is insufficient to support defendant’s claim because it does not show that the prosecutors in defendant’s cases sought the death penalty on the basis of constitutionally impermissible factors.
Initially, we do not agree with the State’s contention that this issue was already decided adversely to defendant on his direct appeals. There, defendant was challenging the facial validity of the statute. In contrast, he is now claiming that the statute is invalid as applied, in that prosecutors are allegedly seeking the death penalty on an arbitrary, capricious ánd discriminatory basis. (See McCleskey v. Kemp (1987), 481 U.S. 279, 308, 95 L. Ed. 2d 262, 289, 107 S. Ct. 1756, 1775 (wherein the Court separately addressed defendant’s claims that the capital punishment statute was both facially invalid, and that it was arbitrarily and capriciously applied).) Nevertheless, we do not believe that defendant has made a substantial showing of a constitutional violation such as to entitle him to an evidentiary hearing on this claim.
Assuming, without deciding, the validity of the survey, it simply evidences that prosecutors have exercised discretion in determining whether to seek imposition of the death penalty. For instance, the survey evidences that not every prosecutor has sought the death penalty in every case where it might statutorily be imposed.
In previously holding that the statutory grant of discretion to prosecutors is valid, this court recognized that the death penalty would not be sought in every “death-*111eligible” case. Rather, the decision to seek a death sentence will depend on various factors, such as an “estimate of what testimony and other evidence will be available and its persuasive effect.” People ex rel. Carey v. Cousins (1979), 77 Ill. 2d 531, 543.
In McCleskey v. Kemp (1987), 481 U.S. 279, 296, 311-12, 95 L. Ed. 2d 262, 281, 291, 107 S. Ct. 1756, 1768, 1777, the United States Supreme Court recognized the propriety of allowing prosecutors to exercise discretion in capital cases, and stated that:
“[T]he policy considerations behind a prosecutor’s traditionally ‘wide discretion’ suggest the impropriety of our requiring prosecutors to defend their decisions to seek death penalties ***.”
“[T]he capacity of prosecutorial discretion to provide individualized justice is ‘firmly entrenched in American law.’ [Citation.] As we have noted, a prosecutor can decline to charge, offer a plea bargain, or decline to seek a death sentence in any particular case. [Citation.] Of course, ‘the power to be lenient [also] is the power to discriminate,’ [citation], but a capital-punishment system that did not allow for discretionary acts of leniency ‘would be totally alien to our notions of criminal justice.’ [Citation.]”
Cf. Gregg v. Georgia (1976), 428 U.S. 153, 199, 49 L. Ed. 2d 859, 889, 96 S. Ct. 2909, 2937 (opinion of Stewart, J., joined by Powell and Stevens, JJ.) (the fact that, under Georgia law, a prosecutor has “unfettered authority” to choose whom he will charge with a capital offense, does not offend the Constitution).
In the instant case, while the survey purportedly evidences the application of prosecutorial discretion, there is virtually no suggestion as to the reasons why the death penalty was or was not sought in any given case. The small minority of prosecutors who provided reasons for seeking this penalty primarily stated that the decision was based upon the available evidence and the *112strength of their case, the nature of the offense, and the prior record of the accused. Thus, we cannot conclude, and will not assume, that the prosecutors’ judgments are based on whim or caprice or otherwise invoke impermissible considerations. We note in this connection that defendant does not claim that the prosecutors in his cases acted arbitrarily in seeking the death sentence. See McCleskey v. Kemp (1987), 481 U.S. 279, 296 n.17, 95 L. Ed. 2d 262, 281 n.17, 107 S. Ct. 1756, 1768 n.17 (“Requiring a prosecutor to rebut a study that analyzes the past conduct of scores of prosecutors is quite different from requiring a prosecutor to rebut a contemporaneous challenge to his own acts”).
Moreover, this court has previously rejected studies preferred by defendants in an effort to show that the death penalty is arbitrarily imposed, holding that, inter alia, “a prosecutor cannot arbitrarily impose the death penalty” since imposition of that penalty is the province and responsibility of the sentencing body. (People v. Lewis (1984), 105 Ill. 2d 226, 252; see also People v. Silagy (1987), 116 Ill. 2d 357, 372-73.) The prosecutor is authorized only to seek the death penalty, and even then his discretion is not unfettered since he may only do so where one or more statutory aggravating factors exist.
Accordingly, since prosecutors are entitled to exercise discretion in seeking imposition of the death penalty, and since defendant has proffered no evidence indicating that such discretion has been exercised on the. basis of impermissible factors, we find that the survey relied upon by defendant is insufficient to establish a substantial showing of a constitutional violation.
Defendant’s final claim for post-conviction relief is that execution by lethal injection violates the eighth amendment because of the alleged absence of “guidelines and procedures” to “protect against a torturous *113death.” Defendant has abandoned, as he must, his related claim asserted in the trial court that approval by the Food and Drug Administration (FDA), for the drugs that would be administered, is required before execution by lethal injection could be performed. See Heckler v. Chaney (1985), 470 U.S. 821, 84 L. Ed. 2d 714, 105 S. Ct. 1649 (the FDA's refusal to take investigatory and enforcement action relating to the drugs to be administered for execution by lethal injection is not subject to judicial review).
Section 119 — 5 of the Code of Criminal Procedure of 1963 (Ill. Rev. Stat. 1983, ch. 38, par. 119 — 5) provides, in relevant part:
“A defendant sentenced to death shall be executed by a continuous, intravenous administration of a lethal quantity of an ultrashort-acting barbituate in combination with a chemical paralytic agent until death is pronounced by a licensed physician according to accepted standards of medical practice.” (Ill. Rev. Stat. 1983, ch. 38, par. 119— 5(a)(1).)
The provision further requires that “[t]he warden of the penitentiary shall supervise such execution which shall be conducted in the presence of 2 physicians and 6 witnesses ***.” Ill. Rev. Stat. 1983, ch. 38, par. 119— 5(a)(2)(d).
Thus, the above-quoted portions of section 119 — 5 do set forth certain general guidelines relating to the “method and manner” of execution. Moreover, defendant fails to suggest what additional guidelines should be required, and fails to cite any authority for the proposition that the absence of these alleged unspecified guidelines renders execution by lethal injection unconstitutional.
We recognize that any method of execution which “involve[s] the unnecessary and wanton infliction of pain” is constitutionally impermissible. (Gregg v. Georgia (1976), *114428 U.S. 153, 173, 49 L. Ed. 2d 859, 875, 96 S. Ct. 2909, 2925.) However, defendant has submitted no evidence which indicates that execution by lethal injection results in protracted death or unnecessary pain. Indeed, this court has previously observed “that commentators and medical experts have urged that death by lethal gas or injection is a more sure, swift, and humane means than [the prior statutory method of] electrocution.” People v. Silagy (1987), 116 Ill. 2d 357, 372.
Defendant’s only “authority” for the proposition that execution by lethal injection may allegedly pose a risk of cruel punishment is language in the United States Court of Appeals’ decision in Chaney v. Heckler (D.C. Cir. 1983), 718 F.2d 1174, which was reversed on other grounds by the United States Supreme Court (Heckler v. Chaney (1985), 470 U.S. 821, 84 L. Ed. 2d 714, 105 S. Ct. 1649).
The issue arose in the context of deciding defendant’s claim that the FDA should be required to take investigatory and enforcement action concerning the drugs administered in performing executions by lethal injection. The Court of Appeals held that the FDA could be judicially compelled to take such action because, inter alia, its refusal to do so “may also implicate the Eighth Amendment’s prohibition of cruel and unusual punishment” insofar as evidence was allegedly presented that execution by injection poses a risk of “cruel, protracted death.” Chaney, 718 F.2d at 1191.
In reversing the Court of Appeals’ decision, the United States Supreme Court determined that the FDA’s decision to refrain from taking enforcement action was not subject to judicial review. Although the Court did not directly rule on defendant’s eighth amendment claim, it did note that “[n]o colorable claim is made in this case that the agency’s refusal to institute proceedings violated any constitutional rights of [defend*115ants].” (Emphasis added.) Heckler, 470 U.S. at 838, 84 L. Ed. 2d at 728,105 S. Ct. at 1659.
For the reasons stated herein, the trial court’s denial of the defendant’s post-conviction petitions is affirmed. The clerk of this court is directed to enter an order fixing Tuesday, May 17, 1988, as the date on which the sentences of death entered in the circuit court are to be executed. The defendant shall be executed by lethal injection in the manner provided by section 119 — 5 of the Code of Criminal Procedure of 1963 (Ill. Rev. Stat. 1985, ch. 38, par. 119 — 5). A certified copy of this order shall be furnished to the Director of Corrections, to the warden of Stateville Correction Center, and to the warden of the institution wherein the defendant is confined.
Judgment affirmed.