delivered the judgment of the court:
Following a jury trial in the circuit court of Jackson County, defendant, Linda Basler, was convicted of driving under the influence and sentenced to 12 months’ probation. The appellate court reversed and remanded for a new trial. We granted the State’s petition for leave to appeal. 177 Ill. 2d R. 315. For the reasons that follow, we now affirm the appellate court’s judgment as modified.
In October of 1996, defendant was arrested by police and charged with driving under the influence of alcohol (625 ILCS 5/11 — 501 (West 1996)). The circuit court appointed the Jackson County public defender to represent her. On the day of her trial, defendant requested a continuance to seek private counsel on the grounds that she and her appointed attorney did not agree on certain matters. Defendant also advised the court that she had been ill, that she did not feel capable of assisting in her defense, and that some of her witnesses were not able to testify that day.
The circuit court denied defendant’s motion, and the matter proceeded to trial before a jury. The jury returned a verdict of guilty. The circuit court then sentenced defendant to 12 months’ probation and fined her $300. The court also ordered defendant to pay $5 per month for the services of the probation office and to pay $25 for the services of her public defender.
Defendant filed a post-trial motion for a new trial, arguing that the State had failed to prove its case beyond a reasonable doubt. That motion was denied, and defendant appealed. As grounds for her appeal, defendant asserted that the circuit court abused its discretion when it denied her motion for a continuance without making further inquiry into the circumstances involved and without making a finding that she had brought the motion to delay trial. Defendant further contended, among other things, that the trial court should not have ordered her to pay a fee to the public defender’s office without holding a hearing on her financial circumstances and her ability to pay.
The appellate court reversed and remanded for a new trial in an unpublished order. No. 5 — 97—0979 (unpublished order under Supreme Court Rule 23). As grounds for its decision, the court held that the circuit court had committed reversible error when it rejected defendant’s motion for a continuance without inquiring further into the circumstances or finding that she had presented the motion merely to delay the trial. The appellate court further held that the trial judge should not have required defendant to pay the $25 fee for her public defender without first holding a hearing on her ability to pay. In disposing of the case, the appellate court directed the circuit court to hold such a hearing on remand and to provide a court reporter to memorialize that hearing.
Defendant petitioned for rehearing, asking the appellate court to consider additional claims she had raised on appeal, including a claim that the circuit court should not have received evidence of the results of a horizontal-gaze-nystagmus (HGN) test without first conducting a hearing under Frye v. United States, 293 F. 1013 (D.C. Cir. 1923). Although the appellate court purported to deny defendant’s petition, it vacated its decision and filed a new, published opinion in its stead.
In its opinion, the court reiterated its prior holdings in the case, but added a discussion regarding the Frye issue. 304 Ill. App. 3d 230. The court noted that it had previously ruled that the HGN test meets the Frye standard and is admissible when a proper foundation is laid. See People v. Buening, 229 Ill. App. 3d 538, 545-46 (1992). The court observed, however, that People v. Kirk, 289 Ill. App. 3d 326 (1997), a subsequent decision from another district of the appellate court, took a different view. In Kirk a divided panel of the Fourth District of the appellate court held that it is necessary to conduct a Frye hearing prior to the admission of the result of a HGN test in a criminal trial for DUI. Kirk, 289 Ill. App. 3d at 331.
Although it cited Kirk with approval, the appellate court in this case stopped short of embracing that decision and overruling Buening. Similarly, it did not expressly hold that the trial court had erred in admitting the HGN test results at the original trial without first conducting a Frye hearing. Instead, it simply suggested that if a new trial is held following remand and the State wishes to introduce evidence of the HGN test results, then “a Frye hearing might well be appropriate.”
On this appeal, the State does not take issue with the appellate court’s decision to reverse and remand for a new trial based on the circuit court’s refusal to grant defendant a continuance. Nor does it contest the appellate court’s determination that the trial judge should not have required defendant to pay the $25 fee for her public defender without first holding a hearing on her ability to pay. The State’s sole concern is the appellate court’s handling of the Frye issue.
The State contends that the appellate court’s decision is problematic because it denied the State the opportunity to address defendant’s request that the appellate court address admissibility of HGN test results under the Frye standard. According to the State, the appellate court’s decision to vacate its original order and file a new opinion in its place was tantamount to granting defendant the relief she requested on rehearing. Where a petition for rehearing is allowed, the opposing party has the right under our rules to respond. 155 Ill. 2d R. 367(d). Because the court in this case purported to deny defendant’s petition for rehearing, however, the State was deprived of that right. 155 Ill. 2d R. 367(d).
The appellate court’s decision is also problematic, according to the State, because its directions to the circuit court are ambiguous and confusing. As we have indicated, the appellate court’s decision neither overrules Buening nor expressly adopts Kirk and gives no clear indication as to whether a Frye hearing is, in fact, required on remand.
Before considering the State’s contentions, we must first address the position taken by defendant. In responding to the State’s arguments, defendant goes beyond the points raised by the State and invites our court to use this matter as a vehicle for considering whether HGN test results should ever be admitted in prosecutions for driving under the influence. This we decline to do. The problem with undertaking such an expansive analysis is that validity of the HGN test was never challenged in the trial court. Defense counsel raised no objection to the admission of the HGN test results against defendant, and use of the HGN test results was not contested by defendant in her post-trial motion.
As a general rule, a defendant must object to an error at trial and include the objection in a post-trial motion in order to preserve it for review on appeal. People v. Enoch, 122 Ill. 2d 176, 186 (1988). A reviewing court may override considerations of waiver where plain errors or defects affecting substantial rights are involved. 134 Ill. 2d R. 615(a). The present case, however, involves neither circumstance. In addition, because validity of the HGN test was not raised below, the record is devoid of the evidentiary material necessary to assess defendant’s challenge. Such material cannot be presented to an appellate court in the first instance. We are not triers of fact. Our function is to serve as a court of review. Accordingly, even if we wanted to take up the matter, we would have no informed basis for doing so.
Given the lack of pertinent evidence in the trial court, the absence of appropriate objections by defense counsel, and the failure of defendant to raise the issue in her post-trial motion, the appellate court’s decision to address the validity of HGN test results on rehearing is difficult to comprehend. Wholly aside from the question of whether the State was prejudiced by its inability to file a response under Rule 367(d) (155 Ill. 2d R. 367(d)), the posture of the case was such that the appellate court could not possibly make a definitive ruling on the matter. At best, its conclusions could only be tentative and conditional. The result is a remand order that obfuscates the law and offers no clear guidance to the trial court. Based upon what the appellate court has written, the trial court has no way to judge whether and under what circumstances a Frye hearing must actually be conducted. Accordingly, the appellate court’s remand directions do little tó advance the court’s stated goal in addressing the issue, which was to prevent error on retrial.
The appellate court’s analysis of the Frye issue is flawed for another, more fundamental reason. Defendant cited research to the court questioning the validity of the HGN test. In ruling as it did, the court apparently believed that requiring a Frye hearing was the appropriate mechanism for bringing that research before the trial court so that the issue of the test’s validity could be reassessed. The court’s concern is legitimate. Science is not static, and methods must exist for reexamining the validity of scientific tests when new information is acquired. What the appellate court failed to appreciate is that the Frye test is not an appropriate vehicle for accomplishing that purpose.
Frye v. United States, 293 F. 1013 (D.C. Cir. 1923), which is followed in Illinois, pertains to the admission of scientific evidence which is novel. It requires that evidence be generally accepted in the relevant scientific community before it can be admitted. People v. Miller, 173 Ill. 2d 167, 187-88 (1996). The HGN test was found to meet the Frye standard in People v. Buening, 229 Ill. App. 3d 538 (1992). That decision, in turn, was followed by the appellate court in People v. Wiebler, 266 Ill. App. 3d 336, 339 (1994).
In the wake of Buening and Wiebler, HGN test results have been routinely admitted in prosecutions for driving under the influence. The tests are no longer “novel” in any meaningful sense. As a result, the State should not be put to the burden of having to reestablish the test’s validity in every case. See People v. Kirk, 289 Ill. App. 3d 326, 335-37 (1997) (Steigmann, P.J., specially concurring). Where, as here, a scientific method has been shown to be generally accepted, a Frye test is no longer necessary each time the State seeks to use evidence obtained by that method. See People v. Rozo, 303 Ill. App. 3d 787, 793 (1999); see also People v. Thomas, 137 Ill. 2d 500, 518 (1990) (trial court did not err by failing to hold a Frye hearing on admissibility of electrophoresis testimony after taking judicial notice of electrophoresis as an accepted scientific procedure based on the prior decision in People v. Partee, 157 Ill. App. 3d 231 (1987), that such testimony was admissible); People v. Johnson, 262 Ill. App. 3d 565, 568-69 (1994) (trial court appropriately relied on precedential case law to determine that the proffered DNA testimony was admissible and had no need to conduct a Frye hearing prior to making its decision on the admissibility of that evidence). To the extent that the appellate court’s decision in People v. Kirk, 289 Ill. App. 3d 326 (1997), suggests otherwise, it is hereby overruled.
Although the State is no longer required to show that the HGN test satisfies the Frye standard before it may introduce the results of an HGN test into evidence, the validity of HGN tests and test results is not beyond challenge. If a defendant has evidence showing that HGN tests are scientifically unsound, then he may interpose the appropriate objection to the HGN test results and present his supporting evidence to the trial court. If the trial court is persuaded by the defendant’s evidence, then the court has the right to bar its admission. Note, however, that it is the defendant’s obligation to show that the test results are infirm. It is not the responsibility of the State to show that the tests and results are scientifically valid. Absent proof by the defense that the HGN test is unsound, the State need only show that the officer who gave the test was trained in the procedure and that the test was properly administered.
For the foregoing reasons, the judgment of the appellate court is affirmed, as modified herein, and the matter is remanded to the circuit court for further proceedings consistent with this opinion.
Affirmed as modified.