Arvin Ritchey Mason and Claudia Mason (hereinafter, the Masons) sued The Home Depot U.S.A., Inc. and The Flecto Company, Inc. in 1997 for injuries Arvin Mason allegedly received in 1996 while using Varathane, a floor covering product manufactured by Flecto and sold by Home Depot, and for Claudia Mason’s resulting loss of *272consortium. Shortly before the case went to trial in February 2005, the Georgia General Assembly enacted the Tort Reform Act of 2005 (hereinafter TRA), including OCGA § 24-9-67.1,* 1 23which governs the qualification of expert witnesses and the admissibility of expert testimony. Based on the new statute, the defense filed a motion to exclude the testimony of two expert witnesses for the plaintiffs, Dr. Grace Ziem and Dr. Ronald Huggins. The trial court denied the motion, holding that application of the new statute after years of discovery under pre-existing rules governing expert witnesses would violate the Georgia Constitution’s proscription against retroactive laws. After a mistrial, the defense renewed their motion to exclude the testimony of the two experts, in response to which the Masons mounted attacks on the constitutionality of OCGA § 24-9-67.1. The trial court rejected the Masons’ contentions the statute denied equal protection, violated the constitutional guarantee of trial by jury, and violated the prohibition against retroactive law, but found a portion of subsection (b) (1) to deny due process because it contradicts part of subsection (a), and cured the problem by excising part of subsection *273(b) (1) and found subsection (f) violative of the principle of separation of powers, but cured that defect by severing subsection (f) from the statute. The trial court entered a second order applying the statute to exclude the testimony of the two experts. This appeal is from those two orders.
1. The Masons contend the statute violates the guarantees of equal protection of the laws found in the constitutions of the United States and Georgia. Specifically, they contend that because the statute imposes more stringent requirements for the admission of expert testimony in their tort action than applicable statutes would in criminal cases and in civil condemnation cases, they are disadvantaged in comparison to the parties in those types of cases.
Standing to challenge a statute on constitutional grounds in Georgia depends on a showing the plaintiff was injured in some way by the operation of the statute or that the statute has an adverse impact on the plaintiffs rights. Tennille v. State, 279 Ga. 884,885 (622 SE2d 346) (2005); Agan v. State, 272 Ga. 540 (1) (533 SE2d 60) (2000); State ofGa. v. Jackson, 269 Ga. 308 (1) (496 SE2d 912) (1998); Ambles v. State, 259 Ga. 406 (1) (383 SE2d 555) (1989). The Masons showed how application of the stricter standards of OCGA§ 24-9-67.1 places them at a disadvantage compared to parties in criminal cases, in which the admissibility of expert testimony is governed by the less strict standard of OCGA § 24-9-67 (“In criminal cases, the opinions of experts on any question of science, skill, trade, or like questions shall always be admissible; and such opinions may be given on the facts as proved by other witnesses.”), and in civil condemnation actions, which are exempted from the requirements of the statute by OCGA § 22-1-14 (b), thereby establishing their appellate standing to assert a claim of denial of equal protection of the law.2
One challenging a statute on equal protection grounds must initially establish that he is similarly situated to members of the class who are treated differently from him. Quarterman v. State, 282 Ga. 383, 384, fn. 4 (651 SE2d 32) (2007). The Masons, therefore, bear the burden of showing that they are situated similarly to persons whom they allege are treated differently with regard to the admissibility of expert opinion testimony than they. Our review of the record and of Georgia law persuades us the Masons cannot shoulder that burden.
*274“[T]he equal protection clause [does not] exact uniformity of procedure. The legislature may classify litigation and adopt one type of procedure for one class and a different type for another.” Dohany v. Rogers, 281 U. S. 362, 369 (50 SC 299, 74 LE 904) (1930). Georgia’s jurisprudence is in line with that quoted above since, far from considering all litigants to be similarly situated, this Court has consistently rejected the argument that all criminal defendants are similarly situated to each other and that all civil litigants are similarly situated to each other for purposes of evidentiary issues. “When considering an equal protection challenge in criminal matters, individuals are ‘similarly situated’ only if they are charged with the same crime or crimes.” Woodard v. State, 269 Ga. 317, 321 (3) (496 SE2d 896) (1998) (1995 amendment to Child Hearsay Statute unconstitutional because it created disparate categories of identically situated defendants charged with molestation, some of whom must defend against more evidence than others). See also Sims v. State, 260 Ga. 782 (1) (399 SE2d 924) (1991) (statute which applies equally to all persons accused of child molestation does not create disparate classifications among similarly situated persons). In Bell v. Austin, 278 Ga. 844 (2) (a) (607 SE2d 569) (2005), this Court rejected an equal protection challenge to OCGA § 24-3-18 (a), which creates a hearsay exception applicable in “the trial of any civil case involving injury or disease,” observing that the plaintiff could not show he was treated differently than similarly situated persons because he was “treated no differently than any other party in a civil action involving injury or disease — ” Thus, just as defendants in criminal cases are situated similarly only to others charged with the same offense, civil litigants are considered similarly situated only to others litigating the same cause of action.
While we have not previously been called upon to decide specifically whether civil law litigants are similarly situated to criminal litigants, we note that procedure in civil cases is governed by the Civil Practice Act while procedure in criminal cases is governed by the provisions of Title 17 of the Official Code of Georgia, thus establishing a separation between the two spheres of litigation. We held in Sims v. State, supra, 260 Ga. at 783, that the defendant, charged with child molestation, had not shown “that child molesters are similarly situated to all other criminals or to civil litigants.” (Emphasis supplied.) From the foregoing, we conclude that, for purposes of evidentiary standards, only those accused of the same offense are similarly situated in the criminal law arena, only those asserting or defending against the same cause of action are similarly situated in the civil law arena, and the parties to civil cases are not similarly situated to those engaged in criminal prosecutions.
*275Thus, the Masons cannot establish the necessary element of an equal protection claim that they are situated similarly to those being treated differently. That being so, we need not pursue the equal protection issue past that point. Quarterman v. State, supra.
The person who is asserting the equal protection claim has the burden to establish that he is similarly situated to members of the class who are treated differently from him. If the person asserting the violation cannot make the foregoing showing, there is no need to continue with an equal protection analysis.
(Citations and punctuation omitted.) Rodriguez v. State, 275 Ga. 283 (1) (565 SE2d 458) (2002). The trial court did not err in rejecting the Masons’ equal protection challenge.
2. The Masons contend that OCGA § 24-9-67.1 violates due process because subsections (a) and (b) (1) are contradictory and, therefore, the entire statute should be stricken. The trial court agreed the subsections were contradictory; however, instead of striking the statute in its entirety, the trial court severed the contradictory language from subsection (b) (l)3 and then proceeded to apply the revised statute, ultimately ruling that the Masons’ experts were precluded from testifying under subsection (b) (2). Appellees contend that the Masons lack standing to challenge the constitutionality of these two provisions because the trial court excluded their expert under subsection (b) (2). We disagree. The Masons have appellate standing because they were harmed by the trial court’s decision not to strike OCGA § 24-9-67.1 in its entirety, thereby enabling the trial court to exclude the Masons’ experts under subsection (b) (2).
As to the underlying merits of the Masons’ constitutional challenges, the trial court was correct in finding that subsection (a) and subsection (b) (1) are contradictory. Specifically, subsection (b) (1) limits experts to relying on potentially admissible facts and data, whereas subsection (a) plainly states that facts and data relied upon need not be admissible. The two provisions cannot be harmonized and, read together, they render the statute unconstitutionally vague. The trial court, however, was not required to strike the statute in its entirety because we have held that invalid portions may be severed if, as here, they are not mutually dependent on the remaining portions and legislative intent is not compromised. Union City Bd. of Zoning Appeals v. Justice Outdoor Displays, 266 Ga. 393, 404 (7) (467 SE2d *276875) (1996) (“When a statute cannot be sustained as a whole, the courts will uphold it in part when it is reasonably certain that to do so will correspond with the main purpose which the legislature sought to accomplish.”); Nixon v. State, 256 Ga. 261, 264 (3) (347 SE2d 592) (1986) (“Where one portion of a statute is unconstitutional, this court has the power to sever that portion of the statute and preserve the remainder if the remaining portion of the Act accomplishes the purpose the legislature intended.”). Therefore, we conclude that the trial court did not err.
3. The Masons contend OCGA § 24-9-67.1 (f) is unconstitutional as a delegation of legislative authority and a denial of due process. Subsection (f) contains two sentences, the first of which is an expression of intent that, “in all civil cases, the courts of the State of Georgia not be viewed as open to expert evidence that would not be admissible in other states.” The second sentence suggests that Georgia courts, “in interpreting and applying this Code section,... may draw from the opinions of the United States Supreme Court in Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 [(113 SC 2786, 125 LE2d 469)] (1993); General Electric Co. v. Joiner, 522 U.S. 136 [(118 SC 512, 139 LE2d 508)] (1997); Kumho Tire Co. Ltd. v. Carmichael, 526 U.S. 137 [(119 SC 1167, 143 LE2d 238)] (1999); and other cases in federal courts applying the standards announced by the United States Supreme Court in these cases.”
Addressing the second sentence, the trial court agreed with the Masons that the suggestion that Georgia courts may consider federal authority in construing OCGA § 24-9-67.1 was a usurpation of judicial authority in that it was “clearly an attempt by the legislature to perform a judicial function by construing a law. . . .” Martin v. Baldwin, 215 Ga. 293, 299 (110 SE2d 344) (1959). Concluding the use of the word “may” did not make the sentence less an attempt to infringe on the province of the courts, the trial court found that provision unconstitutional as a violation of the principle of separation of powers. “The doctrine of separation of powers is an immutable constitutional principle which must be strictly enforced. Under that doctrine, statutory construction belongs to the courts, legislation to the legislature.” (Citation and punctuation omitted.) Allen v. Wright, 282 Ga. 9 (1) (644 SE2d 814) (2007).
We disagree with the trial court’s conclusion. The suggestion in the statute that Georgia “may” consider the decisions of other courts on the subject did not invade the province of the judiciary because it was not couched in mandatory terms and merely stated a principle of law regularly employed by Georgia courts. See, e.g., State v. Mayze, 280 Ga. 5, 8-9 (622 SE2d 836) (2005) (analysis by Alabama court, although not controlling, is persuasive authority for construction of analogous constitutional and statutory provisions); and Barnum v. *277Coastal Health Svcs., 288 Ga. App. 209 (2) (b) (653 SE2d 816) (2007) (because of similarity of Georgia Civil Practice Act to Federal Rules of Civil Procedure from which it was taken, it is proper to consider and give weight to constructions placed on the federal rules by federal courts). The permissive suggestion in OCGA § 24-9-67.1 (f) that the courts consider federal interpretations of the cases on which the federal rules and OCGA § 24-9-67.1 were based contains no words of command and, unlike the resolution condemned in Martin v. Baldwin, supra, does not seek to enforce on the courts a particular construction of the statute. Accordingly, we conclude the trial court was mistaken in declaring subsection (f) unconstitutional on that ground. That conclusion does not require reversal of the trial court’s judgment. The goal of the Masons’ attack on subsection (f) was to have the entire statute declared unconstitutional. Since the trial court, after finding subsection (f) unconstitutional, applied the severability clause enacted with the statute (Ga. Laws 2005, p. 1, § 14), the net effect on the parties was the same as the effect of our ruling: the statute remains in effect.
Having already ruled subsection (f) unconstitutional, the trial court declined to rule on the Masons’ contention that the first sentence of the subsection was also unconstitutional. In the interest of judicial economy, we will address the arguments the Masons raise on appeal.4
The Masons’ argument on this issue identifies two purported infirmities in the first sentence of subsection (f), that it delegates to the courts of Georgia, and ultimately to the courts and legislatures of other states, the legislative authority to prescribe rules of evidence, and that it denies due process of law to litigants because it has no clear guidelines on how Georgia courts are to exercise the delegated powers. At the heart of those arguments is the Masons’ assertion that the first sentence of subsection (f) constitutes a command to the courts of Georgia to rewrite Georgia law anytime any other jurisdiction announces a more strict standard for the admission of expert testimony.iThat assertion is the fatal flaw in their argument because the státement of intent in the first sentence of subsection (f) is no mbre than an explication by the legislature of its goal in enacting the statute. It contains no words of command which would invade the province of the judiciary or would delegate to the judiciary the legislature’s duty. “It is always the duty of a court, in construing a statute, to ascertain and give full effect to the legislative intent....” *278Moore v. Robinson, 206 Ga. 27, 40 (6) (55 SE2d 711) (1949). In that case, the caption of the act contained an expression by the legislature of the act’s purpose and this Court suggested that in construing a doubtful statute there was “no better source to which a court may go for the purpose of finding the legislature’s meaning of an act passed by it.” Id. The present case may contain that “better source” since the legislature has been explicit in the body of the statute itself in stating its intent that the admission of expert testimony in the courts of this State be governed by the strictest standards available. We note that the Georgia Court of Appeals, in construing OCGA § 24-9-67.1, has already recognized subsection (f) as a statement of legislative intent. Cotten v. Phillips, 280 Ga. App. 280, 285-286 (633 SE2d 655) (2006). We conclude, therefore, that the statement of intent in subsection (f) is not a delegation of legislative power.
4. Contending they have a vested right to application of the evidentiary rules in effect prior to the passage of OCGA § 24-9-67.1, the Masons contend the application of the statute to their case violates the constitutional prohibition against retroactive laws. See Ga. Const. 1983, Art. I, Sec. I, Par. X. In support of that contention, they point to Fowler Properties v. Dowland, 282 Ga. 76 (646 SE2d 197) (2007), where this Court held the offer-of-judgment provisions of the TRA could not be applied to actions instituted prior to enactment of the statute. In that case, we drew a distinction between substantive and procedural law: “substantive law creates rights, duties, and obligations while a procedural law prescribes the methods of enforcing those rights, duties, and obligations.” Id. at 78. “[Wjhere a statute governs only procedure of the courts, including the rules of evidence, it is to be given retroactive effect absent an expressed contrary intention.” Polito v. Holland, 258 Ga. 54 (2) (365 SE2d 273) (1988). The Masons’ argument that their expenditures in finding experts and developing their testimony served to vest their right to use the experts contradicts the straightforward rule “that there are no vested rights in any course of procedure.” Foster v. Bowen, 253 Ga. 33 (315 SE2d 656) (1984). The fact that the Masons are disadvantaged by the statutory change does not affect application of the rule stated in Pritchard v. Savannah St. & R. R. R. Co., 87 Ga. 294, 299 (13 SE 493) (1891): “The presumption against a retrospective construction has no application to enactments which affect only the procedure and practice of the courts, even where the alteration which the statutes make has been disadvantageous to one of the parties.” (Citation and punctuation omitted.) The Masons’ reliance on Jones v. Oemler, 110 Ga. 202 (35 SE 375) (1900), for the proposition that one may obtain a vested right in an evidentiary rule is misplaced. The issue there was whether a statute which changed the evidentiary weight to be given to certain charts could be applied to impact the rights a party had *279obtained under a lease. The holding that it could not was based on vested rights in a property interest, not a vested right in an eviden-tiary rule. No such property right is involved here. The application of the evidentiary rules established by the TRA did not violate the constitutional prohibition against retroactive laws.
5. Finally, the Masons assert that the trial court, in applying the statute, erred in its decision to exclude the testimony of the experts used by the Masons at the first trial. They identify two basic decisions of the trial court as error: applying the standards identified in Daubert v. Merrell Dow Pharmaceuticals, supra, and concluding that the Masons’ expert witnesses, Dr. Ziem and Dr. Huggins, could not testify as experts on causation and labeling, respectively, because the methods they employed in reaching their opinions do not meet the standards of OCGA§ 24-9-67.1 (b) (2).
As we noted above, it is proper to consider and give weight to constructions placed on the federal rules by federal courts when applying or construing a statute based on those rules. See, e.g., Turpin v. Todd, 268 Ga. 820 (2) (a) (493 SE2d 900) (1997) (because procedural default standards of OCGA § 9-14-48 (d) are modeled on the federal standards, we look to federal decisions for guidance on this issue). Since OCGA § 24-9-67.1 (b) was based on Fed. R. Evid. Rule 702, which in its present form is based on the holdings in Daubert (“Rule 702 has been amended in response to Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U. S. 579 (1993), and to the many cases applying Daubert, including Kumho Tire Co. v. Carmichael, 119 SC 1167 (1999)” Fed. R. Evid. Rule 702, Advisory Committee Notes, 2000 Amendments), the trial court’s application of the standards of Daubert was proper. Turpin v. Todd, supra.
[WJhether “a witness is qualified to render an opinion as an expert is a legal determination for the trial court and will not be disturbed absent a manifest abuse of discretion.” (Citation and punctuation omitted.) Moran v. Kia Motors America, Inc., 276 Ga. App. 96, 97 (1) (622 SE2d 439) (2005). We perceive no abuse of discretion in the trial court’s conclusion that neither witness’s testimony was shown to be “the product of reliable principles and methods. . . .” OCGA § 24-9-67.1 (b) (2). The trial court’s rationale for excluding the testimony of Dr. Ziem as an expert on the issue of causation was based on a lack of scientific support of her methods of determining causation. Essentially, the trial court faulted Dr. Ziem’s methods as being based only on her own experience and opinions, without any support in published scientific journals or any reliable techniques for discerning the behaviors and effects of the chemicals contained in Varathane. The Masons argue that since Dr. Ziem used the accepted medical methodology of differential diagnosis, the trial court could not properly find her methods to lack scientific support. *280However, “expert opinions employing differential diagnosis must be based on scientifically valid decisions as to which potential causes shouldbe ‘ruled in’ and ‘ruled out.’ [Cit.]” Ervin v. Johnson & Johnson, Inc., 492 F3d 901, 904 (7th Cir. 2007). Dr. Ziem’s testimony did not establish that required basis for supporting the application of a differential diagnosis. The trial court’s findings, based primarily on Dr. Ziem’s own testimony, support the conclusion that Dr. Ziem’s testimony regarding causation was not “the product of reliable principles and methods. . . .” OCGA § 24-9-67.1 (b) (2). Moran v. Kia Motors America, Inc., supra.
The trial court based its exclusion of Dr. Huggins’s testimony as an expert on labeling on the methods he used in reaching his opinion. In considering Dr. Huggins’s methods, the trial court noted that Dr. Huggins based his opinions primarily on toxicity data concerning Varathane’s constituent chemicals without regard to the quantity of each chemical in the product or such factors as evaporation rates, used standards having no specific relevance to consumer use of products, and based his opinion solely on data he obtained from the Internet and from the Masons’ attorneys. Considering those findings, we conclude the trial court did not abuse its discretion in determining Dr. Huggins’s testimony was not “the product of reliable principles and methods. . . .” OCGA§ 24-9-67.1 (b) (2).
Having concluded that the trial court’s decisions on the constitutional issues properly before this Court on appeal were correct or constituted harmless error, and that the trial court’s exclusion of the opinion testimony of Dr. Ziem regarding causation and Dr. Huggins regarding labeling was not an abuse of discretion, we find no basis for reversing the trial court’s judgment.
Judgment affirmed in part and vacated in part.
All the Justices concur, except Melton, J., who concurs specially and Hunstein, P. J., and Carley, J., who dissent.The statute reads, in pertinent part, as follows:
(a) The provisions of this Code section shall apply in all civil actions. The opinion of a witness qualified as an expert under this Code section may be given on the facts as proved hy other witnesses. The facts or data in the particular case upon which an expert bases an opinion or inference may be those perceived by or made known to the expert at or before the hearing or trial. If of a type reasonably relied upon by experts in the particular field in forming opinions or inferences upon the subject, the facts or data need not be admissible in evidence in order for the opinion or inference to be admitted. Facts or data that are otherwise inadmissible shall not be disclosed to the jury hy the proponent of the opinion or inference unless the court determines that their probative value in assisting the jury to evaluate the expert’s opinion substantially outweighs their prejudicial effect.
(b) If scientific, technical, or other specialized knowledge will assist the trier of fact in any cause of action to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education may testify thereto in the form of an opinion or otherwise, if:
(1) The testimony is based upon sufficient facts or data which are or will be admitted into evidence at the hearing or trial;
(2) The testimony is the product of reliable principles and methods; and
(3) The witness has applied the principles and methods reliably to the facts of the case.
(f) It is the intent of the legislature that, in all civil cases, the courts of the State of Georgia not be viewed as open to expert evidence that would not be admissible in other states. Therefore, in interpreting and applying this Code section, the courts of this state may draw from the opinions of the United States Supreme Court in Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993); General Electric Co. v. Joiner, 522 U.S. 136 (1997); Kumho Tire Co. Ltd. v. Carmichael, 526 U.S. 137 (1999); and other cases in federal courts applying the standards announced by the United States Supreme Court in these cases.
To the extent the Masons’ argument on equal protection concerns how criminal defendants are disadvantaged by the TRA, they lack standing to make that argument because they are not criminal defendants. “A party will not he heard to complain of the violation of another person’s constitutional rights.” Sims v. State, 243 Ga. 83 (2) (252 SE2d 501) (1979). For the same reason, the similar arguments made in several briefs filed by amici curiae are of no aid to the Masons.
The trial court severed the phrase “which or will be admitted into evidence at the hearing at trial” from OCGA § 24-9-67.1 (b) (1).
Although this Court generally will not rule on a challenge to the constitutionality of a statute unless the issue has been raised and ruled on in the trial court (Poppell v. Gault, 278 Ga. 437 (2) (603 SE2d 271) (2004)), in the interest of judicial economy we exercise our discretion to decide this question. See Campbell v. State, 268 Ga. 44 (2) (485 SE2d 185) (1997).