concurring in part and dissenting in part.
I agree wholeheartedly with the majority’s conclusion that an expert witness can be questioned both as to the percentage of income attributable to his or her services as an expert witness or consultant and as to the total amount of income that he or she derives from such services.1 Armed with accurate answers to questions concerning those two (2) topics, a jury should be able to assess any possible bias on the part of the expert witness that results from the witness’s compensation by the party who retained him or her. In recognition of that fact, I similarly agree with the majority’s conclusion that an expert witness’s total gross income from all sources should not be subject to routine disclosure through discovery.2 The major*640ity opinion, however, fails to recognize that deposition questioning alone may be insufficient to ensure that a jury receives accurate information regarding the extent of an expert witness’s compensation. In my opinion, the Court of Appeals “hit the nail on the head” in this case when it observed in its opinion below that “[sjince a party is entitled to conduct a fair and complete cross-examination of an expert witness chosen by her adversary, that party is also entitled to [] pursue [fully] the discovery of relevant evidence that has the tendency to assist that process.” As such, I part company with the majority’s ultimate holding that the trial court improperly permitted the plaintiff in this case to seek verification of Dr. Primm’s accounting of his compensation as an expert witness through discovery of portions of his tax returns and financial records. Accordingly, I respectfully concur in part and dissent in part in that I would permit discovery of the documents described in items one (1) and three (3) of the trial court’s Order Regarding Primm Discovery and would remand this case to the Court of Appeals for it to issue a more limited writ than the one ordered by the majority opinion, ie., one prohibiting the trial court from permitting discovery, at this time, of the materials described in items number two (2) and four (4) of its discovery order.
The Kentucky Rules of Civil Procedure authorize the issuance of a subpoena duces tecum to “command the person to whom it is directed to produce and permit inspection and copying of designated books, papers, documents, or tangible things which constitute or contain matters within the scope of the examination permitted by Rule 26.02[.]”3 CR 26.02(1) itself broadly defines the scope of permissible discovery: “[p]arties may obtain discovery regarding any matter, not privileged, which is relevant to the subject matter involved in the pending action, whether it relates to the claim or defense of the party seeking discovery or to the claim or defense of any other party[.]”4 The proper purpose of obtaining accurate information as to the percentage and amount of income that a witness receives from his or her services as an expert witness is not to- annoy or embarrass the witness or to invade unnecessarily the witness’s legitimate privacy interests, but rather, to expose bias or any personal interest the witness may have that affected his or her stated opinion — in other words, to discover relevant evidence. “[EJvidence of bias or lack of bias is substantive, rather than collateral”5 and “it may be developed on direct examination, *641as well as cross-examination, just like any other substantive evidence.”6 Stated differently, “[t]he potential bias of a witness is always relevant testimony[,]”7 and therefore, it may be excluded only “if its probative value is substantially outweighed by the danger of undue prejudice, confusion of the issues, or misleading the jury, or by considerations of undue delay, or needless presentation of cumulative evidence.”8 In the present case, no claim has been made — and no reasonable claim could be made — that evidence of Dr. Primm’s possible bias should be excluded. And, in my opinion, there are no countervailing concerns such as “annoyance, embarrassment, oppression, or undue burden or expense”9 that should prevent the plaintiff from discovering the financial documents that are not only necessary to verify or disprove Dr. Primm’s own calculations, but are themselves relevant evidence for a jury to consider in determining the credibility it should assign to Dr. Primm’s opinion.
The facts of this case furnish ample reasons for permitting independent verification of an expert’s answers when verification is readily available. Dr. Primm’s testimony in the Votaw case,10 as set forth in the majority opinion, was that, in 2001, ten percent (10%) to fifteen percent (15%) of his practice was devoted to CR 35.01 examinations and that he earned either a low of $83,400.00 or a high of $370,500.00 from his services as an expert. Additionally, in the chart attached to Appellant’s brief and referred to in the majority’s opinion,11 Dr. Primm’s services as an expert are shown as representing twenty percent (20%) to twenty-five percent (25%) of his medical practice over an unspecified time frame, and he is therefore shown as earning somewhere between $543,750.00 and $725,000.00 from such services. Is someone who receives $83,400.00 a year from performing CR 35.01 examinations and testifying on behalf of defendants as biased as someone who receives $725,000.00 from such litigation-related services? That is a question for the jury to decide, and in order to do so, it needs to know the actual extent to which the expert’s professional income is derived from such litigation-related services, i.e., whether the witness receives $83,400.00 or $725,000.00 from such services in a given year.12 Hopefully, Dr. Primm’s previous disclosures regarding his litigation-related income do not typify the “accurate information” 13 that the majority anticipates from experts.
If an expert accurately answers deposition questions regarding (1) the percentage of income attributable to his or her expert services and (2) the total amount of *642income that he or she derives from such services, an order that requires that expert to produce redacted tax and financial records, which would only verify the answers otherwise provided through discovery,14 would not cause the expert to suffer embarrassment nor would it invade the expert’s privacy interests. In this case, the relevant inquiries can be accomplished if Dr. Primm produces (1) copies of 1099’s that he received from those paying for his expert services and (2) a redacted copy of the applicable schedule of an income tax return that shows only his name and the gross income from his medical practice. Thus, in my view, items one (1) and three (3) of the trial court’s discovery order properly required Dr. Primm to provide the documents containing that information.15 The materials covered by items (2) and (4) of the trial court’s discovery order, i.e., copies of invoices for litigation-related services and daily and monthly financial records, would no doubt assist the plaintiff in verifying whether Dr. Primm had included all 1099 forms as item (3) required. However, with all due deference to the trial court’s discretion,16 unless there is evidence that a witness’s 1099 disclosures were incomplete, requiring production of the materials described in items two (2) and four (4) would be excessive, and I agree with Dr. Primm’s contention that he was entitled to a protective order as to these discovery requests.17
Trials today are too often a battle of expert witnesses, and juries are called upon to decide which expert’s testimony to accept. As is the case with any witness, evidence relating to an expert witness’s potential bias is important for the jury’s consideration in determining both the credibility and the weight of an expert witness’s testimony. Recently, in Tuttle v. Perry,18 this Court held that it was permissible to cross-examine expert witnesses regarding their compensation in an individual case, and we quoted approvingly from Maryland’s highest court’s opinion in Wro-bleski v. de Lara,19 which had previously held that the amount of money a witness is paid for testifying is proper grounds for cross examination. The Wrobleski Court *643added two (2) important caveats to its holding that warrant repetition:
First, we do not intend by our decision today to authorize the harassment of expert witnesses through a wholesale rummaging of their personal and financial records under the guise of seeking impeachment evidence. The allowance of the permitted inquiry, both at the discovery and trial stages, should be tightly controlled by the trial court and limited to its purpose, and not permitted to expand into an unnecessary exposure of matters and data that are personal to the witness and have no real relevance to the credibility of his or her testimony. Second, the fact that an expert witness devotes a significant amount of time to forensic activities or earns a significant portion of income from those activities does not mean that the testimony given by the witness is not honest, accurate, and credible. It is simply a factor that is proper for the trier of fact to know about and consider.20
I fully agree with both of these caveats, and, in my view, if the permissible discovery were limited to the documents identified in items one (1) and three (3) of the trial court’s order, Dr. Primm would neither be harassed nor have his privacy unnecessarily invaded. Further, given that Dr. Primm would have to consult these same tax and financial documents in order to give a complete and truthful answer to questions that this Court has found relevant and that will no doubt be asked of him at his deposition, the trial court’s discovery order that required Dr. Primm to provide copies of the underlying documents themselves is in no way “unreasonable and oppressive.”21
I wish to emphasize that I have no reason to doubt that Dr. Primm’s testimony is honest, accurate, and credible. But, no witness has the right to testify without challenge to his or her answers. Cross-examination has been accurately described as the " 'greatest legal engine ever invented for the discovery of truth.’ ”22 Only through meaningful cross-examination, however, will the jury be able to assess a witness’s credibility and the weight to assign to the witness’s testimony. And meaningful cross-examination can only occur when the opposing party is allowed access to relevant information. In my view, the majority’s holding, which can be distilled to a determination that experts’ answers to questions regarding positional bias now must be taken at face value, represents a retreat from full disclosure of a witness’s potential bias. Accordingly, today’s majority opinion subverts the goal of ensuring that juries receive accurate information as to potential sources of bias, and I dissent in part because I believe that, in order to ensure meaningful cross-examination in this case, the plaintiff should be permitted to discover the financial and tax documents that are necessary to verify or disprove Dr. Primm’s own accounting of his financial ties to insurance companies and defense lawyers.
. Primm v. Isaac, Ky., 127 S.W.3d 630, 637 (2004) ("[A]n expert can be questioned not only as to the percentage of income attributable to IME and other litigation-related services, but also the total amount of income derived from such activities.”); Id. at 637 (“[T]he jury should be able to assess possible bias on the part of an expert witness if it is made aware of the amount and percentage of gross income attributable to providing expert witness services.” (emphasis added)).
. Id. at 637. Given correct answers to the two (2) questions, it is a simple mathematical task to determine the expert’s gross income from his profession, i.e., the jury need only divide the total amount of income derived *640from such services by the percentage of total overall income those services represent. ' (I disagree, however, with the majority’s suggestion that Dr. Primm’s total income from his medical practice can be extrapolated from the chart attached to Appellant’s brief, which depicts “Dr. Primm’s income from IME’s and depositions, including the percentage of total patients examined for that purpose^]” id. at 638, because, without' evidence that Dr. Primm’s charges for litigation-related examinations are commensurate with his charges for the other examinations he performs, it is a hasty generalization to assume that “the percentage of total patients" is equivalent to the percentage of his total income.) And to verify the expert’s percentage answer, it is necessary to know both the income from services as an expert and the gross income from the expert’s profession. Income from other sources, however, is unnecessary to this inquiry and — as it has slight probative value in exposing bias, particularly in cases such as this, where the professional income alone is substantial — I agree with the majority that an expert’s gross income from all sources is not discoverable as a matter of course.
. CR 45.04(1).
. CR 26.02 (emphasis added).
. United States v. Fusco, 748 F.2d 996, 998 (5th Cir.1984).
. Id.
. United States v. Powell, 124 F.3d 655, 661 (5th Cir.1997); Underhill v, Stephenson, Ky., 756 S.W.2d 459, 461 (1988) ("Evidence to show bias of an expert witness is relevant.”); KRE 104(e) ("This rule does not limit the right of a party to introduce before the jury evidence relevant to weight or credibility, including evidence of bias, interest, or prejudice.” (emphasis added)).
. KRE 403.
. CR 26.03(1).
. Votaw v. Anchor Poods, No. 98-CI-000489, 2001 WL 1782235 (Scott Cir. Ct.2001).
. Id.
. Tuttle v. Perry, Ky., 82 S.W.3d 920, 922 (2002) ("As a primary responsibility of a jury is to determine the weight of evidence and the credibility of witnesses, KRE 104(e) declares that the rule does not limit the right of a party to introduce evidence relevant to weight or credibility 'including evidence of bias, interest or prejudice.’ ” (footnotes omitted)).
. Primm, 127 S.W.3d at 637.
. I disagree with the majority’s suggestion that the routine practice of taking discovery depositions of expert witnesses obviates the need for "more burdensome” forms of discovery. See Primm, 127 S.W.3d at 637 ("As Rhodus has yet to take Dr. Primm’s deposition and question him about the sought after information, the least burdensome route of discovery was simply not followed."). First, CR 26.04 provides that "methods of discovery may be used in any sequence[J” Second, the majority’s suggestion overlooks the fact that the plaintiff seeks discovery of Dr. Primm's tax information and financial records primarily in order to verify his expected deposition testimony.
. I note that the trial court’s discovery order provides that “[a]ll records to be produced as hereinbefore designated shall be labeled "CONFIDENTIAL”, shall be produced UNDER SEAL, and shall be utilized only in connection with the above referenced case; it is further ordered that said information and documentation shall not be disseminated to anyone or to any entity except upon further Orders of this courtf.]” I would observe, however, that, if Dr. Primm so requested, CR 26.03(1) would permit the trial court to afford even greater protection for his privacy by ordering that the documentation that he furnished be filed under seal for an in camera inspection by the trial judge who would then determine if Dr. Primm accurately answered the questions and release the documents to the plaintiff only if the trial judge found discrepancies between Dr. Primm's deposition testimony and financial records.
. Metropolitan Property & Cas. Ins. Co. v. Overstreet, Ky., 103 S.W.3d 31, 36 (2003).
. CR 26.03(1).
. Ky., 82 S.W.3d 920 (2002).
. 353 Md. 509, 727 A.2d 930 (1999).
. Id. at 938.
. CR 45.02.
. California v. Green, 399 U.S. 149, 158, 90 S.Ct. 1930, 1935, 26 L.Ed.2d 489 (1970) (quoting 5 Wigmore § 1367).