OPINION OF THE COURT
ADAMS, Circuit Judge.
James O’Brocta has been convicted of conspiring to defraud the United States by impeding the Internal Revenue Service’s efforts to ascertain and collect revenue in violation of 18 U.S.C. § 371 (1982). He has siso been convicted of three counts of willfully assisting in the filing of and subscribing to false corporate income tax returns in violation of 26 U.S.C. § 7206 (1982).1 O’Brocta was sentenced to concurrent terms of imprisonment on the conspiracy and the substantive counts; these terms were SUSpendec[ by the trial judge, and concurrent periods of probation were imposed for each count. A fine was also imposed
, , J 0 Bro" a ar/ues.on a?Peal tha* the evT dence adduced afmst him at tr'al 13 msuf" ficient to support a conviction for conspir- ........ , ** ^nd that the trial judge s comments du™^ the chf^e so tainted the jury s verdict as to deprive him of an adequate , __ ^ opportunity to defend himself. We cannot agree with O’Brocta’s contentions and the convictions will therefore be affirmed.
I.
O’Brocta is president of a salvage eompa-ny whose business is the buying and selling of scrap metal. Between 1975 and 1978, O’Brocta regularly sold scrap to Penn Iron and Metals Company. Gerald Olgin, the [266]*266president of Penn Iron, testified at O’Broc-ta’s trial that, when a seller of scrap asked for payment in cash, the firm would write a check to a fictitious payee, cash the check, and pay the seller. O’Brocta frequently received payment under this scheme, and he frequently personally cashed the checks drawn to the order of fictitious payees. O’Brocta did not include in his corporate records the receipts from sales of scrap to Penn Iron paid by such checks or cash. Although O’Brocta was responsible for the preparation of the corporate tax returns of his salvage company, he did not inform his accountants of the sales to Penn Iron, which were, consequently, not reported on the tax returns filed by the corporation for 1976, 1977, and 1978.2
O’Brocta’s primary defense at trial was that he lacked the mens rea required for a criminal conviction because he believed that the failure to report cash payments was not improper and would not affect the computation of his tax liability. To support the latter contention, O’Brocta attempted to show that he used the cash received from Penn Iron to purchase scrap for his own business. He claimed that he never recorded these purchases and, consequently, that the cost of goods sold, as reported on the allegedly inaccurate returns, was understated by the same amount as the report of gross receipts. Thus, O’Brocta argued, he did not intentionally violate a known legal duty in understating gross receipts because he believed that he had fulfilled his legal obligation by accurately stating gross profits and correctly computing the income tax owed on this amount.
On appeal O’Brocta argues that the trial judge deprived him of the right to convince the jury that he lacked the intent necessary to conspire or to commit the substantive crimes. He charges that the trial court erred in not allowing him to elicit certain testimony from various witnesses and that this error was compounded by the characterization of his defense as “spurious” in the judge’s remarks to the jury. Finally, O’Brocta maintains that the conspiracy conviction cannot stand because the government has failed to prove that the IRS was actually impeded by the system of cash payments made to him or his corporation by Penn Iron.
II.
The most troubling aspect of this appeal is the claim that the trial judge interfered with O’Brocta’s attempt to persuade the jury of his innocence by characterizing his defense as spurious. That remark was made during the recitation of the jury instructions;
There is a particular statute or two particular sections of the same statute that make the giving of the false information a violation of the law, regardless of whether there was any income tax to be paid or due, regardless of whether the United States suffered any loss or not. No pecuniary loss, no monetary loss, need be suffered by the United States. The violation of the statute is complete if it is shown that the Defendant deliberately and with knowledge that it was contrary to law gave a false account of any material item on the report. Gross receipts are material. There has been an admission here that gross expenditures were also falsely reported and an argument made that they should cancel each other out, and that is spurious, of course, in that it omits the consideration of inventory. If money is paid out to acquire goods, then those goods go into inventory, and so a false statement of any one of these things throws all the rest of the material on the return out of whack and unable to be understood.
App. at 548.
Immediately after these remarks, the judge explained that the jury must deter[267]*267mine whether the defendant had “acted willfully, in the sense that he knew what he was doing was contrary to law, and he knew that the statements were false.” “[N]o one should be convicted,” the judge emphasized, “if he acted by mistake or inadvertence or some other reason. He has to know what he was doing and know that it was contrary to the requirements of law.”3 App. at 548-49. These remarks repeated an earlier and more specific reference to the intent requirement of the statutes under which O’Brocta was being prosecuted:
Do the facts which have been drawn before you, are the things which were done, consistent with a certain state of mind, which is the issue in this case? There is no dispute, there is no denial, that the tax return in question for the three years was false. The Defendant admits that it was, when it say [sic], “Total Receipts,” that the total receipts weren’t there, and that is undisputed. There is no dispute that the Defendant did not record all the receipts that he got. The dispute here is as to his state of mind, because that is an element of the crime for which he is charged, that he knowingly and willfully gave false information.
App. at 528.
The most explicit or at least the most complete explanation of the elements of the crimes with which O’Brocta was charged came later:
If you find beyond a reasonable doubt, from all the evidence in the case, that the Defendant willfully signed the tax returns for 1977 and ’78, that the return was false as to a material matter, and that he knew that the false return was false, then the offense is completed____ [W]ith respect to the ’76 return, you must find that the Defendant aided or assisted in or procured or advised the preparation of a return, that the return was false as to a material matter, and that the Defendant did it willfully, that is, knowing that it was contrary to law, knowing that it was false____
The terminology as used in this statute means that the acts are done voluntarily and purposefully, and not because of a mistake or accident or some other innocent reason____
So remember that the critical element in this case, and the only one in real dispute, is the mental state of the Defendant, that it must be shown that he willfully did the act, knowingly doing something which he knows the law for-bids____ If the evidence leads you to conclude that the Defendant acted in good faith in order to conform to the law, that would not meet the test of willfulness. The Government must prove the Defendant’s conduct in supplying the allegedly false information to his accountant was willfully done, with the specific intent to violate what he knew his legal [268]*268duty to be. He must be shown to have acted voluntarily and intentionally to violate a known duty. The evidence must show that James O’Brocta knew that his conduct was unlawful, that a particular act or failure to act here was a violation of the law.
You may consider all these matters. You may consider if he acted because he, in truth and honesty, believed that his offsetting of purchases would offset the cash received not reported, made his act without any tax consequences. Then, you may consider that in connection with the willfulness.
Is there evidence here which raises a doubt that he acted willfully about any fact by which is shown in the evidence to excuse that state of mind? Did he, in good faith, believe that the method of offsetting expenses that he has testified to rendered it unnecessary for him to give truthful reports? Then, you may consider that for the purpose of determining whether he acted willfully.
So, in summation, a conviction of willful violation of the tax laws must show an intentional violation of a known legal duty, which the Government must prove beyond a reasonable doubt.
App. at 550-53.
We must determine on this appeal whether the trial judge’s use of the word “spurious” in his charge, in light of the circumstances, constitutes reversible error. In order to assist the jury in its deliberations, the trial judge may, within certain limits, comment upon the evidence. See Quercia v. United States, 289 U.S. 466, 469, 53 S.Ct. 698, 698-99, 77 L.Ed. 1321 (1933). When doing so, however, the trial judge must not unduly influence the jury’s decision, so as to deny the defendant his right to trial by jury. The Supreme Court has stated that
In charging the jury, the trial judge is not limited to instructions of an abstract sort. It is within his province, whenever he thinks it necessary, to assist the jury in arriving at a just conclusion by explaining and commenting upon the evidence, by drawing their attention to the parts of it which he thinks important; and he may express his opinion upon the facts, provided he makes it clear to the jury that all matters of fact are submitted to their determination.
Quercia, 289 U.S. at 469, 53 S.Ct. at 699. The Supreme Court added:
The privilege of the judge to comment on the facts has its inherent limitations. His discretion is not arbitrary and uncontrolled, but judicial, to be exercised in conformity with the standards governing the judicial office. In commenting upon testimony he may not assume the role of a witness. He may analyze and dissect the evidence, but he may not either distort it or add to it. His privilege of comment in order to give appropriate assistance to the jury is too important to be left without safeguards against abuses. The influence of the trial judge on the jury “is necessarily and properly of great weight” and “his lightest word or intimation is received with deference, and may prove controlling.” This Court has accordingly emphasized the duty of the trial judge to use great care that an expression of opinion upon the evidence “should be so given as not to mislead, and especially that it should not be one-sided”; that “deductions and theories not warranted by the evidence should be studiously avoided.”
Id. at 470, 53 S.Ct. at 699 (quoting Starr v. United States, 153 U.S. 614, 626, 14 S.Ct. 919, 923, 38 L.Ed. 841 (1894)).
There is no bright line separating remarks that are appropriate from remarks that may unduly influence a jury. Although not explicitly articulated, this Court has balanced a number of factors and has employed what might be termed a sliding scale approach to assess the propriety of a trial judge’s comments during a charge to the jury. Specifically, this Court has considered four factors: the materiality of the comment, its emphatic or overbearing nature, the efficacy of any curative instruction, and the prejudicial effect of the com[269]*269ment in light of the jury instruction as a whole.
On appeal, the materiality of the challenged comment is a preliminary focus of inquiry. The reviewing court should be more concerned with a comment on a matter central to the defense than with a comment on a tangential issue. See United States v. Anton, 597 F.2d 371, 374 (3d Cir.1979). The fact that the judge in Anton commented on the defendant’s credibility, an issue “central to his defense,” was one factor in this Court’s reversal of the conviction. Id.
A second consideration for the reviewing court is the strength of the challenged comment. The reviewing court must be concerned with “emphatic or overbearing remarks” that a jury may accept as controlling. United States v. Gaines, 450 F.2d 186, 189 (3d Cir.1971). For example in Anton, 597 F.2d at 372-73, the trial judge commented that he regarded the defendant “as devoid of credibility” and that he did “not believe [defendant] absolutely and in all respects.”4 Thus, in Anton, the emphatic nature of the judge’s statement provided a second ground for the reversal of the jury verdict. Id. at 374. However, a statement that questions the validity of an assertion, but does not flatly or emphatically dismiss the testimony, is of less concern. See, e.g., United States v. Blair, 456 F.2d at 514, 519 (3d Cir.1972); Gaines, 450 F.2d at 188-89.
The efficacy of any curative instruction must also be considered. In many eases, a trial judge’s comment which potentially could constitute reversible error can be counterbalanced by a curative instruction. See, e.g., Blair, 456 F.2d at 519. For such an instruction to be effective, the judge should clearly explain to the jury that it is free to disregard his remarks and must determine the facts and decide the case on its own. Both the number and clarity of such instructions are to be considered.5
Finally, the reviewing court must assess the prejudicial effect of the comments in light of the jury instruction as a whole. A potentially prejudicial comment cannot be evaluated in isolation, out of context. Rather, the reviewing court must examine the entire jury instruction to assess the true impact of the challenged remark. See Gaines, 450 F.2d at 190.6
Under the sliding scale approach the reviewing court must balance the above-mentioned factors. The stronger the potential prejudicial effect of a statement is, the stronger must be the mitigating factors. There may be cases in which the trial judge’s comments are so out of bounds that no cautionary instruction to the jury could remove their prejudicial effect. For example, in Anton this Court held that the judge’s statement of utter disbelief as to the defendant’s credibility constituted reversible error. Despite clear curative statements before and after the comment, [270]*270the Court stated that “[where] a court has expressed its opinion on a pivitol issue in the case, and has expressed that opinion in a strong, unequivocal and one-sided fashion, abstract instructions regarding the jury’s role as fact finder are not a sufficient remedy.” Anton, 597 F.2d at 375; see also Quercia v. United States, 289 U.S. 466, 469-72, 53 S.Ct. 698, 698-700, 77 L.Ed. 1321 (1933) (judge’s comment, in reference to defendant, that “ ‘wiping’ one’s hands while testifying was ‘almost always an indication of lying’ ” constituted reversible error). In cases in which the prejudicial effect of a judicial comment is not so egregious as to be uncorrectible, the appellate court must weigh the totality of the factors in determining whether the quantum of harm from a statement amounts to reversible error.
III.
The test described above must now be applied to the trial judge’s characterization of the defense testimony on parallel underreporting of gross income and gross expenditures as “spurious.” It is important to note that the charge against O’Brocta was not tax evasion, but the willful filing of inaccurate reports. Thus O’Brocta's state of mind, not the actual tax consequences of such a procedure, was at issue. The trial judge’s comment on the effect of the underreporting might well have been intended merely to alert the jury that this line of defense was not particularly relevant to the charge against O’Brocta. Consequently, any prejudicial effect of the judge’s comment was necessarily less than in Gaines in which the judge’s remarks went to the core of the defense.
The trial judge’s statement was also not nearly as strong as the statement of utter disbelief in Anton. The judge’s opinion that the evidence was spurious is much closer in tone to the statement in Blair that the judge thought the money orders in dispute had been altered. See 456 F.2d at 519.7
In addition to the fact that use of the word “spurious” had limited prejudicial effect, the judge issued a curative instruction to the jury in response to defense counsel’s objection to its use:
I have said, ladies and gentlemen, that the false statements on the returns and the false entries in the books are not in dispute. They have been admitted. The only question left is the question of mental state; that is, the willfulness____ With respect to the court’s comment on whether or not the receipts of money and the expenditures of money and cash — I may have used the word “spurious.” That is solely a conclusion that the court drew. You have heard the testimony of the Government’s expert witness with respect to what the effect of the misstatement of both of those are on other matters here. The opinion of the court there that there is inventory is solely the opinion of the Court and does not bind the jury in any way as to whether or not that would be a logical contention to make, that these two cancel each other out.
App. at 559-60. This cautionary instruction is similar to one which the Court found to be sufficient in Blair, 456 F.2d at 519.
Finally, the judge’s use of the word “spurious” must be evaluated in light of his entire charge. See Gaines, 450 F.2d at 190. In the present case, the judge repeatedly, and correctly, told the jury that its task was to determine whether O'Brocta willfully signed tax returns which he knew were false as to a material matter. The use of one ill-advised word within an entire charge to the jury does not, in this context, constitute reversible error.
Thus, whatever the limits of permissible judicial comment may be, the trial judge’s comment in the case at bar did not transcend the boundary. While the Court does not endorse the judge’s use of the word [271]*271“spurious,” it also does not find that under the test utilized today the inclusion of this one word amounted to reversible error.
IV.
Additionally, we note that the defense failed to object to the effectiveness of the judge’s curative instruction. According to Fed.R.Crim.P. 30,8 it was incumbent upon defense counsel to request a further curative instruction or to register an objection to the efficacy of the one already presented to the jury before the jury retired to consider its verdict. The policy behind rule 30 — to prevent needless retrying of cases when the defect complained of could have been cured at the first trial — is an important one. See United States v. Currens, 290 F.2d 751, 759 (3d Cir.1961).
Thus counsel’s lack of a timely challenge to the efficacy of the judge’s curative instruction, as required by rule 30, forecloses assigning as error that portion of the charge. Our decision today is in keeping with the spirit of Currens. In Currens, this Court noted the important procedural purpose of rule 30, but warned against employing it “woodenly” so as not to become a “trap for the unwary.” Currens, 290 F.2d at 759. The defense strategy in Currens was based on the Durham formula for insanity, yet the trial judge refused the defense’s request to utilize this standard in his charge to the jury. The defense’s challenge to the judge’s instructions was somewhat vague. However, because the entire defense was based on the Durham formula, and the trial judge instructed the jury according to the M’Naghten Rule, we held that rule 30 did not bar defendant’s appeal.
The present case is quite different. Defense counsel did object to the judge’s use of the word “spurious,” but not to the efficacy of his curative instruction. These two points, while related, are different. An appellate court could have ruled for the defendant on either of these grounds separately; thus both points had to be preserved for appeal. As in Anton, we might have found the judge’s comment to be so prejudicial that no cautionary instruction could have removed the taint. Conversely, we could have found that the judge’s remark was sufficiently prejudicial to require a cautionary remark but that the cautionary remark made by the judge was inadequate. In the latter ease, the policy behind rule 30 mandates that defendant raise an objection at trial so that a second trial might be avoided. In light of the totality of the circumstances, the judge’s remark did not amount to reversible error.
V.
O’Brocta also argues that he was deprived of an adequate opportunity to persuade the jury of his innocence as a result of the trial judge’s decision not to allow certain questions to be put to government and defense witnesses. In general, the trial court’s rulings precluded extensive inquiry into two matters: 1) the effect on tax liability of equally underreported receipts and purchases, and 2) the results of the government’s efforts to locate the funds O’Brocta had not reported in his tax returns.
On cross-examination, defense counsel sought to question O’Brocta’s accountant and Special Agents Busse and Waldemar-son about the tax consequences of using unreported gross receipts to make unreported purchases of scrap metal. Walde-marson was permitted to testify that equal, unreported receipts and purchases would lead to a correct computation of gross profits on an income tax return. Busse was allowed to testify that O’Brocta had informed him that the Penn Iron payments “were used for cash purposes and [therefore] netted out.” App. at 272. Nevertheless, it is true that the trial court generally limited inquiry into this area, having con-[272]*272eluded that such questioning was irrelevant.9
It is well settled that a tax deficiency is not an element of the offenses with which O’Brocta was charged. See, e.g., United States v. Cohen, 617 F.2d 56, 58 (4th Cir.), cert, denied, 449 U.S. 845, 101 S.Ct. 130, 66 L.Ed.2d 55 (1980); United States v. Johnson, 558 F.2d 744, 746 (5th Cir.1977), cert, denied, 434 U.S. 1065, 98 S.Ct. 1241, 55 L.Ed.2d 766 (1978). As these cases reveal, evidence of tax liability is generally inadmissible in prosecutions under I.R.C. § 7206, and a trial judge is certainly correct in taking care that a jury does not misperceive a charge of false reporting as an allegation of tax evasion. Moreover, as the district court repeatedly emphasized, the sole issue at trial was O'Brocta’s state of mind when he helped prepare or signed the incorrect tax returns. If the returns were submitted in good faith, the defendant would be entitled to an acquittal on the charges specified in the indictment, whether or not his tax liability had been correctly computed. Conversely, if the defendant knowingly and willfully filed incorrect returns in violation of a legal duty, he would be guilty of the offenses charged, again regardless of the tax consequences of the inaccurate information. Particularly since defense counsel was permitted to make the somewhat questionable point about the effect of dual underreporting on tax liability,10 we can perceive no error in the trial judge’s decision to limit testimony on this issue.
O’Brocta also asserts that he was not allowed to testify fully that he had not converted the unreported gross receipts to his personal use and that he was not permitted to question Agent Busse about the government’s efforts to locate these receipts. During direct examination, O’Broc-ta did testify at length about his spartan lifestyle.11 The trial court properly terminated this testimony after the prosecution objected on the grounds of irrelevance and repetitiveness. The questions to Agent Busse as to the government’s failure to locate the unreported funds were likewise irrelevant to O’Brocta’s defense because they only peripherally bore on his state of mind in filing an incorrect tax statement and had the potential of confusing the members of the jury into thinking that the case before them turned on tax evasion. We can find no error in the trial court’s evidentiary rulings.
VI.
Count I of the indictment charges O’Brocta with conspiring with several other defendants to defraud the United States by impeding the IRS’ efforts to ascertain and assess revenue through a scheme designed to conceal “the true nature and correct amounts of monies paid by Penn Iron ... for the purchase of scrapmetals.” O’Brocta claims that the conspiracy conviction must fall because the government has not proven that the IRS was impeded by the cash payment scheme.
O’Brocta insists that the Internal Revenue agents who prepared the case against him were not impeded by the checks drawn to the order of fictitious payees. By examining those checks and comparing them [273]*273with invoices Penn Iron regularly kept as part of its business records, the agents were indeed able to identify with apparently relative ease those sellers of scrap metal paid by means of the check cashing scheme. But to say that a fruitful investigation absolves those who participated in the scheme is equivalent to saying that a conspiracy must be successful before it becomes illegal. The illegality of a conspiratorial agreement, however, does not depend on the conspirators’ achieving their end; even the objective impossibility of attaining the goals of a conspiracy is irrelevant to the guilt of those who conspire. See United States v. Jannotti, 673 F.2d 578, 591 (3d Cir.1982) (en banc), cert, denied, 457 U.S. 1106, 102 S.Ct. 2906, 73 L.Ed.2d 1315 (1982). Consequently, we are not persuaded by the argument that the conviction for conspiracy cannot stand because the government was not actually impeded in its efforts to assess and collect revenue.
VII.
For the reasons explained above, the judgment of the district court will be affirmed.