opinion: Justice White
  delivered the opinion of the Court.
  The issue here is whether a bargaining impasse justifies an employer’s unilateral withdrawal from a multiemployer bargaining unit. The National Labor Relations Board (Board) concluded that an employer attempting such a withdrawal commits an unfair labor practice in violation of §§ 8(a)(5) and 8(a)(1) of the National Labor Relations Act (Act), 29 U. S. C. §§ 158(a)(5) and 158(a)(1), by refusing to execute the collective-bargaining agreement later executed by the union and the multiemployer association. The Court of Appeals for the First Circuit enforced the Board’s order. 630 F. 2d 25 (1980). Both the Board and the Court of Appeals recognized that several other Courts of Appeals had previously rejected the Board’s position on this issue. We granted certiorari, 450 U. S. 979 (1981), to resolve the conflict among the Circuits on this important question of federal labor law. We affirm the judgment of the Court of Appeals.
  I
  The factual findings of the Administrative Law Judge were affirmed by. the Board and are undisputed. Petitioner, Charles D. Bonanno Linen Service, Inc. (Bonanno), is a Massachusetts corporation engaged in laundering, renting, and distributing linens and uniforms. Teamsters Local No. 25 (Union) represents its drivers and helpers as well as those of other linen supply companies in the area. For several years, Bonanno has been a member of the New England Linen Supply Association (Association), a group of 10 employers formed to negotiate with the Union as a multiem-ployer unit and a signatory of the contracts negotiated between the Union and the Association. On February 19, 1975, Bonanno authorized the Association’s negotiating committee to represent it in the anticipated negotiations for a new contract. Bonanno’s president became a member of the committee.
  The Union and the Association held 10 bargaining sessions during March and April. On April 30, the negotiators agreed upon a proposed contract, but four days later the Union members rejected it. By May 15, according to the stipulations of the parties, the Union and the Association had reached an impasse over the method of compensation: the Union demanded that the drivers be paid on commission, while the Association insisted on continuing payment at an hourly rate.
  Several subsequent meetings failed to break the impasse. On June 23, the Union initiated a selective strike against Bonanno. In response, most of Association members locked out their drivers. Despite sporadic meetings, the stalemate continued throughout the summer. During this period two of the employers met secretly with the Union, presumably in an effort to reach a separate settlement. These meetings, however, never reached the level of negotiations.
  Bonanno hired permanent replacements for all of its striking drivers. On November 21, it notified the Assocation by letter that it was “withdrawing from the Association with specific respect to negotiations at this time because of an ongoing impasse with Teamsters Local 25.” Pet. for Cert. 58. Bonanno mailed a copy of its revocation letter to the Union and read the letter over the phone to a Union representative.
  Soon after Bonanno’s putative withdrawal, the Association ended the lockout. It told the Union that it wished to continue multiemployer negotiations. Several negotiating sessions took place between December and April, without Bonanno participating. In the middle of April, the Union abandoned its demand for payment on commission and accepted the Association’s offer of a revised hourly wage rate. With this development, the parties quickly agreed on a new contract, dated April 23, 1976, and given retroactive effect to April 18, 1975.
  Meanwhile, on April 9, 1976, the Union had filed the present action, alleging that Bonanno’s purported withdrawal from the bargaining unit constituted an unfair labor practice. In a letter dated April 29, the Union informed Bonanno that because the Union had never consented to the withdrawal, it considered Bonanno to be bound by the settlement just reached. In a reply letter, Bonanno denied that it was bound by the contract.
  An Administrative Law Judge concluded, after a hearing, that no unusual circumstances excused Bonanno’s withdrawal from the multiemployer bargaining unit. The Board affirmed, ordering Bonanno to sign and implement the contract retroactively. In a supplemental decision, the Board explained the basis of its decision that Bonanno’s attempt to withdraw from the multiemployer unit was untimely and ineffective. Charles D. Bonanno Linen Service, Inc., 243 N. L. R. B. 1093 (1979). The Court of Appeals enforced the Board’s order. 630 F. 2d 25 (1980).
  II
  The standard for judicial review of the Board’s decision m this case was established by NLRB v. Truck Drivers, 353 U. S. 87 (1957) (Buffalo Linen). There, the Union struck a single employer during negotiations with a multiemployer bargaining association. The other employers responded with a lockout. Negotiations continued, and an agreement was reached. The Union, claiming that the lockout violated its rights under §§ 7 and 8 of the Act, then filed charges with the Board. The Board rejected the claim, but the Court of Appeals held that the lockout was an unfair practice.
  This Court in turn reversed. That the Act did not expressly authorize or deal with multiemployer units or with lockouts in that context was recognized. Nonetheless, multiemployer bargaining had “long antedated the Wagner Act” and had become more common as employers, in the course of complying with their duty to bargain under the Act, “sought through group bargaining to match increased union strength.” 353 U. S., at 94-95 (footnote omitted). Furthermore, at the time of the debates on the Taft-Hartley amendments, Congress had rejected a proposal to limit or outlaw multiemployer bargaining. The debates and their results offered “cogent evidence that in many industries multi-employer bargaining basis was a vital factor in the effectuation of the national policy of promoting labor peace through strengthened collective bargaining.” Id., at 95. Congress’ refusal to intervene indicated that it intended to leave to the Board’s specialized judgment the resolution of conflicts between union and employer rights that were bound to arise in multiemployer bargaining. In such situations, the Court said:
  “The ultimate problem is the balancing of the conflicting legitimate interests. The function of striking that bal-
  anee to effectuate national labor policy is often a difficult and delicate responsibility, which the Congress committed primarily to the National Labor Relations Board, subject to limited judicial review.” Id., at 96.
  Thus, the Court of Appeals’ rejection of the Board’s justification of the lockout as an acceptable effort to maintain the integrity of the multiemployer unit and its refusal to accept the lockout as a legitimate response to the whipsaw strike had too narrowly confined the exercise of the Board’s discretion.. Id., at 97.
  Multiemployer bargaining has continued to be the preferred bargaining mechanism in many industries, and as Buffalo Linen predicted, it has raised a variety of problems requiring resolution. One critical question concerns the rights of the union and the employers to terminate the multi-employer bargaining arrangement. Until 1958, the Board permitted both employers and the union to abandon the unit even in the midst of bargaining. Bearing & Rim Supply Co., 107 N. L. R. B. 101, 102-103 (1953); Stamford Wall Paper, 92 N. L. R. B. 1173 (1951); Morand Bros. Beverage Co., 91 N. L. R. B. 409, 413, 416-418 (1950), enf’d in part on other grounds, 190 F. 2d 576, 581-582 (CA7 1951). But in Retail Associates, Inc., 120 N. L. R. B. 388 (1958), the Board announced guidelines for withdrawal from multiemployer units. These rules, which reflect an increasing emphasis on the stability of multiemployer units, permit any party to withdraw prior to the date set for negotiation of a new contract or the date on which negotiations actually begin, provided that adequate notice is given. Once negotiations for a new contract have commenced, however, withdrawal is permitted only if there is “mutual consent” or “unusual circumstances” exist. Id., at 395.
  The Board’s approach in Retail Associates has been accepted in the courts, as have its decisions that unusual circumstances will be found where an employer is subject to extreme financial pressures or where a bargaining unit has become substantially fragmented. But as yet there is no consensus as to whether an impasse in bargaining in a multi-employer unit is an unusual circumstance justifying unilateral withdrawal by the Union or by an employer. After equivocating for a time, the Board squarely held that an impasse is not such an unusual circumstance. Hi-Way Billboards, Inc., 206 N. L. R. B. 22 (1973). The Court of Appeals for the Fifth Circuit refused enforcement of that decision, 500 F. 2d 181 (1974), although it has since modified its views and now supports the Board. Similar decisions by the Board were also overturned by the Courts of Appeals in three other Circuits. NLRB v. Beck Engraving Co., 522 F. 2d 475 (CA3 1975); NLRB v. Independent Assn. of Steel Fabricators, 582 F. 2d 135 (CA2 1978); H. & D., Inc. v. NLRB, 665 F. 2d 257, 105 LRRM 3070 (CA9 1980), cert. pending, No. 80-1498. After again considering the question in this case, the Board issued its decision reaffirming its position that an impasse is not an unusual circumstance justifying withdrawal. Its decision was sustained and enforced by the Court of Appeals for the First Circuit.
  Ill
  We agree with the Board and with the Court of Appeals. The Board has recognized the voluntary nature of multi-employer bargaining. It neither forces employers into multiemployer units nor erects barriers to withdrawal prior to bargaining. At the same time, it has sought to further the utility of multiemployer bargaining as an instrument of labor peace by limiting the circumstances under which any party may unilaterally withdraw during negotiations. Thus, it has reiterated the view expressed in Hi-Way Billboards that an impasse is not sufficiently destructive of group bargaining to justify unilateral withdrawal. As a recurring feature in the bargaining process, impasse is only a temporary deadlock or hiatus in negotiations “which in almost all cases is eventually broken, through either a change of mind or the application of economic force.” 243 N. L. R. B., at 1093-1094. Furthermore, an impasse may be “brought about intentionally by one or both parties as a device to further, rather than destroy, the bargaining process.” Id., at 1094. Hence, “there is little warrant for regarding an impasse as a rupture of the bargaining relation which leaves the parties free to go their own ways.” Ibid. As the Board sees it, permitting withdrawal at impasse would as a practical matter undermine the utility of multiemployer bargaining.
  Of course, the ground rules for multiemployer bargaining have not come into being overnight. They have evolved and are still evolving, as the Board, employing its expertise in the light of experience, has sought to balance the “conflicting legitimate interests” in pursuit of the “national policy of promoting labor peace through strengthened collective bargaining.” Buffalo Linen, 353 U. S., at 95, 96. The Board might have struck a different balance from the one it has, and it may be that some or all of us would prefer that it had done so. But assessing the significance of impasse and the dynamics of collective bargaining is precisely the kind of judgment that Buffalo Linen ruled should be left to the Board. We cannot say that the Board’s current resolution of the issue is arbitrary or contrary to law.
  If the Board’s refusal to accept an impasse, standing alone, as an unusual circumstance warranting withdrawal were the only issue in this case, we would affirm without more. But several Courts of Appeals have rejected Hi-Way Billboards on the grounds that impasse may precipitate a strike against one or all members of the unit and that upon impasse the Board permits the union to execute interim agreements with individual employers. These Courts of Appeals consider the possibility of such events as sufficient grounds for any employer in the unit to withdraw.
  In Beck Engraving Co., for example, the Court of Appeals for the Third Circuit held that an impasse followed by a selective strike justified unilateral withdrawal from the bargaining unit. Because at that juncture labor relations law, as interpreted by the Board, would permit the union to execute an interim agreement with the struck employer, the Court of Appeals concluded that the union and the employer entering into such an agreement would be given unfair advantage against other employers if the latter were not permitted to withdraw from the unit. The Court of Appeals thought the employer’s right to withdraw and the union’s privilege of executing interim contracts should mature simultaneously. It concluded that the Board’s approach too drastically upset the bargaining equilibrium to be justified in the name of maintaining the stability of the bargaining unit.
  The Board’s reasons for adhering to its Hi-Way Billboards position are telling. They are surely adequate to survive judicial review. First, it is said that strikes and interim agreements often occur in the course of negotiations prior to impasse and that neither tactic is necessarily associated with impasse. Second, it is “vital” to understand that the Board distinguishes “between interim agreements which contemplate adherence to a final unitwide contract and are thus not antithetical to group bargaining and individual agreements which are clearly inconsistent with, and destructive of, group bargaining.” 243 N. L. R. B., at 1096. In Sangamo Construction Co., 188 N. L. R. B. 159 (1971), and Plumbers and Steamfitters Union No. 323 (P. H. C. Mechanical Contractors), 191 N. L. R. B. 592 (1971), the agreements arrived at with the struck employers were only temporary: both the union and the employer executing the interim agreement were bound by any settlement resulting from multiemployer bargaining. “[I]n both cases, since the early signers maintained a vested interest in the outcome of final union-association negotiations, the multiemployer unit was neither fragmented nor significantly weakened,” 243 N. L. R. B., at 1096, and unilateral withdrawal was not justified.
  On the other hand, where the union, not content with interim agreements that expire with the execution of a unit-wide contract, executes separate agreements that will survive unit negotiations, the union has so “effectively fragmented and destroyed the integrity of the bargaining unit,” ibid., as to create an “unusual circumstance” under Retail Associates rules. Cf. Typographic Service Co., 238 N. L. R. B. 1565 (1978). Furthermore, the Board has held that the execution of separate agreements that would permit either the union or the employer to escape the binding effect of an agreement resulting from group bargaining is a refusal to bargain and an unfair labor practice on the part of both the union and any employer executing such an agreement. Teamsters Union Local No. 378 (Olympia Automobile Dealers Assn.), 243 N. L. R. B. 1086 (1979). The remaining members of the unit thus can insist that parties remain subject to unit negotiations in accordance with their original understanding.
  The Board therefore emphatically rejects the proposition that the negotiation of truly interim, temporary agreements, as distinguished from separate, final contracts, is “inconsistent with the concept of multiemployer bargaining units.” 243 N. L. R. B., at 1096. Although interim agreements establish terms and conditions of employment for one or more employer members of the unit pending the outcome of renewed group bargaining, all employers, including those executing interim agreements, have an “equivalent stake” in the final outcome because the “resulting group agreement would then apply to all employers, including each signer of an interim agreement.” Ibid. Such interim arrangements “preclude a finding that the early signers had withdrawn from the unit.” Ibid. Although the Board concedes that interim agreements exert economic pressure on struck employers, this fact should no more warrant withdrawal than the refusal of one employer to join with others in a lockout. In any event, the Board’s view is that interim agreements, on bal-anee, tend to deter rather than promote unit fragmentation since they preserve a continuing mutual interest by all employer members in a final associationwide contract.
  The Board also rests on this Court’s admonition that the Board should balance “conflicting legitimate interests” rather than economic weapons and bargaining strength. Its conclusion is that the interest in unit stability, recognized as a major consideration by both Buffalo Linen and NLRB v. Brown, 380 U. S. 278 (1965), adequately justifies enforcement of the obligation to bargain despite the execution of a temporary agreement.
  Of course, no interim or separate agreements were executed in this case. But neither did the impasse initiate any right to execute an agreement inconsistent with the duty to abide by the results of group bargaining. Some Courts of Appeals, taking a different view of the interests involved, question the legitimacy of enforcing the duty to bargain where impasse has occurred and interim agreements have been or may be executed. We think the Board has confined itself within the zone of discretion entrusted to it by Congress. The balance it has struck is not inconsistent with the terms or purposes of the Act, and its decision should therefore be enforced.
  IV
  The Chief Justice, in dissent, is quite right that this case turns in major part on the extent to which the courts should defer to the Board’s judgment with respect to the critical factors involved. He is also correct in restating the Court’s admonition in Brown, swpm, at 291, that “[Reviewing courts are not obliged to stand aside and rubber-stamp their affirmance of administrative decisions that they deem inconsistent with a statutory mandate or that frustrate the congressional policy underlying a statute.” But The Chief Justice does not suggest that the Board seeks here to promote illegitimate ends. Both he and the Board strive to further labor peace through effective collective bargaining. Hence, if the Board’s assessment of the impact of impasse and interim agreements on those goals is accepted, it is plain that its decision in this case is consistent with its mandate and promotes the underlying congressional purpose.
  The Chief Justice, candidly accepting that the issue is one of balancing the legitimate interests involved, nonetheless disputes the Board’s judgment regarding the underlying factors with respect to what would best serve the statutory goals. He rejects the Board’s assessment of the significance of impasse and interim agreements in the multiemployer bargaining context and substitutes his own views. For example, he finds that the impasse in this case “was no ‘temporary deadlock or hiatus in negotiations’ as the Board claims; this was instead a complete breakdown in negotiations coupled with a prolonged strike and lock-out.” Post, at 422. He also states, contrary to the Board’s judgment, that when the parties have remained at impasse for a long period, “withdrawal of one or a few employers may facilitate rather than frustrate bargaining.” Post, at 424. Thus, The Chief Justice avers, it would be “more consistent with [the goals of industrial peace] to permit withdrawal and allow negotiation of separate agreements than to force the parties into escalated economic warfare.” Post, at 425.
  The Chief Justice may be quite right. There is obviously room for differing judgments, however, as the conflicting judgments of the Courts of Appeals and the strong views of the Board on the issues now before us make clear. But the dissenting Justices would have us substitute our judgment for those of the Board with respect to the issues that Congress intended the Board should resolve. This we are unwilling to do. If the courts are to monitor so closely the agency’s assessment of the kind of factors involved in this case, the role of the judiciary in administering regulatory statutes will be enormously expanded and its work will become more complex and time consuming. We doubt that this is what Congress intended in subjecting the Board to judical review. Indeed, we so held in Buffalo Linen.
  We agree that the National Labor Relations Act does not constitute the Board as an “arbiter of the sort of economic weapons the parties can use in seeking to gain acceptance of their bargaining demands,” NLRB v. Insurance Agents, 361 U. S. 477, 497 (1960), or give “the Board a general authority to assess the relative economic power of the adversaries in the bargaining process and to deny weapons to one party or the other because of its assessment of that party’s bargaining power,” American Ship Building Co. v. NLRB, 380 U. S. 300, 317 (1965). But the Board has refused to enter that proscribed area, despite the urging of several Courts of Appeals. Instead, it looked at its statutory mandate and duty — to promote labor peace through strengthened collective bargaining — in developing its rule.
  In Brown itself the Court disagreed with the Board and held that no unfair labor practice occurred when members of a multiemployer unit hired temporary replacements following a lockout. Maintaining the stability of the multiemployer unit was the key to that decision: the Court reasoned that without temporary replacements “the prospect that the whipsaw strike would succeed in breaking up the employer association was not at all fanciful.” Brown, 380 U. S., at 284. In contrast to its action in Brown, the Board in this case has developed a rule which, although it may deny an employer a particular economic weapon, does so in the interest of the proper and pre-eminent goal, maintaining the stability of the multiemployer unit. Because the Board has carefully considered the effect of its rule on that goal, we should defer to its judgment.
  Affirmed.
  Section 8(d) of the Act, 61 Stat. 142, 29 U. S. C. § 158(d), states, in relevant part:
  “[T]o bargain collectively is the performance of the mutual obligation of the employer and the representative of the employees to meet at reasonable times and confer in good faith with respect to wages, hours, and other terms and conditions of employment, or the negotiation of an agreement, . . . and the execution of a written contract incorporating any agreement reached if requested by either party, but such obligation does not^ompel either party to agree to a proposal or require the making of a concession . . . .”
  Section 8(a)(5) of the Act, 61 Stat. 141, 29 U. S. C. § 158(a)(5), declares it an unfair labor practice for an employer to “refuse to bargain collectively with the representatives of his employees, subject to the provisions of section 9(a).” Section 8(b)(3), 61 Stat. 141, 29 U. S. C. § 158(b)(3), declares it an unfair labor practice for a labor organization “to refuse to bargain collectively with an employer, provided it is the representative of his employees subject to the provisions of section 9(a).” Section 9(a), 61 Stat. 143, 29 U. S. C. § 159(a), in turn, specifies that “[Representatives designated or selected for the purposes of collective bargaining by the majority of the employees in a unit appropriate for such purposes, shall be the exclusive representatives of all the employees in such unit for the purposes of collective bargaining. . . .” Finally under §2(2), 61 Stat. 137, 29 U. S. C. § 152(2), the term “employer” includes “any person acting as an agent of an employer, directly or indirectly,” and the term “person” is defined in § 2(1), 61 Stat. 137, 29 U. S. C. § 152(1), to include “associations.”
  See NLRB v. Independent Assn. of Steel Fabricators, Inc., 582 F. 2d 135 (CA2 1978), cert. denied, 439 U. S. 1130 (1979); NLRB v. Beck Engraving Co., Inc., 522 F. 2d 475 (CA3 1976); NLRB v. Associated Shower Door Co., Inc., 512 F. 2d 230 (CA9), cert. denied, 423 U. S. 893 (1975); NLRB v. Hi-Way Billboards, Inc., 500 F. 2d 181 (CA5 1974); Fairmont Foods Co. v. NLRB, 471 F. 2d 1170 (CA8 1972).
  As the Court of Appeals explained in this case:
  “Multiemployer bargaining offers advantages to both management and labor. It enables smaller employers to bargain ‘on an equal basis with a large union’ and avoid ‘the competitive disadvantages resulting from nonuniform contractual terms.’ NLRB v. Truck Drivers Local 449, 353 U. S. 87, 96 . . . (1957). At the same time, it facilitates the development of industry-wide, worker benefit programs that employers otherwise might be unable to provide. More generally, multiemployer bargaining encourages both sides to adopt a flexible attitude during negotiations; as the Board explains, employers can make concessions ‘without fear that other employers will refuse to make similar concessions to achieve a competitive advantage,’ and a union can act similarly ‘without fear that the employees will be dissatisfied at not receiving the same benefits which the union might win from other employers.’ Brief, at 10. Finally, by permitting the union and employers to concentrate their bargaining resources on the negotiation of a single contract, multiemployer bargaining enhances the efficiency and effectiveness of the collective bargaining process and thereby reduces industrial strife.” 630 F. 2d, at 28.
  A recent survey of major collective-bargaining agreements (those covering 1,000 or more employees) found that of 1,536 major agreements, 648 (42%) were multiemployer agreements and that 3,238,400 employees were covered by these agreements. U. S. Bureau of Labor Statistics, Dept, of Labor, Bull. No. 2065, Characteristics of Major Collective Bargaining Agreements — January 1, 1978, p. 12, table 1.8 (1980).
  See, e. g., NLRB v. Custom Wood Specialties, Inc., 622 F. 2d 381 (CA8 1980); Carvel Co. v. NLRB, 560 F. 2d 1030 (CA1 1977), cert. denied, 434 U. S. 1065 (1978); NLRB v. Central Plumbing Co., 492 F. 2d 1252 (CA6 1974); NLRB v. Brotherhood of Teamsters, Local No. 70, 470 F. 2d 509 (CA9 1972), cert. denied, 414 U. S. 821 (1973); NLRB v. Johnson Sheet Metal, Inc., 442 F. 2d 1056 (CA10 1971); NLRB v. Dover Tavern Owners’ Assn., 412 F. 2d 725 (CA3 1969); NLRB v. John J. Corbett Press, Inc., 401 F. 2d 673 (CA2 1968).
  See, e. g., NLRB v. Southwestern Colorado Contractors Assn., 447 F. 2d 968 (CA10 1971); NLRB v. Spun-Jee Corp., 385 F. 2d 379 (CA2 1967); Connell Typesetting Co., 212 N. L. R. B. 918 (1974); Atlas Electrical Service Co., 176 N. L. R. B. 827 (1969); U. S. Lingerie Corp., 170 N. L. R. B. 750 (1968).
  In NLRB v. Marine Machine Works, Inc., 635 F. 2d 522 (CA5 1981), it was concluded that the Board’s policy wisely and properly weighed the competing policy concerns.
  The Board explains that if withdrawal were permitted at impasse, the parties would bargain under the threat of withdrawal by any party who was not completely satisfied with the results of the negotiations. That is, parties could precipitate an impasse in order to escape any agreement less favorable than the one expected. In addition, it is precisely at and during impasse, when bargaining is temporarily replaced by economic warfare, that the need for a stable, predictable bargaining unit becomes acute in order that the parties can weigh the costs and possible benefits of their conduct. Brief for Respondent NLRB 24-25.
  The Board adopts the language of the First Circuit below: “the uneven application of economic pressure per se is not inconsistent with multi-employer bargaining.” 630 F. 2d, at 33. In addition, it points out that the employer also has additional weapons at its disposal for exerting economic pressure. It can engage in a lockout, make unilateral changes in working conditions if they are consistent with the offers the union has rejected, hire replacements to counter the loss of striking employees, and try to blunt the effectiveness of an anticipated strike by stockpiling inventories, readjusting contract schedules, or transferring work from one plant to another. The Board further notes that interim agreements do not always have the effect the Union desires. The signing of an interim agreement may not weaken the association’s determination to resist the union’s demands, see Plumbers & Steamfitters Union No. 323 (P. H. C. Mechanical Contractors), 191 N. L. R. B. 592 (1971), and the eventual contract settlement may have terms more favorable to the employers than the interim agreements, requiring the union to give up its temporary gains, see Associated Shower Door Co., 205 N. L. R. B. 677 (1973), enf’d on other grounds, 512 F. 2d 230 (CA9), cert. denied, 423 U. S. 893 (1975). Brief for Respondent NLRB 33, nn. 48 and 49.
  The dissent here ignores Buffalo Linen’s recognition of whipsawing as a legitimate weapon of economic persuasion in the course of collective bargaining. See Buffalo Linen, 353 U. S., at 90, n. 7.
choices: case did not arise on cert or cert not granted federal court conflict federal court conflict and to resolve important or significant question putative conflict conflict between federal court and state court state court conflict federal court confusion or uncertainty state court confusion or uncertainty federal court and state court confusion or uncertainty to resolve important or significant question to resolve question presented no reason given other reason