What follows is an opinion from the Supreme Court of the United States. Your task is to determine the bases on which the Supreme Court rested its decision with regard to the legal provision that the Court considered in the case. Consider "judicial review (national level)" if the majority determined the constitutionality of some action taken by some unit or official of the federal government, including an interstate compact. Consider "judicial review (state level)" if the majority determined the constitutionality of some action taken by some unit or official of a state or local government. Consider "statutory construction" for cases where the majority interpret a federal statute, treaty, or court rule; if the Court interprets a federal statute governing the powers or jurisdiction of a federal court; if the Court construes a state law as incompatible with a federal law; or if an administrative official interprets a federal statute. Do not consider "statutory construction" where an administrative agency or official acts "pursuant to" a statute, unless the Court interprets the statute to determine if administrative action is proper. Consider "interpretation of administrative regulation or rule, or executive order" if the majority treats federal administrative action in arriving at its decision.Consider "diversity jurisdiction" if the majority said in approximately so many words that under its diversity jurisdiction it is interpreting state law. Consider "federal common law" if the majority indicate that it used a judge-made "doctrine" or "rule; if the Court without more merely specifies the disposition the Court has made of the case and cites one or more of its own previously decided cases unless the citation is qualified by the word "see."; if the case concerns admiralty or maritime law, or some other aspect of the law of nations other than a treaty; if the case concerns the retroactive application of a constitutional provision or a previous decision of the Court; if the case concerns an exclusionary rule, the harmless error rule (though not the statute), the abstention doctrine, comity, res judicata, or collateral estoppel; or if the case concerns a "rule" or "doctrine" that is not specified as related to or connected with a constitutional or statutory provision. Consider "Supreme Court supervision of lower federal or state courts or original jurisdiction" otherwise (i.e., the residual code); for issues pertaining to non-statutorily based Judicial Power topics; for cases arising under the Court's original jurisdiction; in cases in which the Court denied or dismissed the petition for review or where the decision of a lower court is affirmed by a tie vote; or in workers' compensation litigation involving statutory interpretation and, in addition, a discussion of jury determination and/or the sufficiency of the evidence.

Opinion:
MISSOURI ex rel. SOUTHERN RAILWAY CO. v. MAYFIELD, CIRCUIT COURT JUDGE.
NO. 15.
Argued October 16, 1950.
Decided November 6, 1950.
Floyd E. Thompson argued the cause for petitioners. Sidney S. Alderman, Bruce A. Campbell and H. G. Hed-rick were on the brief for the Southern Railway Co. Mr. Thompson, J. C. Gibson and R. S. Outlaw were on the brief for the Atchison, Topeka & Santa Fe Railway Co.
Roberts P. Elam argued the cause for respondents. With him on the brief was Harvey B. Cox.
Mr. Justice Frankfurter
delivered the opinion of the Court.
These two cases had their origin in suits based on the Federal Employers’ Liability Act, 35 Stat. 65, as amended, 45 U. S. C. § 51 et seq., brought in the Circuit Court of the City of St. Louis, Missouri. It is superfluous to give concrete details regarding the parties, the circumstances of the injuries, and the considerations affecting the choice of forum. It suffices to state that in both cases the plaintiff was not a resident of Missouri, the carrier was a foreign corporation, and the accident which gave rise to the claim of liability for negligence took place outside Missouri. In both, the doctrine of jorum non conveniens was invoked; in both, the trial court denied the motion to dismiss the suit on that ground as beyond the jurisdiction of the court to grant. In both cases original proceedings in mandamus were thereupon begun in the Supreme Court of Missouri to compel the trial court to exercise discretionary jurisdiction in disposing of the motions. After alternative writs of mandamus had issued and the causes had been consolidated for consideration, the writs were quashed by a single judgment. 359 Mo. 827, 224 S. W. 2d 105. We brought the proceedings here for review, 339 U. S. 918, because they involved questions important to the enforcement of the Federal Employers’ Liability Act by the courts of the States.
A decision by the highest court of a State determining that the doctrine of jorum non.conveniens cannot bar an action based on the Federal Employers’ Liability Act, in the circumstances before us, may rest on one of three theories. (1) According to its own notions of procedural policy, a State may reject, as it may accept, the doctrine for all causes of action begun in its courts. If denial of a .motion to dismiss an action under the Federal Employers’ Liability Act is rested on such a general local practice, no federal issue comes into play. (It is assumed of course that the State has acquired jurisdiction over the defendant.) (2) By reason of the Privileges- and-Immunities Clause of the Constitution, a State may not discriminate against citizens of sister States. Art. IV, § 2. Therefore Missouri cannot allow suits by nonresident Missourians for liability under the Federal Employers’ Liability Act arising out of conduct outside that State and discriminatorily deny access to its courts to a non-resident who is a citizen of another State. But if a State chooses to “[prefer] residents in access to often overcrowded Courts” and to deny such access to all nonresidents, whether its own citizens or those of other States, it is a choice within its own control. This is true also of actions for personal injuries under the Employers’ Liability Act. Douglas v. New York, N. H. & H. R. Co., 279 U. S. 377, 387. Whether a State makes such a choice is, like its acceptance or rejection of the doctrine of forum non conveniens, a question of State law not open to review here.
But, (3), a State may reject the doctrine of forum non conveniens in suits under the Federal Employers’ Liability Act because it may deem itself compelled by federal law to reject it. Giving the opinion of the Supreme Court of Missouri in these cases a scope most favorable to reliance on a non-federal ground, doubt still remains whether that Court did not deem itself bound to deny the motions for dismissal on the score of forum non con-veniens by its view of the demands of our decisions in Baltimore & O. R. Co. v. Kepner, 314 U. S. 44, and Miles v. Illinois Central R. Co., 315 U. S. 698.
But neither of these cases limited the power of a State to deny access to its courts to persons seeking recovery under the Federal Employers’ Liability Act if in similar cases the State for reasons of local policy denies resort to its courts and enforces its policy impartially, see McKnett v. St. Louis & S. F. R. Co., 292 U. S. 230, so as not to involve a discrimination against Employers’ Liability Act suits and not to offend against the Privileges-and-Immunities Clause of the Constitution. No such restriction is imposed upon the States merely because the Employers’ Liability Act empowers their courts to entertain suits arising under it. There was nothing in that Act even prior to § 1404 (a) of the 1948 revision of the Judicial Code, Title 28, U. S. C., which purported “to force a duty” upon the State courts to entertain or retain Federal Employers’ Liability litigation “against an otherwise valid excuse.” Douglas v. New York, N. H. & H. R. Co., supra, at 388.
Therefore, if the Supreme Court of Missouri held as it did because it felt under compulsion of federal law as enunciated by this Court so to hold, it should be relieved of that compulsion. It should be freed to decide the availability of the principle of jorum non conveniens in these suits according to its own local law. To that end we vacate the judgment of the Supreme Court of Missouri and remand the cause to that Court for further proceedings not inconsistent with this opinion. State Tax Comm’n v. Van Cott, 306 U. S. 511; Minnesota v. National Tea Co., 309 U. S. 551; Herb v. Pitcairn, 324 U. S. 117; 325 U.S. 77.
Judgment vacated.
Section 1404 (a) reads, “For the convenience of parties and witnesses, in the interest of justice, a district court may transfer any civil action to any other district or division where it might have been brought.” See Ex parte Collett, 337 U. S. 55.

Question: What is the basis of the Supreme Court's decision?

Choices:
judicial review (national level)
judicial review (state level)
Supreme Court supervision of lower federal or state courts or original jurisdiction
statutory construction
interpretation of administrative regulation or rule, or executive order
diversity jurisdiction
federal common law

Answer: 1