While res judicata encompasses questions which might have been litigated, collateral estoppel applies only to questions actually litigated in a prior suit. Once a court has decided an issue essential to its judgment, collateral estoppel precludes the issue from being relitigated in another suit on a different cause of action involving a party to the first cause. The doctrine has the effect of establishing conclusively questions of law or fact that have received a final judgment for the purposes of a later suit. In addition to protecting litigants from the burden of relitigation, collateral estoppel serves the purpose of promoting judicial economy. State Farm Mut.
Federal law determines the preclusive effect of a prior federal court judgment.
The issues raised and decided in Boston I and Boston II are conclusive for purposes of any subsequent attempt to relitigate the same. If the plaintiff is attempting to relitigate issues already adjudicated, her action should be dismissed. Therefore, the court must determine the issue before the court in Boston I and Boston II . The court came to the conclusion that substantive due process under the Constitution was not determined by state law. In so doing, the court found that the county did not deny Boston of her substantive or procedural due process rights.