Fallacy of composition
From Mises Wiki, the global repository of classical-liberal thought
The fallacy of composition arises when one infers that something is true of the whole from the fact that it is true of some part of the whole (or even of every proper part). An example would be assuming that, because good weather enables a grain farmer to produce larger crops, and because a grain farmer's producing larger crops results, ceteris paribus, in his receiving more income proportionate to the crop increase, therefore good weather means that he will earn more income proportionate to the crop increase. In practice, the fact that other farmers' yields will also increase due to the good weather could make this conclusion false, since their yields will tend to push down the market price due to the law of supply.