Simpson's paradox is a paradox from statistics. It is named after Edward H. Simpson, a British statistician who first described it in 1951. The statistician Karl Pearson described a very similar effect in 1899.- Udny Yule's description dates from 1903. Sometimes, it is called the Yule–Simpson effect. When looking at the statistical scores of groups, these scores may change, depending on whether the groups are looked at one by one, or if they are combined into a larger group. This case often occurs in social sciences and medical statistics. It may confuse people, if frequency data is used to explain a causal relationship. Other names for the paradox include "reversal paradox" and "amalgamation paradox".