Let us start with an example. Consider math and biology teachers working in a school. Every year, they train students and send to school final examinations. Some students pass. Some students do not make it. Every year, each teacher gets 'percentage of passes' score. The pass percentages for each teacher for two years are given below. Math teacher Biology teacher Year I 70/80 = 87.5% 20/20 = 100% Year II 10/20 = 50% 50/80 = 62.5% Total 80/100 = 80% 70/100 = 70% Here, the denominator is the number of students appeared and the numerator is the no.of students passed. Every year biology teacher seems to show good performance than the math teacher. But when we look at the total of 2 years, math teacher wins. This is Simpson's paradox. Simpson's paradox occurs when groups of data show one particular trend, but this trend is reversed when the groups are
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