A sociologist uses a 2 4 contingency table to compare four groups on a binary response variable. For group i, let i denote the

A sociologist uses a 2 × 4 contingency table to compare four groups on a binary response variable. For group i, let πi denote the population proportion of response in the first outcome category, i = 1, 2, 3, 4. Explain what df = 3 means in the context of comparing these four population proportions.