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Z test diff of proportions

From OPOSSEM

<math>Z_\text{ob} = \frac{p_2 - p_1}{s_{p_2-p_1}}</math>, where <math>s_{p_2-p_1} = \sqrt{\frac{(p_1 n_1 + p_2 n_2)((1-p_1) n_1 + (1-p_2) n_2)}{n_1 + n_2}({\frac{1}{n_1}}+{\frac{1}{n_2}})}</math>