 If we are studying difference in means between two groups, that is, for example, when we are studying mean difference of hemoglobin between lung cancer and bladder cancer patients, then the formula is this. The Z value at the alpha level, which is 1.96, instead of P1 and Q1, it is replaced by pooled variance, which is given by the formula. These values of N1 and N2, standard deviations of 1 and 2, will be arrived from previous literature or pilot study. D is the allowable error. Here is one example how we calculate. Among lung cancer, this is the mean and standard deviation. Among 35 patients with lung cancer, the mean hemoglobin. Among 55 bladder cancer patients is 6 grams and standard deviation of 2 grams per deciliter. First, we need to calculate this pooled variance. Then if we substitute in the formula, then we will get like this. So here they have assumed D as 20 percentage of the mean, 1 gram. So if we keep it as 10 percentage, then we need to have it as 0.5. Then if we substitute 0.5 into 0.5, then the sample size will increase. So likewise, we can decrease the precision value and increase the sample size.