A psychologist wanted to investigate whether or not people are influenced by the opinions of others. The psychologist selected 100 pupils from a secondary school to be participants in the study. The psychologist showed participants a cake which weighed 350 grams. The task for the participants was to estimate the weight of the cake in grams. Participants were allocated randomly to one of two groups. In Group A, 50 participants were asked individually to estimate the weight of the cake. The psychologist gave each participant a blank piece of paper on which to write his or her estimate. In Group B, the other 50 participants were asked individually to estimate the weight of the cake. This time, the psychologist gave each participant a piece of paper which contained a list of five weights (493 grams, 512 grams, 502 grams, 485 grams and 601 grams). The participants were told that these were the estimates given by five people and that they should write their own estimate below these other estimates.

The psychologist expected that participants in Group B would be influenced by the five other estimates. She expected that they would write down a weight similar to the five estimates on the piece of paper.

21. Explain how stratified sampling might have been used to select the participants in this study.