Greetings everyone,

a friend needs some stats help on her doctoral thesis. She is conducting a longitudinal, follow up, study on the same participants, and we need some help about the sample size. She measured the values 3 times, which means the study has 3 repeated measures.

We were looking for a general rule of thumb for calculating the size. We used the p value of 0.05, power 0.8, and the effect size of 0.25. The calculated sample size was 159. Is the calculation done correct? Due to the limitations of time and finance, we would be happy with a smaller size. Can anybody suggest what would be a reasonable smaller, but still significant sample size?