Design and Data Analysis in Psychology I Salvador Chacón Moscoso Susana Sanduvete Chaves School of Psychology Dpt. Experimental Psychology 1

Lesson 7 Sample size 2

1. Introduction  The higher the sample size is, the lower the standard error is (statistic closer to the parameter).  Aim: to know the sample size required in order to obtain a standard error small enough. 3

2. Sample size. Means 4

2. Sample size. Means. Example 5 We would like to estimate the mean opinion about abortion using a questionnaire with scores from 0 to 10. A previous study found that the standard deviation in the population was 4.5. Calculate the sample size to use, with a probability of making a mistake of 0.05 and an error of 1.

2. Sample size. Means. Example 6 We will need a sample size equal to 78 in order to estimate the mean opinion in the population with α=0.05 and an error of 1

3. Sample size. Proportions 7 σ and π come from the population or a previous study. If π is unknown, we will take π(1- π)=0.25

3. Sample size. Proportions. Example 8 Calculate the sample size in a study of proportions knowing that the parameter is about 0.7, we would like to obtain a error equal to 0.1, and we would like to have a probability of getting right of 0.95.

3. Sample size. Proportions. Example 9 We needed a sample size equal to 81 in order to estimate the proportion in the population (0.7) with α=0.05 and an error of 0.1