What caution should I take when calculating an effect size?

Effect sizes are more accurate when calculated from a large sample. Thus, caution should be taken when interpreting effect sizes from small samples (<30 students). One should also take particular caution when the sample group has outliers i.e. students with exceptionally high or low scores. When calculating an effect size, it is best to compare your results including and excluding outliers to see if it makes a difference. When comparing pre-test and post-test scores, it is more useful to ensure that all students are tested and that scores from the same group of students are compared. Finally, one should always look at the context when interpreting effect sizes.