Question

It has become more important to present effect size in reporting the results of a research project. Discuss the importance of effect size in significance testing and provide two examples of common effect size measures.

Answer 1

It has become more important to present effect size in reporting the results of a research project. Discuss the importance of effect size in significance testing and provide two examples of common effect size measures.

'Effect size' is simply a way of quantifying the size of the difference between the two groups. Effect size is an important tool in reporting and interpreting effectiveness.

Example:

Consider an experiment conducted by Dawson (2000) to investigate the time of day effects on learning: do children learn better in the morning or afternoon? A group of 38 children were included in the experiment. Half were randomly allocated to listen to a story and answer questions about it (on tape) at 9 am, the other half to hear exactly the same story and answer the same questions at 3 pm. Their comprehension was measured by the number of questions answered correctly out of 20.

The average score was 15.2 for the morning group, 17.9 for the afternoon group: a difference of 2.7. But how big a difference is this? If the outcome were measured on a familiar scale, such as GCSE grades, interpreting the difference would not be a problem. If the average difference were, say, half a grade, most people would have a fair idea of the educational significance of the effect of reading a story at different times of the day.