How is effect size calculated between two sets of scores?

The calculation of effect size between two sets of scores is fundamentally about understanding the magnitude of difference between those scores relative to their variability. The correct method to calculate effect size is to take the difference between the means of the two groups and divide that by the standard deviation, typically of the combined groups or the pooled standard deviation if applicable.

In this case, by using the formula of (mean1 - mean2) / standard deviation, you are effectively measuring how many standard deviations apart the two mean scores are. This provides a standardized way to understand the size of the difference in context to how variable the scores are within those groups. Effect size is instrumental in research to compare differences and assess the strength or significance of those differences, regardless of sample size, making the approach both robust and informative in statistical analyses.

Other options do not follow the standard approach to calculating effect size, with some misrepresenting the relationship or failing to properly contextualize the scores. Thus, using the difference in means divided by the standard deviation is the correct and accepted method for calculating effect size in this context.