Final answer:
A d value of 0.8 or greater indicates a large effect size according to Cohen's conventions. Effect size measures the strength of the difference between two means, with values above 0.8 signifying significant differences.
Step-by-step explanation:
According to Cohen's conventions, a d value of 0.8 or greater indicates a large effect size. Cohen's d is a measure of effect size that evaluates the magnitude of the difference between two means. When the d value is greater than 0.8, it suggests a significant difference exists between the means of two populations.
For instance, if we calculate a Cohen's d and find the value to be 0.834, this would be considered a large effect because it exceeds 0.8. It indicates that the observed difference in means is quite large and therefore potentially of practical significance. An effect size of 0.625 would be considered medium as it falls between the conventional threshold for medium (0.5) and large (0.8) effect sizes.