Effect size

Effect size shows how large a difference is, regardless of how many people took part. For two groups it is often Cohen's d: the distance between the means, measured in standard deviations.

Example: d = 0.57. The two phone groups are a little more than half a standard deviation apart.

Caution: The labels small (0.2), medium (0.5) and large (0.8) are a rough rule of thumb from Jacob Cohen. What counts as large in one field can be small in another.