Why do researchers use correlated groups differences in scor

Why do researchers use correlated groups differences in scores rather than the raw scores.

Solution

The correlated group data comes from the paired data. If we want to see the effect of any variable (say the lightening condition) on the score of the students, then getting the data of same students in two condiation (with or without extra lightening) will help us draw better conclusion rather than the raw scores because the effect of variation between the groups will be zero.

So, we will get a better knowledge of effect of variable on the score from paired data rather than the raw data.