1.   A major problem is how to make the comparison groups as similar to the experimental group as possible.

2.   Self-Selection Another problem in selecting a comparison group is self-selection.

3.   Records of patients with multiple sclerosis were selected as a neurological comparison group.

4.   Records of previous admission for peptic ulcer were sought for people in the case and the comparison groups.

5.   Patients reported about three times as many undesirable events in the six months preceding their illness as the comparison group over the same period.

6.   Interpersonal arguments and difficulties were also a more common feature of the experiences of depressed patients than of the comparison group.

7.   A similar-aged comparison group of women was found who had also participated in a previous study.

8.   Two-fifths of the ex-care sample had a rating of poor parenting, compared to one in nine of the comparison group.

9.   Both mothers and fathers of children admitted into care were more likely than a comparison group to have a psychiatric disorder.