I have a study comparing depression among middle school and high school children. After finishing the data collection for this study, we decided to look and see how the sample we took (n=300) represent in relation to the source population (all children in the school). So we obtained records about the demographic distributions of children in the school and we found that there is selection bias based on the socioeconomic status which is different in the groups (one group is similar to the source population while the other is not). How do we address this bias in the analysis of the data? Do you have ideas of ways we can adjust for it (knowing that low socioeconomic status is a potential confounder to the relationship between school group (middle school/ high school) and depression?