I need to know the difference between these two in a psychological research paper I am studying, in which there are two levels of independent variable. The first level is a group with reads in silence, the second group reads aloud. However, there is 48 males and only 26 females in the entire sample. The procedure does not indicate that there was random allocation of groups. Would this mean that we could suspect gender as a potential confounding variable? As there is no evidence that one of the groups was not a group made up of entirely males! However, a confounding variable needs to systematically vary across the levels of independent variable, and correlate with the dependent variable, but I cannot tell if it does or not! Is gender a confounding variable here?