1. scores sampled at random 2. Some statisticians also say that the data must be on an interval or ratio scale. Cases in each sample must be independent and unrelated. Looks at variance due to treatment vs. variance due to sampling. Think of the different variances like this.
Scores vary around the subgroup means.
This is inherent variance, within groups variance, or error variance . And the same holds true for the way each subgroup's scores vary anound the mean of that subgroup. Since the subjects in each subgroup all got the same treatment, the variance of scores around the mean of that subgroup is due to sampling. So we call that variance inherent variance, within groups variance, or error variance. There is another kind of variance we can look at.