The more homogenious the population, less reliable. Validity degree to which test measures what it is supposed to measure. Criterion-related extent to which a test agrees with other measures of other theoretically related constructs. The degree to which the entire body of literature using the test supports theoretical statements concerning the construct. Homogeneity of population, less validity.
Reliability is a prerequisite for validity.
As a result, the best estimate of the true score is the mean of scores. The standard deviation of the mean scores is the standard error of the measurement . This is useful to get an accuracy judgement of the estimate of the mean. Basic test theory stuff pg. Generalizability Theory addresses subjects mean score on tests of a particular type.