In longitudinal studies data is collected in a series of waves. Each wave after the first suffers from attrition. Therefore it can be difficult to discriminate between changes in sample parameters due to a longitudinal process (e.g. ageing) and changes due to attrition. The problem is particularly vexing if one of the purposes is to compare…

Theoretical predictions, based on the confluence model, were made for data from six national surveys of intellectual performance, each relating intellectual performance scores to family configuration variables. The confluence model was capable of accurate prediction in all cases when three parameters were estimated. (Author/RD)