  Tetrad correlations were obtained historically for Gaussian distributions
when tasks are designed to measure an ability or attitude so that a single
unobserved variable may generate the observed, linearly increasing dependences
among the tasks. We connect such generating processes to a particular type of
directed graph, the star graph, and to the notion of traceable regressions.
Tetrad correlation conditions for the existence of a single latent variable are
derived. These are needed for positive dependences not only in joint Gaussian
but also in joint binary distributions. Three applications with binary items
are given.
