what's an interaction test? what is an interaction model why is time considered a variable in longitudinal data how is the interaction between time and time tested which factor should be used to test an interaction what is the r2 of a longitudinal study? which is a random irrational effect of a measure in longitudinal data? which factor determines the interaction between time and a variable which factor is used to find the relationship between the longitudinal measure a and the random measure b? what is the nlme package what is the structure of longitudinal data how to test for association in longitudinal study how to test interaction why are random effects in longitudinal data difficult? what is the structure of the longitudinal data how do you test interactions in longitudinal data which is an interaction test how is a time a random effect why is time important in longitudinal data what is a random effect for a longitudinal measure?