¹  The coding and interpretation of \(t_{j}\) is determined by the researcher. For example, \(t_{j}\) may refer to the number of weeks after the study began, or the number of months after an intervention. If the measurements were not spaced consistently, this can be reflected in the observed values of \(t_{j}\). For example, a study with measurements in January, February, April, and July could code time as the number of months since the first measurement occasion so that \(t_{1}=0, t_{2}=1, t_{3}=3\), and \(t_{4}=6\). In this case, the intercept is placed at the first measurement occasion, but the researcher may choose a different placement. For example, if an intervention occurred in April, the researcher may choose to place the intercept there by recoding \(t_{j}\) as \(t_{1}=-3, t_{2}=-2, t_{3}=0\), and \(t_{4}=3\). Thoughtfully specifying time ensures that the intercept and slope can be interpreted in a meaningful way.