^{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.