Studies using a pre/post design are not technically considered trajectory studies because trajectory analyses were not applied to the data, based as they were on two measures. In discrete mathematics, a trajectory is based on a sequences of values. Accordingly, trajectory analysis typically requires inclusion of data for individuals with three or more observations. Two previous studies did involve the analysis of visual P300 collected at up to three points in time at ages 17, 20, and 23 (Carlson and Iacono, 2006, 2008). These studies found a decrease in P300 over time and reported that P300 amplitude was most related to the severity of externalizing disorders in parents (Carlson and Iacono, 2008), though parents with internalizing disorder were not included. In the other study, biometrical modeling in twins was used to assess the genetic contribution to intercept and slope finding significant heritability for both though specific genes were not tested (Carlson and Iacono, 2006).
