i have this following situation. I have a dataset of four measurement occasions. In the first three occasions the same test was introduced. This three timepoints are equidistant. The fourth data aqcuisition was some years later. The time distance to the fourth measurement occasion is much longer, and another test, though a similiar psychological construct, was conducted.
At first i tried to fit a model on the data of the first three measurement occasions. On theese, a linear model fits perfect to the data. To take this special fourth measure into account, i thought about using the information of the slope and intercept mean and variation (of the first three measurements) as a predictor for the fourth measurement occasion in a regression.
So i wrote a model: i s | y1@0 y2@.1 y3@.2;
4th_var ON i s;
But the model fit decreased drastically.
So i was wondering, if this is the false solution for this problem? Are there other, elegant solutions for this problem?
Thank you very much!