1. At the first time point, they were no missing values and the estimated intercept means did not differ very much between the models. Only the listwise deletion will overestimate the intercept since the weak students with low score drop out.
2. From table 2, it is clear that the dual-process NMAR model were not substantially different from those that from MAR. Therefore, it was assumed that missing data are MAR.
The listwise-delection approach will overestimate the intercept mean than the NMAR model. Moreover, when there were no risk factors, the slope are not different between the listwise delection and NMAR model. But, where there were risks, the slopes were overestimated with listwise-delection than NMAR model.
3. Exactly.
4. Thank you for your carefulness. I even did not notice it.
5. Yes. I'm thinking how to solve the problem within the framewok of IRT.
6. It was assumed that the intercept have equal varaiance 1 across three timepoint. Therefore, in figure 1 growth model, there are three 1 under intercept. However, the authors did not explain why the residual variance will be different for linear slope and quadratic slope.