I am building autoregressive latent trajectory models. I follow the method of Morin et al, 2011 (The longitudinal interplay of adolescents' self-esteem and body image: a conditional autoregressive latent trajectory analysis), which I think is very helpful.
However, there are two things I do not understand. Firstly, in their syntax (supplementary material), the LCM part specifies BI1@-.4. I know that in ALTs, the first measurement point is treated as predetermined. However, I am not sure about why it is predetermined. Furthermore, I am not sure why the number -.4 is used. Can someone explain what number you should use?
Secondly, in the autoregressive parts of the syntax (supplementary material), BI1; or SE1; is specified, before specifying the autoregressive components with the ON statement. Can someone explain the rational behind specifying this statement is? I am not sure whether I should include it.