# Mplus multilevel model with variables of different length

Let's say I have 4 variables on the within person level - xa, ya, xb, yb. xa and ya have each 100 trials, xb and yb have 200, and there are 150 subjects. I want to build the following multilevel model in Mplus:

CLUSTER = subject;
WITHIN = ya, yb;

ANALYSIS:
TYPE = TWOLEVEL RANDOM;

MODEL:
%WITHIN%
sa | ya ON xa;
sb | yb ON xb;

%BETWEEN%
G BY sa sb;


So basically, I extract the random slopes from the within person level regression a and b and use them to model another factor G on the between person level. The issue is that Mplus calculates the whole model only on 100 rows where all 4 variables have non-missing values, although regressions ya on xa, and yb on xb are done independently of each other. I hoped that Mplus will calculate the a regression on 100 trials and extract 150 random slopes for all people, and that it will calculate the b regression on 100 trials, and again extract 150 random slopes for all people. Then, on the between person level we would be able to model the latent variable G with two variables that each have 150 values....

However, Mplus always calculates the random effects only on 100 trials. How can I correct this?

This is the warning I get

*** WARNING
Data set contains cases with missing on x-variables.
These cases were not included in the analysis.
Number of cases with missing on x-variables:  100
1 WARNING(S) FOUND IN THE INPUT INSTRUCTIONS

SUMMARY OF ANALYSIS

Number of groups                                                 1
Number of observations                                        100