Hi again,
I think the issue has to do with your original question-- what to enter as the time values. It is not correct to use the same time value for both time points. I suggest you set time to 0 for the first visit and keep the time value as it is for the second visit. Another alternative is to create a date variable that equals the value of ECHO_DATE_1 for the first visit and equals ECHO_DATE_2 for the second visit. Then- you would use that date variable in the model in place of your time variable. Doing so would give you a slightly different interpretation, since the model is now taking into account the actual dates and their spacing, instead of just their spacing.
An alternative that I think is better than either of the above, though (which I mentioned in another post) is to use AGE as your time variable. Age would need to be calculated by subtracting the visit dates from birth date, so the age is a decimal, not rounded to a year. Age would also need to be calculated at both visits. This would lead to a really nice interpretation of your results, so that you can talk about how your outcome is changing for different ages of children, while simultaneously talking about the outcome over time. Note- using age to measure time would not be the correct choice if this was an experimental study where an experimental treatment is applied at the first visit. In such a case, your primary interest is time from baseline. Your study sounds like it is purely observational, though.
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