I have a basic question about growth curve models.
I want to analyse the factors that affect crop production. For this, lets assume that I have the crop production per administrative unit (100 counties) per year (four years). Among the variables that affect crop productions, some vary among counties but not in time, such as surface of the county; others varies among counties and in times, such as crop surface; and I have another variable type that varies in time but not among counties, such as crop price.
My question is, how do I treat this last variable?
Lets assume that
- CP0 to CP3 is crop production (in tons) per county for the first to the fourth year;
- S is county surface (in hectares);
- C0 to C3 is crop surface (in hectares) per county for the first to the fourth year; and
- D0 to D3 is the crop price (in USD/tons of crop) for the first to the fourth year.
Base on the lavaan tutorial, I set the model in this way
model <- '
# intercept and slope with fixed coefficients
i =~ 1*CP0 + 1*CP1 + 1*CP2 + 1*CP3
s =~ 0*CP0 + 1*CP1 + 2*CP2 + 3*CP3
i ~ S
s ~ S
# time-varying covariates
CP0 ~ C0 + D0
CP1 ~ C1 + D1
CP2 ~ C2 + D2
CP3 ~ C3 + D3
D will be constant for all counties (it means that in the data frame, D0 will be the same for all counties). Is that correct?