I am trying to produce a single value representing social vulnerability based on 6 z-score normalized demographic variables for 55 geographic areas. I have successfully done a factor analysis on the 6 variable and computed the factor scores (using Bartlett) in SPSS for the 2 factors generated from my inclusion criteria (eigenvalues > 1, break in scree plot and cumulative variance over 80%). Now I need to turn these two factor scores into one useful number representing the social vulnerability of each area. Do I just sum the two factor scores? Should I z-score normalize them?
I will then be multiplying the vulnerability index with other target layers (heat stress and population density), how do I make sure the vulnerability value is on a useful scale compared with the other target layer values?