I ran SEM model in Mplus and initially, I did not use any latent variables defined with command BY. CFI and TLI was 1.000 and X² and RMSEA of 0.000. However when I replaced one of the predictors with a latent concept (BY) then fit indices looked more ‘normal’ (e.g. X² = 936.01(10), CFI = .744, TLI = .448, RMSEA = .120). Could anyone explain why this has happened. Is it simply that because not including latent concepts in the Mplus makes it impossible to obtain RMSEA and X² , but then CFI and TLI look as perfect… Please if someone could shed some light.