same level of the construct. This case is often described in terms of latent variable theory (Bollen, 2002; Bollen & Lennox, 1991; Borsboom, Mellenbergh, & van Heerden, 2003). Although a complete review of latent variable models is beyond the scope of this article (see Borsboom et al., 2003), the theoretical position is that variation in the latent variable causes variation in its indicators; each indicator is an expression of the latent variable.
