  We investigate the relative size of various twist-3 quark-gluon correlation
functions relevant to single transverse spin asymmetries (SSAs) in a
quark-diquark model of the nucleon. We calculate the quark-gluon correlation
function $T_{q,F}(x, x)$ that is responsible for the gluonic pole contribution
to the SSAs, as well as $T_{q,F}(0, x)$ and $T_{\Delta{q}, F}(0, x)$
responsible for the fermionic pole contributions. We find in both cases of a
scalar diquark and an axial-vector diquark that at the first nontrivial order
only the $\tq(x, x)$ is finite while all other quark-gluon correlation
functions vanish. Using the same model, we evaluate quark Sivers function and
discuss its relation to the $\tq(x, x)$. We also discuss the implication of our
finding to the phenomenological studies of the SSAs.
