differential operator

I've read journal "On the Comparison of Several Mean Values: An Alternative approach" (Welch, 1951). I don't understand this expression: $$E\left(\exp\left[ \sum_t ( w_t - \omega_t ) D_t\right]\right),$$

where $$D_t^r = \left(\frac{\partial^r}{\partial z_t^r}\right)_{z=\omega}$$ and $E$ is expectation over $w_t$

Why is $D_t$ written in the last equation?

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What kind of mathematical objects want $r,t,w,\omega$ denote? We can have some guesses, but please specify them a bit. –  Berci Nov 30 '12 at 11:05
For more explanation, you can read this journal: google.co.id/… –  Maifiana Sari Dec 4 '12 at 5:28