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Specifically, the second moments, do they change much from say 100 to 1000 GeV? Why or why not?

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You can check directly yourself using the Durham HEPData page, which lets you plot PDFs as a function of $x$ or $Q^2$.

At large $x = 0.1$, they're fairly constant over a range of $Q^2$.

large-x PDFs

But at small $x = 10^{-4}$, they rise quickly with $Q^2$:

small-x PDFs

This is an illustration of Bjorken scaling at large $x$ and its breaking at small $x$.

I'd also mention that the $Q^2$-dependence of the PDFs is predicted from QCD by the DGLAP equations, so you can look at those to further understand the behavior.

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As usual the review of particle physics has some useful figures.

From the 2011 edition the chapter on Structure Functions (PDF link) has this figure enter image description here which is not quite the bounds you asked for but does show considerable change.

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Well they're at least qualitatively the same.. and that's over a 100x bigger range than I'm considering, so would it be a reasonable approximation to say they don't change much (say within 5-10%) from 10^2-10^3 GeV? –  user21675 Mar 6 '13 at 1:38

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