As a percentage? No. But qualitatively speaking, "often."

The most recent book I read discusses this particularly with respect to medicine, where the problem is especially pronounced because a majority of studies are conducted or funded by an industry with a financial stake in the results, with considerable leeway to influence them even without committing formal violations of procedure. But even in fields where this is not the case, issues like non-publication of data (a large proportion of all studies conducted are not published, and those which are not published are much more likely to contain negative results) will tend to make the available literature statistically unrepresentative.