This one is interesting. It has numbers.
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0005738
How Many Scientists Fabricate and Falsify Research? A Systematic Review and Meta-Analysis of Survey Data
The frequency with which scientists fabricate and falsify data, or commit other forms of scientific misconduct is a matter of controversy. Many surveys have asked scientists directly whether they have committed or know of a colleague who committed research misconduct, but their results appeared difficult to compare and synthesize. This is the first meta-analysis of these surveys.
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A pooled weighted average of 1.97% (N = 7, 95%CI: 0.86–4.45) of scientists admitted to have fabricated, falsified or modified data or results at least once –a serious form of misconduct by any standard– and up to 33.7% admitted other questionable research practices. In surveys asking about the behaviour of colleagues, admission rates were 14.12% (N = 12, 95% CI: 9.91–19.72) for falsification, and up to 72% for other questionable research practices.
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Considering that these surveys ask sensitive questions and have other limitations, it appears likely that this is a conservative estimate of the true prevalence of scientific misconduct.
(Also found at the nih.gov WWW site:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2685008/)