So travellers/gypsy/pikeys are criminals etc. Anybody got any data?

Apparently travellers are criminals, at least that is what I’m told. I’m also aware that prejudice against travellers is pretty much also the only prejudice which it is acceptable (expected?) to hold. I try not to form opinions on things on anecdote, so I’m looking for some data. Can anyone help?

Travellers provide perfect controlled experiments. We can look at year by year crime stats and see if the arrival (or disappearance) of travellers causes a break in trend. If they are more criminally active than average it should show up and it should be easy to detect.

Now there will be confounding factors, for example, people may be more likely to notice and report low level crime if they know travellers are nearby, likewise they may report crimes which did not happen to gain leverage to have the moved on, similarly you would need to control for travellers merely as an increase in population and density, plus you’d need to control for socioeconomic status (ugly phrase) of the travellers. There are doubtless other things I’ve not thought of.

So, has anybody carried out these tests? If so, why don’t we hear about them, if not, why not?