It certainly would depend on the kind of research you are conducting, but there are ethical and legal considerations to be thought through first before any information gathering takes place.
Does the research violate a Constitutional right to privacy that is still guaranteed to the accused and the convicted? Does the research intrude on the privacy and trauma of the victims, some of which are victims of violent crimes and assaults? In that case, their rights as humans to heal and avoid any public humiliation possible would trump any need or right to conduct research. Does the research involve crimes against minors? Add another layer of privacy rights.
Finally, while research can give you accurate sets of numbers, it can not often explain them. A survey of convicts will find a disproportionate number of ethnic minorities and the poor in prison, but the numbers themselves will not explain why, which leaves people to easily make erroneous judgments about the nature of who commits crimes and why.