I can't really respond to that question in a detailed way because it depends on the context in which the information arises or the issue arises.
The reason I would decide to commence an inquiry would be because the information that came to me, one way or another, seemed to me to be convincing enough that an issue of public ethics was at stake and should therefore be investigated. It's a very vague response, but that is in fact what I do. I ask myself whether or not the information is sufficiently convincing and sufficiently substantive to make an inquiry worth while.
I try not to do this often because one of the difficulties that I find we work under is that the launching of an inquiry all by itself creates problems for the people about whom the inquiry is launched. Irrespective of the outcome or irrespective of any wrongdoing, clearly that's the case. It's the sheer mentioning of something.
To choose another context, it's as if one accused a kindergarten teacher of some sort of sexual harassment. Whether or not it's true, damage is created by the sheer mention of the possibility.
I try to do it quite infrequently, and I have done it quite infrequently simply because of that.