Comps/Precedent: When to compare outside of one industry?

In terms of using comps/precedent, in building the "universe" of companies, we want to choose companies that have the same business models. Qualitatively and quantitatively, this can be the same industry, same geography, market size, etc. But, when is the case when it would be better to compare OUTSIDE of one industry? Is it only when there are not similar companies within one industry we can compare the target company to, or is it really just dependent upon assumptions (ie. one company might span various industries and so comparing a hybrid company to a tech company makes more sense than comparing it to a consumer retail company)