Famously, scientific theories are underdetermined by their evidence. This occurs in the factor analytic model, which is often used to connect concrete data to hypothetical notions. After introducing FA, three general topics are addressed. Underdetermination: the precise reasons why FA is underdetermined illuminates various claims about underdetermination, abduction, and theoretical terms. Uncertainties: FA helps distinguish at least four kinds of uncertainties. The prevailing practice, often encoded in statistical software, is to ignore the most difficult kinds, which are essential to FA's (...) underdetermination. What to do: some suggestions for dealing with these hardest types of uncertainty are offered. (shrink)