Prob. PER is only really useful for a firm with a stable and positive net income (think of things like industrials where growth capex isn't such a major factor)

Prob. PER is only really useful for a firm with a stable and positive net income (think of things like industrials where growth capex isn't such a major factor). For something like a firm fluttering around zero EPS you would use a more forward measure of expected profit. The problem with PER is that stock price is forward looking while EPS is backwards looking.