The forward price-earnings (P/E) multiple has limited value during normal times.

And the metric arguably has even less value during periods of elevated uncertainty.

That's because the E is based on analysts' estimates for the near future. And when the outlook for business is increasingly uncertain and rapidly changing, it can take time for many analysts to adjust that E.

Assuming tariffs are negative for earnings — which is what everyone assumes — this means the E is being distorted higher by stale estimates.

Forward earnings estimate haven't really moved amid the market sell-off. (Source: FactSet)

With stock prices falling the way that they have been in recent weeks, the P/E ratio could be creating the illusion that stocks have gotten cheaper than they are in reality.

Forward P/E ratios have come down. But is the E accurate? (Source: FactSet)

Unfortunately, we might not get a clean E any time soon.

"There is a reasonable probability that absent some resolution/clarity, transparency could be compromised," BofA's Savita Subramanian wrote on Thursday. "Companies tend to shut down guidance amid uncertainty."

If you're going to trade, be careful about trading based on expectations for the near future. The savviest minds in the market caution this is a guessing game.