In a paper published last month, the team of Eugene Fama and Ken French—credited with identifying the value premium back in 1992—analyzed how the value factor has fared over the past 28 years and what the outlook might be going forward. This according to a recent article in Institutional Investor.

What did they find? “The short answer, French told II in a phone interview, “is you pretty much can’t say anything.”

The study, which involved examining actual returns of a value portfolio between July 1963 and June 2019, found that value premiums were on average much lower in the second half of the period. Still, French explained, even nearly three decades of data on falling returns isn’t enough to conclude whether the value factor has stopped working or is simply suffering an extended rough patch.

French said, “It drives many investors nuts,” but that “the reality is you can’t look at what turn out to be pretty brief periods from a statistical perspective.”

The team concluded that there was too much volatility in monthly returns to “decisively determine whether or not the expected premium from the factor has changed.” According to French, “there is no way to tell” whether the expected value premium has declined or disappeared.