While the success of retailers on Black Friday can send stocks up or down in the very short term, Mark Hulbert says investors shouldn’t think that such market moves are a harbinger of things to come for the rest of the year.

“The initial reports of how retailers are doing on Black Friday are an unreliable guide to how the stock market performs through the end of the year,” Hulbert writes for MarketWatch. “In fact, more often than not, it’s been a bad omen whenever those initial reports are especially positive and stocks soar.”

Hulbert says that’s the conclusion he reached after studying stock market returns back to 1975, when the term “Black Friday” became popular. He says he looked at the correlation between a) the Dow Jones Industrial Average’s performance on the Friday and Monday following Thanksgiving and b) the Dow’s performance the rest of the year.

“Surprisingly, I found an inverse correlation,” Hulbert writes. “That is, whenever the Dow was strongly up over the two post-Thanksgiving trading sessions, the market tended to fall from then through New Year’s Day. And just the opposite tended to be the case whenever the Dow lost ground over these two post-Turkey sessions.” What’s more, he says the correlation is “significant at the 95% confidence level that statisticians often use to determine if a pattern is genuine.”