At this point, shopping on Black Friday is every bit a Thanksgiving tradition as turkey dinners and pumpkin pie.

But certain discounts you see online may not be as good a deal as advertised. In fact, 36% of items on sale for Black Friday offer no savings compared with how they were priced between Oct. 27 and Nov. 17, according to new data from WalletHub.

“There’s some trickery happening,” says consumer savings expert Andrea Woroch. “Retailers are constantly fluctuating prices. So they may repackage something and say it’s on sale for Black Friday when, in reality, it was the same discount a few weeks ago.”

That’s not to say that there aren’t legitimate savings to be had. For items that are on sale, the average markdown is about 24%, according to WalletHub’s data. You’ll just have to do a little digging to make sure that you’re making the most of the discounts you see.

“The consumer has to be proactive,” says Chip Lupo, a writer and analyst at WalletHub. “Because retailers do more or less rely on people rushing in at the last minute and taking advantage of anything that appears to be a sale.”