Before lining up at a retail store or firing up your computer for Black Friday bargains this week, ask yourself one question: Have you ever scored a great deal from the annual post-Thanksgiving shopping bonanza?

If you were hard-pressed to think of something, you would not be alone. Black Friday, which has traditionally been the moment to flock to stores for steep discounts, and which has evolved to also include major online sales events for retailers like Amazon, Best Buy and Walmart, is not all that it is billed to be. We asked J. D. Levite, the deals editor of the product recommendations website The Wirecutter, for some data on just how beneficial the deals are on Black Friday — and the answer was not encouraging.

Year round, Mr. Levite and his team track product prices across the web to unearth discounts on goods of all types, from gadgets to kitchenware. They also look at whether the product is high quality and durable based on their own testing and other reviews, and whether the seller or brand has a reasonable return or warranty policy. By those measures, Mr. Levite said, only about 0.6 percent, or 200 out of the approximately 34,000 deals online, which typically carry the same price tags inside retailers’ physical stores, will be good ones on Black Friday.