Researchers at the Bureau of Labor Statistics periodically conduct surveys with shoppers to figure out what products they buy and where, and based on those surveys, build a basket of goods. Then each month, inspectors go out to stores to check the prices of those goods — about 94,000 individual items.

To try to keep the basket of goods current, the bureau updates about an eighth of the basket every six months. If consumers report buying items online, those items have a chance of being picked for the basket.

But the bureau’s own economists are the first to acknowledge there are several drawbacks to the process. Because these surveys take time, the composition of the basket of goods probably reflects consumer behavior from a few years ago, said Steve Reed, an economist with the bureau’s consumer price program.

So the index could lag in reflecting the shift in shopping to the web — and the lower prices that follow.

“It’s always been the case that we’ve trailed actual consumer behavior,” Mr. Reed said.

In comparison, the Adobe index tracks about 1.7 million consumer goods, all online, in real time. It can incorporate new goods quickly, and pick up price trends over days, not just months. For example, data from Black Friday last year showed that TV prices really did fall over all during that holiday weekend. So Black Friday may really be the best time to buy a new TV.

Some Internet deals, of course, can be deceptive. Still, the lower prices make sense, said Tamara Gaffney, the principal research analyst on Adobe’s digital index project, because websites lets shoppers browse and shop anywhere, comparing prices across multiple sellers and taking advantage of the best bargains available.

And not all online prices were lower. Groceries bought online logged a cumulative inflation of 0.75 percent in the 12 months to February. Over the same period, the Consumer Price Index for that category fell 0.31 percent.