U.S. shoppers no longer blow the bulk of their holiday budgets on the day after Thanksgiving known as Black Friday.

It's a major shift that has made it difficult for stores to track and learn from shoppers' spending habits during the traditional start to the busy holiday shopping season.

Many U.S. shoppers like to make purchases on their desktops and smartphones nowadays, they insist on getting big discounts whenever they shop, and they don't feel pressured to shop on particular days.

That shift has caused the National Retail Federation, the nation's largest retail trade group, to overhaul the way it tracks shopper spending and visits during the four-day Thanksgiving weekend - something it's been doing for more than a decade. The group said the changes are aimed at getting a fuller picture of shoppers' habits, including their growing affinity for online buying.

The group, which has long been criticized for its estimates of total spending over the extended weekend because they're based on shopper surveys and not actual sales data, said it will no longer track total spending for the weekend. It also said it's asking shoppers different questions, which has led it to report numbers that vary significantly from those it's gotten in previous years.

The moves are significant because retail experts, investors and economists look to the group's Thanksgiving weekend numbers to provide an indication of the mood of consumers heading into the holiday shopping season, a period of historically strong spending.

The retail group on Sunday said more than 151 million people shopped in stores or online over the weekend, according to a survey of 4,281 shoppers. That compares with 133.7 million last year, but the group said those figures are not comparable because of its change in methodology.