I asked some of the boycott organizers how effective they thought they had been. They were able to provide detailed information about their roles in promoting the protests on social media but had no data about the effects of their efforts on retailers.

Rahiel Tesfamariam, the founder and publisher of the online magazine Urban Cusp, created the hashtag #NotOneDime on Twitter on Nov. 24, after a St. Louis County grand jury decided not to indict Darren Wilson, the police officer who this summer shot and killed Michael Brown, an unarmed man in Ferguson, Mo. Her slogan has been repeated across the Internet.

“The power structure isn’t listening to us in the streets or in the courts, so we are going to have to do it with our buying power,” she said in an interview. With a new hashtag, #BlackDecember, she is calling for people to buy from black-owned businesses.

Separately, a Los Angeles-based network called the Blackout for Human Rights used the hashtag #BlackoutBlackFriday on Twitter in early November, according to Michael Latt, a spokesman for the group. The group decided to include a Black Friday boycott in its protests “because America speaks the language of money, and that’s something that everybody can understand,” Mr. Latt said. But he and Ms. Tesfamariam said that they had not tried to come up with any numbers that might prove they were having an economic impact.

“We think the impact is obvious,” Mr. Latt said. And, he said, protests and, perhaps, boycotts will continue. They are now focused on the decision on Wednesday of a Staten Island grand jury not to indict a police officer in the death of Eric Garner, another unarmed black man, who died after being placed in a chokehold.

There’s no doubt that such protests can galvanize public opinion. But what effects are they having in this prime shopping season? This brings us back to the data. Early shopping numbers are notoriously unreliable, and the National Retail Federation’s 11 percent decline was only an estimate based on a survey of 4,631 people. It divided respondents by income level and age — but not by race — and asked them about their intended and actual purchases, both in bricks-and-mortar stores and online. On the basis of their answers, it extrapolated aggregate numbers for the entire American economy.

The questions in this year’s survey were decided on Nov. 17, Ms. Grannis said, before the grand jury decision in the Ferguson case. In any event, no questions about social unrest have ever been included in the survey. “It’s just an estimate,” she said. “It’s our best shot.”