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By Peter Coy
Two widely followed surveys “significantly overestimated” how many American adults got their first dose of a Covid-19 vaccine last spring, says an article in the scientific journal Nature that was published online on Wednesday.
The surveys have large numbers of participants but nevertheless aren’t representative of the U.S. adult population, the article says. This flaw is a symptom of a bigger problem, which is the belief that asking lots of people to respond to a survey can make up for deficiencies in its design. It’s what one of the Nature authors, the statistician Xiao-Li Meng, calls the big data paradox. “The more the data, the surer we fool ourselves,” he wrote in 2018 in The Annals of Applied Statistics.
The Nature article analyzes two surveys of Covid-19 vaccine uptake: One is the Delphi-Facebook survey, which gathers about 250,000 responses a week and which its backers describe as unmatched in “detail and scale” for a public health emergency. The other is the Census Bureau’s Household Pulse Survey, which gathers about 75,000 responses every two weeks. The article says that earlier this year the Delphi-Facebook survey overestimated the uptake of Covid-19 vaccines by 17 percentage points (70 percent versus the actual 53 percent), while the Census Bureau’s Household Pulse Survey overestimated the uptake by about 14 percentage points.
Those are big misses, far bigger than the surveys’ own estimates of their margins of error. The reason we know the surveys were wrong is that we can compare their estimates for Jan. 9 to May 19 of this year with verified records of vaccinations for that period that were published on May 26 by the Centers for Disease Control and Prevention.
A much smaller survey conducted by Axios and Ipsos was off by only 4 percentage points, the Nature article says. That survey is based on an online panel with only about 1,000 responses per week, but it uses best practices for obtaining a representative cross-section.
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