The number of visitors to the federal government’s Web site dropped 88 percent between Oct. 1 and Oct. 13, according to a new analysis of America’s online use, while less than half of 1 percent of the site’s visitors successfully enrolled for health insurance the first week.

The new numbers on the health-care law — released by Kantar US Insights and based on an assessment conducted by the nonpartisan research firm Millward Brown Digital — provide a partial snapshot of how the federal health-care exchange has fared since it launched at the start of the month.

Of the 9.4 million unique visitors to the site during the first week, roughly a third attempted to register and 1.01 million completed registration, according to the analysis. Millward Brown Digital — which tracks the online activity of 2 million Americans, or 1 percent of all Internet users in the United States — said that roughly 36,000 Americans signed up for an insurance plan online the first week.

Obama administration officials challenged the independent assessment but did not release any specific enrollment figures.

“ received 14.6 million unique visits in the first 11 days, showing the intense demand for quality, affordable health insurance,” Health and Human Services Department spokeswoman Joanne Peters said in an e-mail. “While traffic is down somewhat from its peak on day one, it remains high as Americans continue to seek to learn more about their new coverage options. We will be releasing enrollment figures on a monthly basis similar to how we release data for the [Children’s Health Insurance Program] and Medicare programs.”

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An administration official who spoke on the condition of anonymity because enrollment figures were not yet public said the numbers in the Kantar US Insights report were “inaccurate.”

Aneesh Chopra, who served as the White House’s first chief technology officer during President Obama’s first term, said the analysis actually captured the public’s sustained interest in signing up for health insurance on the federal exchange. He said that although the total number of visitors to the site is interesting, it matters much more that 1 million Americans created an online account.

“Account creation is always the holy grail. That’s the moment that matters,” Chopra said in an interview. “In one week, a million people began a process that will result in affordable coverage. That means a lot of people are going to ultimately get the product.”

In a blog post, Millward Brown Digital Vice President Matthew Pace, who called the first week’s performance “atrocious,” said the numbers could change dramatically once the federal government improves how the Web site functions.

“The fact that millions of Americans not only visited the exchanges but also took what actions they could to register and enroll suggests that demand indeed exists for new health-care options,” Pace wrote. “As the site improves and more consumers are able to compare their coverage options, they’ll decide with their wallets how truly affordable Obamacare turns out to be.”

Pace said HHS’s visitor totals varied slightly from his firm’s because they used “different timeframes and different methodologies” but were “pretty close.”

He compared the initial number of users with the kind of traffic experiences, noting that about 0.9 percent of all online users logged onto the site during the first week. As of Oct. 13, that had fallen to 0.1 percent, he said. Last week, he estimated, the site attracted 4.1 million visitors, or less than half of what it garnered its first week.