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Medicare Chief Marilyn Tavenner testified Tuesday on Capitol Hill that the online rollout of President Obama’s health care overhaul has improved since its disastrous debut on Oct. 1 and continues to do so.

Tavenner told a Senate committee that the site healthcare.gov is now able to process 17,000 sign-ups per hour with almost no errors.

During her opening remarks before the Senate Committee on Health, Education, Labor and Pensions, Tavenner says her department expects “the user experience to continue to improve with each passing week.”

She added that over the past month, millions of Americans have visited healthcare.gov to look at their new coverage options under the Affordable Care Act – commonly referred to as ObamaCare.

Republicans on the Senate Committee emphasized their longstanding criticism of the law, citing examples of cancellations and increased costs while raising questions about cyber-security for healthcare.gov.

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    One, Sen. Lamar Alexander of Tennessee, asked if the administration would support legislation introduced in the Senate to allow individuals to retain their existing coverage if they want to. He recalled Obama's pledge to that effect, first made in 2009.

    In response, Tavenner said she hadn't read the legislation, and took no position on it.

    White House officials declined comment on Monday on similar bills introduced in both houses by other lawmakers.

    During the hearing, Sen. Johnny Isakson, R-Ga., had a poster displayed behind his seat saying, `Tip of the iceberg' that showed a pale blue iceberg floating in water. Above the waterline, the iceberg was labeled `website failures.' Below  were examples of reported health care law problems including canceled coverage, higher co-pay and deductibles, premium increases and fraud and identify theft.

    Committee Democrats were less pointed, although Sen. Barbara Mikulski of Maryland cited consumer confusion.

    "I think it's very confusing about where you go," she said. "I can tell you, people really don't know, they really, really don't know."

    Tavenner, who last week apologized to the public for the poor quality of the website in its earliest days, invited the public to go online to see how it was now functioning.

    Tavenner said the site would be "fully functioning" by the end of the month.

    She seemed reluctant to concede the widespread cancellations that some senators referred to.

    "Some of the 5 percent of Americans who currently get insurance on the individual market have recently received notices from their insurance companies suggesting that their plans will no longer exist," she said.

    "These Americans do have a choice. They can choose a different plan being offered by their insurer or they can shop for coverage in the marketplace or outside the marketplace. As insurers have made clear, they are not dropping consumers; they're improving their coverage options, often offering better-value plans with additional benefits."

    Despite her general assurances, several senators raised detailed questions about experiences their own constituents have had.

    Sen. Lisa Murkowski, an Alaska Republican, said as of Monday, only three people in her state had been able to enroll, and she said there were concerns that they had done so on the basis of incorrect information.

    Sen. Tim Scott, R-S.C., cited an example of a man whose personal financial information had been inadvertently disclosed.

    Tavenner appeared at a time when Democrats remain uneasy about the implementation of a program they created over unanimous Republican opposition in 2010.

    The website went down again in the middle of the day Monday for about 90 minutes. And the administration still refuses to divulge enrollment statistics until mid-November.

    Tavenner began her career as a nurse and built a successful record as a hospital executive before entering public service. Seen as a businesslike manager, she has enjoyed support from lawmakers across the political spectrum.

    HealthCare.gov was supposed to provide one-stop shopping for people who don't have a health plan on the job. Its target audience is not only uninsured Americans but those who already purchase coverage individually. Middle-class people can sign up for private coverage made more affordable by tax credits that act like a discount on premiums. Lower-income people will be steered to an expanded version of Medicaid in states that agreed to expand that safety net program.

    A new study released Tuesday estimates the potential size of the market nationally at 28.6 million people. The nonpartisan Kaiser Family Foundation says three out of five, or more than 17 million people, will be eligible for tax credits. That includes both uninsured people and those switching over from current individual plans. Texas, California and Florida have the highest numbers of residents eligible for the credits.

    Earlier, the Congressional Budget Office estimated that 7 million uninsured people would gain coverage through the marketplaces, a statistic the Obama administration adopted as its own enrollment target.

    The Associated Press contributed to this report