Washington (CNN) White House economic adviser Kevin Hassett acknowledged Tuesday that the White House underestimated the negative impact on the United States' economic growth from the ongoing government shutdown.

Hassett discussed the consequences of the shutdown, which is in its fourth week , with Fox Business Network's Melissa Francis on "After the Bell."

"Does that sound right? 1.7% for the quarter?" Francis asked of Hassett's revised estimates for GDP growth in the first quarter of the year.

"Right. So what happens is that when the Bureau of Economic Analysis, when they calculate GDP, what they do is they basically have to estimate how much the government produces, and their estimate of how much the government produces is based on just government workers going to work and getting paid," Hassett replied.

"And so if government workers don't go to work, then they reduce their output calculations accordingly. We made an early estimate right at the beginning of the crisis that was a little bit lower than the estimate you just cited and have been studying hard as this has gone on and have found that actually the damage is a little bit worse because of government contractors, something that was excluded from our first analysis."

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