For 2006-2010, the households and noninstitutional group quarters selected for interview each week in the NHIS are a probability sample representative of the target population. Beginning in 2011, the minimum time length for a probability sample changed from a week to a month. With four sample panels and no sample cuts or augmentations, the expected NHIS sample size (completed interviews) is approximately 35,000 households containing about 87,500 persons. [...] The annual response rate of NHIS is close to 90 percent of the eligible households in the sample.
Other countries’ (pdf) surveys seem to have similarly large and representative samples.
The US data is from the NHIS 2009
http://www.cdc.gov/nchs/nhis/about_nhis.htm
Other countries’ (pdf) surveys seem to have similarly large and representative samples.