From the Introduction: "The National Health Interview Survey (NHIS) is in the field continuously, collecting survey responses throughout the year by making personal visits to respondents' homes. One sample adult and one sample child (if any children are present in the household) are randomly selected to be the subjects of the interview. Interviewing is divided into calendar year quarters, and each quarter's data are representative of the overall population. To produce each annual public data file, four quarters of data are combined and weighted to represent the civilian noninstitutionalized population of the United States. The NHIS Early Release (ER) Program publishes preliminary estimates of key indicators on a quarterly basis in advance of the eventual annual public-use data file release. The COVID-19 [coronavirus disease 2019] pandemic significantly impacted NHIS data collection beginning in quarter 2 (Q2) of 2020. The U.S. Census Bureau collects data for the NHIS through an interagency agreement. The Census Bureau suspended all personal visits on March 19, 2020. Telephone interviews were used to conduct all NHIS interviews in Q2. The Census Bureau made extensive efforts to match telephone numbers to the addresses in the NHIS sample; however, collecting data by telephone rather than personal visit resulted in a reduction in the household response rate from 60.0% in quarter 1 (Q1) to 42.7% in Q2. The sample adult response rate declined from 57.9% in Q1 to 41.1% in Q2. The sample child response rate declined from 57.6% to 40.1%."