First Published: October 31, 2024
Context: For NHANES cycles prior to the August 2021-August 2023 cycle, the first day of dietary intake (Day 1) was collected in-person during the mobile exam center (MEC) visit, while the second dietary intake (Day 2) was collected over the phone.
Mode change: To minimize in-person interactions during the COVID-19 pandemic during data collection for the August 2021-August 2023 cycle, NHANES changed the mode of administration for the Day 1 interview to be over the phone. This phone interview was scheduled during the MEC visit. That is, both the Day 1 and Day 2 dietary interviews occurred by phone after the MEC examination for the first time in the August 2021-August 2023 cycle. Based on the condensed window of time to get NHANES operational and back in the field as quickly as possible, there was not time to formally pilot or test the mode change of the Day 1 recall.
Response rate and sample size changes: Response rates in August 2021-August 2023 were substantially lower than in previous cycles for NHANES at all stages of sampling, including for the MEC visit. Due to the lower response rate for the MEC visit compared to past cycles, the change in timing, and the mode change, the sample size for the August 2021-August 2023 dietary intake data was also lower.
Dietary recall response rates, sample size, and food and nutrient intake in August 2021-August 2023 were compared to past cycles (2011-2012 through 2017-2018).
Some comparisons of response rates, sample sizes, and mean and distribution of nutrient intakes are described below. This information should be considered when using August 2021-August 2023 dietary data, particularly in conjunction with other cycles of NHANES data.
- The August 2021-August 2023 Day 1 dietary interview response rate for those examined in the MEC was 76.2%. In past recent cycles this rate ranged from 91.2% (2011-2012) to 87.8% (2017-2018). When response rates for all stages of NHANES are accounted for--household screening, health interview and MEC exams --the response rate for the Day 1 dietary interview was 19.6% compared to 68.3% in 2011-2012 and 42.8% in 2017-2018 (see analytic guidance below).
- The mean number of days between dietary data collection on Day 1 and Day 2 was higher in the August 2021-August 2023 cycle (9.3 days). By comparison, mean number of days between Day 1 and Day 2 ranged from 6.4 in 2013-2014 to 8.7 in 2015-2016.
- The mean number of foods reported on Day 1 in the August 2021-August 2023 cycle (14.9) was slightly lower than the mean number of foods reported on Day 2 (15.2), while in past cycles more foods were reported on Day 1 compared to Day 2 (for example, in 2017-2018, Day 1 mean number of foods was 16.2 and Day 2 was 15.4). Mean absolute differences were less than one food for all cycles, including August 2021-August 2023.
- The absolute and relative differences between Day 1 and Day 2 distributions of nutrient intake, number of foods reported, percent of energy from carbohydrates, fat, and protein, and iron, vitamin A, and calcium per 1,000 calories were similar in August 2021-August 2023 compared to past cycles.
Analytic Guidance: The information presented here is intended to alert data users to the changes to the Dietary Recall data in the August 2021-August 2023 NHANES. Given the changes and the resulting analyses described here, analysts may want to consider these issues when analyzing the August 2021-August 2023 dietary data, particularly when examining trends over time. There are some further considerations for analysts:
- The analyses could not discern whether differences in mean nutrient intake and distributions for some nutrients were due to changes that occurred over time, the mode of data collection, lower response rates, or any combination thereof. Analysts should consider and acknowledge these changes and the possibility that the mode change may impact findings.
- As a result of lower response rate and sample sizes, estimates may have lower precision and there may be lower statistical power to discern significant differences between groups. For details pertaining to sample design, weighting methods, and analytic guidelines, please refer to the brief overview that accompanied the initial data release for the August 2021-August 2023 cycle.
- Because the Day 1 dietary interviews were included in the MEC exam in past cycles, dietary interview response rates are not provided on the NHANES website for past cycles. However, since August 2021-August 2023 was the first cycle that the Day 1 dietary interview has been conducted after the MEC visit and thus, the dietary interviews became an additional stage of the survey, response rates for the Day 1 dietary interview are now provided on the NHANES website—these account for household screening, health interview and MEC exam response rates in addition to response to the Day 1 dietary interview.
- As for all cycles, dietary interview weights account for nonresponse to the dietary interviews and should be used in all analyses of dietary interview data.