National Health and Nutrition Examination Survey

The objective of the dietary interview component is to obtain detailed dietary intake information from NHANES participants. The dietary intake data are used to estimate the types and amounts of foods and beverages (including all types of water) consumed during the 24-hour period prior to the interview (midnight to midnight), and to estimate intakes of energy, nutrients, and other food components from those foods and beverages. Following the dietary recall, participants are asked questions on salt use, whether the person’s overall intake on the previous day was much more than usual, usual or much less than usual, and whether the respondent is on any type of special diet. Questions on frequency of fish and shellfish consumed during the past 30 days were asked of survey participants 1 year or older, with the use of proxies or adult assistants for young children (see the MEC In-Person Dietary Interviewers Procedures Manual for more information on proxy interview).

The dietary interview component, called What We Eat in America (WWEIA), is conducted as a partnership between the U.S. Department of Agriculture (USDA) and the U.S. Department of Health and Human Services (DHHS). Under this partnership, DHHS’ National Center for Health Statistics (NCHS) is responsible for the sample design and data collection and USDA’s Food Surveys Research Group (FSRG) is responsible for the dietary data collection methodology, maintenance of the databases used to code and process the data, and data review and processing.

All NHANES examinees are eligible for two 24-hour dietary recall interviews. The first dietary recall interview is collected in-person in the Mobile Examination Center (MEC) and the second interview is collected by telephone 3 to 10 days later.

As in previous years, two types of dietary intake data are available for the 2007-2008 survey cycle - Individual Foods files and Total Nutrient Intakes files. Starting with the 2007-2008 data release, data documentation will be published only in HTML format. The document in its new format contains the same information as it has in previous years.

What’s New with the 2007-2008 WWEIA Release:

Vitamin D has been added to the list of nutrients. The vitamin D values in this dataset reflect the sum of ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3) content of foods reported by survey participants. The contribution of another form of vitamin D, 25-hydroxycholecalciferol, which may be present in certain foods, was not included because adequate data from validated methods for this form of vitamin D were not available. Caution should be used when comparing these data to earlier reported vitamin D intakes by various investigators, since some differences may be attributed to discrepancies in analytical methods and food sampling.

A variable has been added in each of the four files (DR1DBIH for day 1 files and DR2DBIH for day 2 files) to indicate the number of days between the intake day and the day of family questionnaire administered in the household. NHANES has collected information on the receipt of food stamps, including the last benefit received date as part of the family interview. Previous research has indicated that there is a monthly cycle in food consumption behavior for different levels of benefits. These new variables will be critical in understanding dietary behavior of food stamp recipients over the month.

In 2007-2008, a new sample design was implemented. All Hispanics were oversampled, not just Mexican Americans. In addition, for each of the race/ethnicity domains, the 12-15 and 16-19 year age domains were combined and the 40-59 year age minority domains were split into 10 year age domains 40-49 and 50-59. This has led to an increase in the number of participants aged 40+ and a decrease in 12-19 year olds from previous cycles. Lastly, pregnant women were no longer oversampled. Based on these changes, unweighted frequencies of certain variables may differ slightly in this survey cycle compared to earlier survey cycles. Refer to the NHANES 2007–2008 Public Data General Release File Documentation (https://wwwn.cdc.gov/nchs/nhanes/continuousnhanes/generalrelease.aspx?BeginYear=2007) on the NHANES website for more detail.

NHANES has been collecting data on study participants’ use of dietary supplements in the past 30 days, during the Dietary Supplements Section (DSQ) in the household interview. In 2007-2008, additional information on supplement and antacid use for the previous 24 hours was collected to provide data of the same timeframe as the food and beverage intake. This will allow researchers to improve the total nutrient intake estimations as well as examine associations by nutrient source such as from foods versus supplements.

With a similar protocol, the 24-hour dietary supplement interview was collected following the 24-hour dietary recall. All NHANES examinees responding to the dietary recall interview were eligible for the dietary supplement and antacid use questions. Information was obtained on all vitamins, minerals, herbals and other dietary supplements that were consumed during a 24-hour time period (midnight to midnight), including the name and the amount of dietary supplement taken.

The 24-hour dietary supplement data are released in separate files available on the NHANES website. They can be linked to the dietary files by the respondent sequence number (SEQN). For more information on the data, refer to the documentation accompanying the 24-hour dietary supplement datasets.

Further details about these and other changes for 2007-2008 are provided within this document. Appendix 1 provides a summary of changes among the 3 latest cycles of data collection.

Dietary Interview Data Files: Four data files were produced from the information collected in the dietary interview: two Individual Foods files and two Total Nutrient Intake files. Each file includes one day of intake data. The number “1” or “2” in the file name identifies the day (and mode) of the interview: 1 = first day (in-person), 2 = second day (phone). File names are the following:

The nutrient amounts in these files reflect only nutrients obtained from foods, beverages, and water including tap and bottled water. They do not include nutrients obtained from dietary supplements, antacids, or medications.

Individual Foods Files (DR1IFF_E and DR2IFF_E): Detailed information about the types and amounts of individual foods reported by each participant, as well as amounts of nutrients from each food are included in the Individual Foods files. The names for both Day 1 and Day 2 variables are listed in Appendix 2.

The Individual Foods files include, for each interview day, one record for each food consumed by a survey participant. Each food record is sequentially numbered and contains the information listed below:

• Number of days of complete intake obtained from participant
• Day of the week of the intake
• Whether the food was eaten in combination with other foods, such as in a sandwich
• Time of eating occasion when the food was eaten
• Eating occasion name
• Where the food was obtained
• Whether the food was eaten at home or not
• Food, water, or beverage identified by a USDA food code
• Whether nutrients were calculated directly from the food as identified in FNDDS 4.1 or the FNDDS item was modified by adjusting recipe ingredients
• Amount of food, water, and beverages consumed, in grams
• Amounts of energy and 64 nutrients/food components (listed in Appendix 3) from each food, as calculated using USDA's Food and Nutrient Database for Dietary Studies, 4.1 (FNDDS 4.1)

Beginning with the 2005-2006 WWEIA and continued in 2007-2008 WWEIA, the consumption of all types of water (including tap and bottled water, plain and carbonated, sweetened and unsweetened water) has been collected during the 24-hour recall. As a result, each report of water intake throughout the day was collected and coded as a separate food item. These water intake items now appear in the Individual Foods file with food codes designating the type of water. As with other foods, a variable is present to indicate if the water was consumed at home or away. For bottled waters, a variable is present indicating where the water was obtained. For tap water, the variables DR1TWS and DR2TWS in the Total Nutrient Intake files indicate the main source of tap water drunk by the participant. In 2003-2004, only sweetened bottled waters were collected during the 24-hour recall and included in the Individual Foods file. Information on daily intake of non-sweetened waters (total plain water, total tap water and the source of tap water, total bottled water, and plain carbonated water) were collected after the 24-hour dietary recall and included in the Total Nutrient Intake file for 2003-2004. Starting in 2005-2006, summary water variables that are compatible with previous data cycles are included in the Total Nutrient Intake files with the exception of plain carbonated water. These water totals (DR1_320Z, DR2_320Z, DR1_330Z, DR2_330Z, DR1BWATZ and DR2BWATZ) represent total water consumed as a beverage by itself. Therefore, they do not include the moisture content of foods and beverages (such as watermelon, coffee, lettuce) that are available for each food reported in the Individual Foods files.

Two supporting files are also included with the Individual Foods files: the Food Code Description file (DRXFCD_E) and the Modification Code Description file (DRXMCD_E). The DRXFCD_E file includes complete descriptions (up to 200 characters) associated with each USDA food code identified in the Individual Foods files. The DRXMCD_E file includes descriptions (up to 200 characters) associated with each modification code identified in the Individual Foods files. Modification codes represent adjustments to predefined recipe ingredients that reflect more closely the food as described by the respondent. Appendix 4 provides SAS code examples that may be used to link the food code or the modification code description to the Individual Foods file.

Total Nutrient Intakes Files (DR1TOT_E and DR2TOT_E): For each participant, daily total energy and nutrient intakes from foods and beverages and whether the amount of food consumed was usual, much more than usual, or much less than usual are included in the Total Nutrient Intakes files. The Day 1 file also includes information on salt use in cooking and at the table; whether the participant is currently on any kind of diet to lose weight or for other health-related reason and, if so, the type of diet; and for participants 1 year or older, information on frequency of fish and shellfish consumption. The names for both Day 1 and Day 2 variables are listed in Appendix 5.

The Total Nutrient Intakes files provide a summary record of total nutrient intakes for each individual. Each total intake record contains the following information:

• Number of days of complete intake obtained from participant
• Day of the week of the intake
• Type of salt used and how often added at the table and in food preparation (Day 1 file only)
• Whether the participant is currently on any kind of diet to lose weight or for other health-related reason and, if so, the type of diet (Day 1 file only)
• Total number of foods reported for that participant for that day’s intake
• The daily aggregates of water (moisture), DR1TMOIS and DR2TMOIS, consist of all moisture present in foods and beverages, including tap and bottled waters consumed as beverages
• Daily aggregates of food energy and 64 nutrients/food components (listed in Appendix 3) from all foods, as calculated using USDA’s Food and Nutrient Database for Dietary Studies 4.1 (FNDDS 4.1)
• Whether the amount of food consumed was usual, much more than usual, or much less than usual
• Total amount of water consumed (plain water, tap water and the source of tap water, and bottled water)
• Frequency of fish and shellfish consumption in the past 30 days (examinees one year or older, Day 1 file only)

All NHANES examinees are eligible for the dietary interview component. However, only examinees one year or older are eligible for the frequency of fish and shellfish consumption questions following the 24-hour recall.

The examination protocol and data collection methods are fully documented in the NHANES dietary interviewers procedures manuals (In-person interview: https://wwwn.cdc.gov/nchs/data/nhanes/2007-2008/manuals/manual_dietarymec.pdf; phone follow-up interview: https://wwwn.cdc.gov/nchs/data/nhanes/2007-2008/manuals/manual_dietarypfu.pdf).

Interviews were conducted for survey participants less than six years of age with a proxy who was generally the person most knowledgeable about the survey participant’s intake. With children ages 6 to 11, the interviews were conducted with the child and the assistance of an adult familiar with the child’s intake. Dietary interviews were conducted in English and Spanish. Translators were used to conduct interviews in other languages.

The in-person interview was conducted in a private room in the NHANES mobile examination center (MEC). A set of measuring guides (various glasses, bowls, mugs, drink boxes and bottles, household spoons, measuring cups and spoons, a ruler, thickness sticks, bean bags, and circles) was available in the MEC dietary interview room for the participant to use for reporting amounts of foods (NHANES Measuring Guides for the Dietary Recall Interview: https://www.cdc.gov/nchs/nhanes/measuring_guides_dri/measuringguides.htm). Upon completion of the in-person interview, participants were given measuring cups, spoons, a ruler, and a food model booklet, which contained two-dimensional drawings of the various measuring guides available in the MEC, to use for reporting food amounts during the telephone interview. Telephone dietary interviews were collected 3 to 10 days following the MEC dietary interview but not on the same day of the week as the MEC interview. Any participant who did not have a telephone was given a toll-free number to call so that the recall could be conducted.

What We Eat in America data were collected using USDA's dietary data collection instrument, (the Automated Multiple Pass Method (AMPM) (http://www.ars.usda.gov/ba/bhnrc/fsrg). The AMPM was designed to provide an efficient and accurate means of collecting intakes for large-scale national surveys. The AMPM is a fully computerized recall method that uses a 5-step interview outlined below:

1. Quick List - Participant recalls all foods and beverages consumed the day before the interview (midnight to midnight).
2. Forgotten Foods - Participant is asked about consumption of foods commonly forgotten during the Quick List step.
3. Time and Occasion - Time and eating occasion are collected for each food.
4. Detail Cycle - For each food, a detailed description, amount eaten, and additions to the food are collected. Eating occasions and times between eating occasions are reviewed to elicit forgotten foods.
5. Final Probe - Additional foods not remembered earlier are collected.

The AMPM includes an extensive compilation of standardized food-specific questions and possible response options. Routing of questions is based on previous responses. The AMPM is updated yearly to reflect the changing food supply and to address research needs from the data user community. Additional information about the AMPM is provided in Raper et al. (Raper et al., 2004).

The AMPM was validated in a large study and shown to be an effective method for collecting accurate group energy intake of adults. Completed in 2004, this extensive research project included 524 healthy, weight-stable volunteers, aged 30-69 years. The accuracy of the AMPM was evaluated by comparing reported energy intake (EI) to total energy expenditure (TEE) using the doubly labeled water technique (Moshfegh et al., 2008). Among the findings were that EI compared to TEE was under-reported by 11% overall, by less than 3% for normal weight subjects with body mass index (BMI) <25 and 16% for overweight subjects with BMI ≥ 25.

Additional studies provide evidence that the AMPM accurately measures group energy intake. Blanton (Blanton et al., 2006) reported that EI was not significantly different from TEE for a sample of 20 adult females. Rumpler and colleagues found that mean EIs were accurately reported for a sample of 12 adult males (Rumpler et al., 2008).

For additional information about the dietary interview component and related survey protocols, please go to the Survey Operations Manual, Consent Documents, Brochures site at:
https://wwwn.cdc.gov/nchs/nhanes/continuousnhanes/default.aspx?BeginYear=2007

All dietary interviewers were required to complete an intensive one-week training course and to conduct supervised practice interviews before working independently in the field. Retraining sessions were conducted annually to reinforce the proper protocols and technique.

Interviewers were monitored throughout the data collection period. Monitoring consisted of the following:

• Data transmittal sheets were reviewed to verify receipt of data files.
• Reviews of audio-taped interviews or in-person observations were conducted for approximately 5% of each interviewer's work.
• Interviews were checked for completeness of the recalls, missing information, inconsistent reports, and unclear notes. Written notification and feedback were provided to the interviewers.

Interview data files were sent electronically from the field and were imported into Survey Net, a computer-assisted food coding and data management system developed by USDA (Raper et al., 2004).

USDA's Food and Nutrient Database for Dietary Studies, 4.1 (FNDDS 4.1), was used for processing the 2007-2008 intakes (http://www.ars.usda.gov/ba/bhnrc/fsrg). The FNDDS includes comprehensive information that can be used to code individual foods and portion sizes reported by participants and also includes nutrient values for calculating nutrient intakes. The underlying nutrient values for FNDDS 4.1 were based on values in the USDA National Nutrient Database for Standard Reference, release 22, produced by USDA’s Nutrient Data Lab (http://www.ars.usda.gov/nutrientdata). FNDDS values are updated for every 2-year WWEIA, NHANES release cycle. FNDDS 4.1 corresponds with WWEIA 2007-2008. Additional information about the FNDDS and related tools is available on the Food Surveys Research Group website (Raper et al., 2004; Anand et al., 2006; Bodner-Montville et al, 2006; Ahuja et al., 2008).

Coders were required to pass a certification test after the initial training. They were routinely monitored to ensure the quality and completeness of their work. Approximately 10 percent of the coder's work was randomly selected to be independently coded by another coder. Results from the two codings were compared and adjudicated, if necessary.

After intake data were coded, various types of reviews were conducted to ensure the quality of the data. An overview of quality assurance procedures conducted during the data processing stage is available in Anand, et al. (Anand et al., 2006). Examples of reviews include the following:

• Overall acceptability of each recall. This review determined if the recall met the two minimum criteria listed below. A recall was considered unacceptable if it failed to meet either of these criteria:
  1. The first 4 steps of the 5-step AMPM are completed. Failure to meet this criterion occurs infrequently and is usually due to the participant stopping the interview before completion of the fourth step. This step collects the details (description and amount consumed) for each reported food.
  2. Foods consumed for each reported meal must be identified.
• Interviewers' and coders' questions and comments are reviewed to ensure that they have been accounted for in coding.
• Decisions are made about how to code new or unusual foods or food quantities reported by participants. Foods or portions that could not be matched to items in the database are resolved by FSRG scientists. New food items and new portion sizes are added as needed to the FNDDS. Information about new foods and package sizes are collected using internet resources, direct contact with manufacturers, or food labels. Unusual food mixtures are coded using multiple food codes to represent the mixture, linking those foods with a combination food number, and specifying the type of food mixture (such as a salad or soup).
• Specific data integrity checks for reasonableness, consistency, and logic. Many quality control features are built into the AMPM software, including automated routing based on the participant's previous answers and extensive checks which prevent illogical responses. Nevertheless, over 50 unique checks are conducted across all dietary data. Examples are meals reported at unusual times (e.g., school lunch at 12:30 a.m.), foods not usually consumed by certain age groups (e.g., baby foods consumed by respondents over 2 years of age), and extremely large quantities of foods.
• Intakes with extreme levels for individual nutrients. Nutrient intakes are reviewed separately for various age and sex groups. Unusual values are examined and corrected when necessary.

During data processing, the following edits were made to ensure the logical consistency and analytic usefulness of the data:

• Adjusted sodium values for certain foods. Sodium values for home-prepared foods are based on the sodium values of recipe ingredients in the FNDDS. In some cases, the amount of salt in recipes was reduced or eliminated if the participant answered dietary interview questions about salt use in cooking or preparing foods with the response "occasionally", "rarely" or "never", respectively.
• Modified nutrient values for some food mixtures. During the food coding process, predefined recipes for some food mixtures are modified to match more closely the food as described by the respondent. Nutrients are modified by substituting ingredients in a predefined recipe for the mixture. An example of a modified recipe is an egg fried in butter instead of margarine. Each modification is assigned a unique 6-digit identification number. Recipe modification numbers appear in the variable DR1MC in the DR1IFF_E file and in the variable DR2MC in the DR2IFF_E file. Descriptions for each modification are provided in a separate file called DRXMCD.

Each Individual Foods file (Day 1 and Day 2) is comprised of food records. For most survey participants, there are multiple records in each file. For each Total Nutrient Intakes file (Day 1 and Day 2) there is one record for each participant. These files can be linked with other NHANES files by the respondent sequence number (SEQN).

Variable names: For data collected on both Day 1 and Day 2, variable names are differentiated by having the number "1" or "2" in the third position of the variable name to identify the collection day. For example, the USDA food code variable (in the Individual Foods File), which identifies the food reported by the participant, is named DR1IFDCD in the Day 1 file and DR2IFDCD in the Day 2 file. Appendices 2 and 5 list the Day 1 and Day 2 variable names for the Individual Foods file and the Total Nutrient Intakes file, respectively.

Names for the following variables are the same for both days in the Individual Foods file and the Total Nutrient Intakes file: 

Number of days of intake: Because two days of data are included in the 2007-2008 release, a variable has been included to indicate the number of days of intake available for each participant. The variable name is DRDINT. For 2007-2008, 9,255 respondents provided complete dietary intakes for Day 1 and of those providing the Day 1 data, 7,838 provided complete dietary intakes for Day 2.

Dietary recall status code: A status code (DR1DRSTZ or DR2DRSTZ) is used in both the Individual Foods and Total Nutrient Intake files to indicate the quality and completeness of a survey participant's response to the dietary recall section. The codes are the following: 

1 = Reliable and met the minimum criteria

For individuals with a code 1, all relevant variables associated with the 24-hour dietary recall contain a value.

2 = Not reliable or did not meet the minimum criteria

Individuals with a code 2 have incomplete records. No data on total nutrient intakes and the total number of foods reported are provided for these cases. These individuals have no records in the Individual Foods files.

3 [Code 3 is not included in the current datasets. It was only used for data from the 1999-2000 survey cycle.]

4 = Reported consuming breast milk

For infants and children who consumed human milk, there is a record in the Individual Foods files for each report of human milk. However, because amounts of human milk intake are not quantified, these records contain missing values for the amount consumed and for the amounts of energy and nutrients from human milk. Also, records of human milk have a missing value for the food source variable (DR1FS, DR2FS) and the eaten at home variable (DR1_040Z, DR2_040Z) in the Individual Foods files. Records for any other foods and beverages consumed by breast-fed infants and children are included in the Individual Foods files along with their amounts and nutrient information. Because of the missing nutrient information for human milk, no total nutrient intakes (contained in the Total Nutrient Intakes files) were computed for participants with a code 4.

A variable that identifies breast-fed children, DRABF, is included. This variable has a code of 1 if a child consumed breast milk in either intake day.

5 = Not done

This code is assigned when the dietary recall section of the interview did not take place due to various reasons (such as arrived late/left early, refusal, illness, emergency, or equipment failure). These individuals have no records in the Individual Foods files. These individuals have a record in the Total Nutrients file with values only for the following variables: the respondent sequence number (SEQN), the dietary recall status code (DR1DRSTZ or DR2DRSTZ) and for participants one year or older, the fish and shellfish questions in the DR1TOT_E file (DRD340, DRD350A-K, DRD350AQ-JQ, DRD360, DRD370A-V, and DRD370AQ-UQ).

Although there are four possible values, only codes 1 and 4 appear in the Individual Foods file. In addition to the status code described above, the variable DR1_300 and DR2_300 in the Total Nutrients file, denotes the participant's assessment of whether the amount of food he/she consumed on the recall day was usual, much more than usual, or much less than usual.

Participants who reported consuming only water, no food or other beverages: Beginning in 2005-2006, information on all waters were collected during the 24-hour recall and reported in the Individual Foods file, including tap water and bottled water. Therefore, records are now included in the Individual Foods file for participants who consumed only water. There are 5 such individuals in the 2007-2008 datasets, all in the Day 2 data. Their dietary recall status variable for the day is coded as "1" (complete and reliable) in the Total Nutrients file and the total number of foods is coded based on how many times water was reported. There are 5 nutrients for the tap water and plain bottled water codes that have values greater than zero - calcium, magnesium, sodium, zinc, and copper. Individuals with just water intake and no food intake will have zero energy intake for the day. Depending on the type of analysis, these individuals may need to be excluded from the analysis.

Participants who reported consuming no water, food or other beverages: In addition to the survey respondents consuming just water, there are 2 intakes (1 intake in Day 1 and 1 intake in Day 2) with no water, food or other beverage records for the day. These records are not included in the Individual Foods File for these intakes. However, their dietary recall status is coded as complete and reliable and their records will have zero values for all nutrients in the Total Nutrients file.

Number of days between the intake day and the day of family interview: A variable has been added in each of the four files (DR1DBIH for day 1 files and DR2DBIH for day 2 files) to indicate the number of days between the intake day and the day that the family questionnaire was administered in the household. A positive value in DR1BHIH or DR2BHIH indicates the family interview occurred prior to the day the intake was recalled for. In the survey, most of the family interviews were done before the participant came to the MEC and received the dietary interview. A value of “0” in DR1BHIH or DR2BHIH indicates the family interview occurred on the same date the intake was recalled for. A negative value (i.e., DR1BHIH<0 or DR2BHIH<0) means that the family interview occurred after the day the intake recalled for.

Food source: The source of each food (where it was obtained, e.g., from a store, fast food restaurant, cafeteria) is included with the 2007-2008 release as it was for 2005-2006. The variable names are DR1FS and DR2FS and are located in the Individual Foods file. The code descriptions for this variable are:

Eating occasion: The variables DR1_030Z and DR2_030Z are located in the Individual Foods file. The code descriptions for the eating occasion variables are shown in the table below. 

Eating occasion is designated by the respondent. During the interview, a list of eating occasion names is available to the respondent for reference. However, eating occasion names are not defined for the respondent and the interpretation may differ from one person to another.

Combination foods: Foods that are eaten in combination with other foods, such as cereal with milk, are identified by a combination food type (DR1CCMTX) and a combination food number (DR1CCMNM). They are located in the Individual Foods file. About half of the foods reported in 2007-2008 were consumed in combination with other foods. The code descriptions for this variable are the following:

Special diet: Information on whether the participant is currently on any kind of diet to lose weight or for other health-related reason and, if so, the type of diet, is included. The variable DRQSDIET identifies whether a participant is on a special diet. The variables DRQSDT1 through DRQSDT10 and DRQSDT91 identify the type of diet(s) that the participant is following. These variables can be found in the Total Nutrient Intakes file.

Note: A participant could report more than one type of diet, and all the responses were recorded. In 2003-2004 and 2005-2006, the variable DRQSDT1 denotes the type of diet the participant followed specifically for weight loss purposes, including a variety of low calorie diets, low carbohydrate diets, and/or high protein diets. In 2007-2008, 2 new variables have been added to allow participants to report "low carbohydrate diet (DRQSDT9)" and "high protein diet (DRQSDT10)" individually. The variable "DRQSDT1" is now used to denote a weight loss or low calorie diet.

Sample weights for dietary intake data: The NHANES participants were selected on the basis of a national probability design. In order to increase the number of participants for specific demographic groups, a multi-stage, unequal probability of selection design was implemented. All Hispanics were oversampled, not just Mexican Americans. In addition, for each of the race/ethnicity domains, the 12-15 and 16-19 year age domains were combined and the 40-59 year age minority domains were split into 10 year age domains 40-49 and 50-59. This has led to an increase in the number of participants aged 40+ and a decrease in 12-19 year olds from previous cycles. Lastly, pregnant women were no longer oversampled.

Sample weights are constructed that encompass the unequal probabilities of selection, as well as adjustments for non-participation by selected sample persons. In order to produce national, representative estimates, the appropriate sample weights must be used.

For the 2007-2008 NHANES, there were 12,943 persons selected; of these 9,762 were considered respondents to the MEC examination and data collection. However, 9,255 of the MEC respondents provided complete dietary intakes for Day 1. Furthermore, of those providing the Day 1 data, 7,838 provided complete dietary intakes for Day 2.

Most analyses of NHANES data use data collected in the MEC and the variable WTMEC2YR should be used for the sample weights. However, for the WWEIA dietary data, different sample weights are recommended for analysis. Although attempts are made to schedule MEC exams uniformly throughout the week, proportionally more exams occur on weekend days than on weekdays. Because food intake can vary by day of the week, use of the MEC weights would disproportionately represent intakes on weekends.

A set of weights WTDRD1 is provided that should be used when an analysis uses the Day 1 dietary recall data (either alone or when Day 1 nutrient data are used in conjunction with MEC data). The set of weights WTDRD1 is applicable to the 9,255 respondents with Day 1 data. Day 1 weights were constructed by taking the MEC sample weights (WTMEC2YR) and further adjusting for (a) the additional non-response and (b) the differential allocation by day of the week for the dietary intake data collection. These Day 1 weights are more variable than the MEC weights, and the sample size is smaller, so estimated standard errors using Day 1 data and Day 1 weights are larger than standard errors for similar estimates based on MEC weights.

When analysis is based on both days of dietary intake, only 7,838 sample persons have valid data. The NHANES protocol requires an attempt to collect the second day of dietary data at least 3 days after the first day, but the actual number of days between the two interviews is variable. A set of adjusted weights, WTDR2D, is to be used only when analysis uses both Day 1 and Day 2 dietary data. This two-day weight was constructed for the 7,838 respondents by taking the Day 1 weights (WTDRD1) and further adjusting for (a) the additional non-response for the second recall and (b) for the proportion of weekend-weekday combinations of Day 1 and Day 2 recalls.

Note that all sample weights are person-level weights and each set of dietary weights should sum to the same population control total as the MEC weights (WTMEC2YR). In addition, the MEC weights (WTMEC2YR) are appropriate for use in the analysis of the fish and shellfish consumption data (i.e., variables DRD340, DRD350A-K, DRD350AQ-JQ DRD360, DRD370A-V, and DRD370AQ-UQ) located in the Day 1 Total Nutrient Intake File (DR1TOT_E), if no other dietary data are included in the analysis. Additional explanation of sample weights and appropriate uses are included in the NHANES Analytic Guidelines (https://wwwn.cdc.gov/nchs/nhanes/analyticguidelines.aspx). Please also refer to the on-line NHANES Tutorial (https://www.cdc.gov/nchs/tutorials/) for further details on other analytic issues.