Dietary Assessment Primer

Glossary of Key Terms

A

Accuracy

The degree of closeness of measurements of a quantity to that quantity's true value. See also: Precision.

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Association

A dependent relationship between two or more variables. The relationship does not establish causality. See also: Correlation.

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Attenuation

Underestimation of the true correlation or association between two different measures because of measurement error. See also: Deattenuation.

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Attrition

The loss of participants during a study.

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Automated Self-Administered 24-Hour Dietary Assessment Tool (ASA24)

A freely available web-based tool developed by the National Cancer Institute (NCI) that enables multiple automated self-administered 24-hour dietary recalls.

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B

Between-person variation

Differences between individuals for a given variable of interest.

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Bias (also known as systematic error)

A source of measurement error in which measurements consistently depart from the true value in the same direction. Bias affects the sample mean as well as percentiles and can result in incorrect estimates and conclusions. See also: Intake-related bias, Person-specific bias.

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Biomarker

A measurable substance in an organism whose presence is indicative of some phenomenon such as disease, infection, or environmental exposure (such as diet). See also: Concentration biomarker, Predictive biomarker, Recovery biomarker.

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Body mass index (BMI)

A measure of relative weight, calculated as mass (kg)/height (m)2. Standards for defining underweight, normal weight, overweight, and obesity are often defined in terms of BMI cut-points.

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Box-Cox transformation

A method for determining the best power transformation of data from non-normally distributed to approximately normally distributed. See also: Normal (Gaussian) distribution.

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C

Calibration

The process of using equations to re-scale data obtained from a more biased, less accurate instrument based on information obtained from a less biased, more accurate instrument.

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Calibration study

A study that collects data from a less biased, more accurate instrument to calibrate a more biased, less accurate instrument. These studies can be conducted using data from a subset of participants within a study or from an external study. See also: Internal calibration, External calibration, Reference instrument, Regression calibration.

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Case-control study (also known as a retrospective study)

A study that compares two groups of people: those with the disease or condition under study (cases) and a very similar group of people who do not have the disease or condition (controls).

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Cohort study

A study in which exposures of interest are assessed at baseline in a group (cohort) of people and health outcomes occurring over time are then related to baseline exposures.

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Combining instruments

The analytical strategy of using information from two or more dietary assessment instruments to estimate intake.

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Comparative validation (also known as relative validation)

A type of study that compares two or more imperfect instruments, for example, food frequency questionnaire performance relative to other self-report instruments, such as 24-hour dietary recalls and food records.

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Concentration biomarker

A type of biomarker that reflects the concentration of a specific chemical or compound in blood, urine, or tissues that is subject to substantial inter-individual differences in metabolism; related to and can be used as an indirect measure of dietary intake. See also: Predictive biomarker, Recovery biomarker.

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Correlation

A measure of linear association between two variables. See also: Association.

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Covariate

A variable that is possibly predictive of the outcome under study. A covariate may be of direct interest or it may be a confounder or effect modifier. See also: Nuisance effects.

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Cross-sectional study

A study that includes measurements on a group of individuals at a single interval in time.

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D

Day-of-week effect

A phenomenon indicating how overall mean intake varies according to the day of the week. See also: Nuisance effects.

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Day-to-day variation

Fluctuations that occur in dietary intake from one day to the next.

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Deattenuation

A statistical procedure to correct for the weakening effect of measurement error. See also: Attenuation.

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Dependent variable

The response or outcome variable that is hypothesized to be affected by the independent or exposure variable. See also: Independent variable.

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Dietary supplements

Vitamins, minerals, herbs or other botanicals, amino acids, and other substances taken orally, which are intended to supplement the diet. See also: Total nutrient intake.

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Diet history

A dietary assessment instrument that collects detailed information about typical intakes of foods and beverages consumed by meal; information includes usual cooking and preparation practices.

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Differential error (also known as differential response bias)

Reporting error that is different in nature or magnitude between two groups. This type of error is of particular concern in intervention studies. See also: Intervention study.

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Doubly labeled water (DLW)

Water with identifiable isotopes of the hydrogen (2H) and oxygen (18O) that can be used as a recovery biomarker for measuring energy expenditure. DLW is assumed to be equal to energy intake in weight-stable individuals.

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E

Energy adjustment

Adjustment of nutrient and food group intake for total energy intake. See also: Nutrient density.

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Episodically consumed dietary components

Nutrients and foods that are not consumed on a daily basis by nearly everyone in the population, and whose intake may therefore commonly be reported as zero on a particular day. See also: Non-episodically consumed dietary components.

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Exposure

The explanatory variable; also referred to as the independent variable in a regression model, it is often a behavior such as diet. See also: Dependent variable, Outcome.

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External calibration

Calibration performed using data from an external study (that is, a study that is conducted with participants who are not in the main study). See also: Calibration study, Internal calibration.

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Extreme values (also known as outliers)

Values in a dataset that are implausible or so high or low as to have undue influential effects on findings.

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F

Feeding study

A study in which all foods and drinks available to the participants during the study period are provided, measured, and controlled by the investigators, and the amounts consumed are recorded by observers.

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Flattened slope

A phenomenon in which participants with a high level of intake tend to underreport their intake and participants with a low level of intake tend to overreport their intake. See also: Intake-related bias.

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Food frequency questionnaire (FFQ)

A dietary assessment instrument that asks respondents to report their usual frequency of consumption of each food in a list of foods over a specific period of time. See also: Long-term instrument.

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Food propensity questionnaire

A term sometimes used to define a food frequency questionnaire that does not include portion size.

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Food record

A dietary assessment instrument in which a respondent records the types and amounts of all foods and beverages consumed over one or more days; also called a food diary. See also: Short-term instrument.

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G

Generic memory

General knowledge or information that can be recalled. See also: Specific memory.

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I

Imputation

The process of replacing missing data with a probable value based on other available information.

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Independent variable

The exposure or other variable that is hypothesized to effect the dependent or outcome variable. See also: Dependent variable.

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A type of systematic error in which deviation from true intake arising from a correlation between error and true intake. See also: Flattened slope.

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Internal calibration

Calibration performed using data from an internal study. That is, a study that is conducted with participants who are in the main study. See also: External calibration, Calibration study.

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Intervention study

A study in which the researcher tests the effectiveness of a condition with a defined timing, dose or intensity assigned to an experimental group in comparison to the control group (i.e., the group that is not assigned the condition). For example, in dietary research, the condition can be a supplement or a program to affect dietary behaviors or intakes.

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L

Long-term diet

Diet over a long period of time; for example, over a year or a month rather than a day.

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Long-term instrument

A dietary instrument that captures intake over a long period of time, such as a food frequency questionnaire or screener. See also: Short-term instrument.

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M

Main dietary assessment instrument

The primary dietary assessment instrument used in a study, sometimes referred to as the study instrument; may be calibrated or validated using a reference instrument. See also: Calibration study, Validation study.

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Mean

An indicator of central tendency, derived as the average of a set of values (the sum of the values divided by the number of values in the set).

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Measurement error

The difference between the observed or measured value and the true value. See also: Random error, Systematic error.

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Median

An indicator of central tendency, it is the numerical value separating the higher half of a data distribution from the lower half.

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Misreporting

Reporting on a dietary assessment instrument that does not match true intake.

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Mode of administration

The method of administration of an instrument (e.g., telephone, in-person, self-administered). See also: Nuisance effects.

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Mode

An indicator of central tendency, it is the most common value in a data distribution.

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N

National Cancer Institute (NCI) method

A unified approach for estimating usual dietary intake distribution from 24-hour dietary recalls and predicting individual intakes for use in diet and health models; can be used for dietary components consumed nearly daily by nearly all persons as well as those consumed episodically.

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National Health and Nutrition Examination Survey (NHANES)

A program of studies designed to assess the health and nutritional status of adults and children in the United States. The survey is unique in that it combines interviews and physical examinations.

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n-day

The number of days in a particular administration of a dietary assessment instrument.

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Nested case-control analytic design

An analysis strategy in which cases of a disease that occur in a defined cohort are identified and, for each, a specified number of matched controls who have not developed the disease are selected from among the same cohort.

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Non-episodically consumed dietary components

A term describing nutrients and foods that are consumed on a daily basis by nearly everyone in the population, and whose intake may therefore rarely, if ever, be reported as zero on a particular day. See also: Episodically consumed dietary components.

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Normal (Gaussian) distribution

A probability distribution that is symmetrical around the mean and has few values outside of two standard deviations. The density function resembles a bell-shaped curve with only one peak. See also: Skewed distribution.

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Nuisance effects

Variables that have an effect on observations but are of no intrinsic interest themselves. See also: Day-of-week effect, Mode of administration, Season effect, Sequence effect.

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Null hypothesis

A general statement or default position that there is no relationship between two measured phenomena.

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Nutrient density

Nutrient intake (in kilocalories) divided by total energy intake (also in kilocalories), often expressed either as a percentage or as amounts per 1,000 kilocalories.

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O

Observational study

A type of study in which individuals are observed and variables of interest are measured. No attempt is made to affect the outcome.

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Outcome

The target variable; also referred to as the dependent variable in a regression model; often a health outcome, such as the occurrence of a specified disease. See also: Exposure, Independent variable.

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Outliers (also known as extreme values)

Values in a dataset that are implausible or so high or low as to have undue influential effects on findings.

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P

Parametric statistics

Statistics that assume that the data are normally distributed; in contrast to non-parametric statistics, which assume no particular distribution.

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Person-specific bias

The portion of total systematic error that is not intake-related bias. It may be related to individual characteristics, such as social/cultural desirability, that affect how a particular individual reports dietary intakes. See also: Within-person random error.

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Person-specific random effect

A term specific to an individual that describes how that individual's value deviates from the average value. Both parts of the two-part National Cancer Institute (NCI) method model include a person-specific random effect.

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Power

In statistics, the probability that a test correctly rejects the null hypothesis when the alternative hypothesis is true.

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Precision

The degree to which repeated measurements under unchanged conditions show the same results. See also: Accuracy.

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Predictive biomarker

A type of biomarker whose relationship with intake is much stronger, relatively stable, time-related, and sensitive to intake in a dose-response manner than for a concentration biomarker. Currently, the only known predictive biomarker is a urinary sugar biomarker. See also: Recovery biomarker.

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Prospective study (also known as prospective cohort study)

A study that assesses exposures of interest for a group of individuals at baseline, follows them over time, and compares them for a particular outcome.

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R

Random error

A type of measurement error that contributes variability (reduces precision) but does not influence the sample mean or median. See also: Systematic error.

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Ratio

A relationship between two numbers that defines the amount of one measure relative to another measure.

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Reactivity

A change in behavior due to awareness that behavior is being or will be measured. If attempting to measure typical diet, reactivity bias results.

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Recovery biomarker

A type of biomarker that is directly related to intake and not subject to homeostasis or substantial inter-individual differences in metabolism; for example, doubly labeled water for energy intake and urinary nitrogen for protein intake. See also: Concentration biomarker, Predictive biomarker.

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Reference instrument

An instrument that is administered in a sub-study and is used to calibrate or validate the main dietary instrument; examples include recovery biomarkers. The reference instrument is assumed to provide estimates that are closer to truth than the main instrument. See also: Calibration study, Validation study.

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Reference period

The time period to which a dietary assessment instrument pertains, such as 24 hours for a 24-hour dietary recall, some number of days for a food record, or the period the participant is asked to recall (usually 30 days to 1 year) for a food frequency questionnaire. See also: Long-term instrument, Short-term instrument.

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Regression calibration

A statistical method for correcting estimated regression coefficients for bias due to measurement error in one or more continuous covariates. In diet it is used to adjust relative risk estimates for measurement error in studies of the association of diet and health outcomes. See also: Bias, Calibration study.

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Relative validation (also known as comparative validation)

A type of study that compares two or more imperfect instruments, for example, food frequency questionnaire performance relative to other self-report instruments, such as 24-hour dietary recalls and food records.

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Residual confounding

Occurs when a confounder has not been adequately adjusted for in analyses.

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Response bias

A deviation from an accurate report that is related to a variety of factors that affect an individual as they respond to a verbal or written question.

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Retrospective study (also known as case-control study)

A study that compares two groups of people: those with the disease or condition under study (cases) and a very similar group of people who do not have the disease or condition (controls).

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S

Scoring algorithm

In reference to dietary screeners, an analytical procedure used to convert responses to estimates of dietary intake for food groups of interest, such as fruits and vegetables or added sugars.

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Screener

In dietary assessment, an instrument that asks a limited number of questions in order to estimate intake of one or a few food components. See also: Long-term instrument.

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Season effect

Variation in diet due to differing seasonal consumption patterns. See also: Nuisance effects.

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Self-reported intake

Intake as reported by the individual who actually consumed the dietary component; can be measured using various dietary assessment instruments or methods. See also: True intake.

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Sequence effect

The effect of repeated administration of an instrument on reported intakes. See also: Nuisance effects.

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Short-term instrument

A dietary assessment instrument that captures intake over a short period of time, such as a 24-hour dietary recall or food record. See also: Long-term instrument.

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Skewed distribution

A distribution that is not symmetrical. See also: Normal distribution.

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Specific memory

Memory of details of a specific behavior. In diet it is used to describe the type of memory required to remember types and amounts of foods and drinks consumed when reporting on a 24-hour dietary recall. See also: Generic memory.

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Surrogate reporter

A person who responds to survey questions when the study participant (e.g., young child) is not able to respond for him/herself. The surrogate reporter should be knowledgeable about that person's diet.

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Systematic error (also known as bias)

A type of measurement error in which measurements consistently depart from the true value in the same direction; affects the sample mean as well as percentiles and can result in incorrect estimates and conclusions. See also: Intake-related bias, Person-specific bias, Random error.

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T

Total nutrient intake

A term referring to nutrient intake from all sources, including food, beverages, and dietary supplements. See also: Dietary supplements.

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Transformation

The application of a mathematical function (e.g., the logarithm or the square root) to a set of values to create a new set of values usually performed in order to obtain normal distributions. See also: Box-Cox transformation.

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True intake

Actual intake, which usually cannot be measured among free-living individuals. See also: Self-reported intake.

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Twenty-four-hour dietary recall (24HR)

A dietary instrument that asks the respondent to remember and report all foods and beverages consumed in the preceding 24 hours or during the preceding day. See also: Automated Self-Administered 24-Hour Dietary Assessment Tool (ASA24), Short-term instrument.

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U

Unbiased instrument

An instrument that collects data containing only random error.

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Underreporting

A type of misreporting in which an individual reports less than their true intake. See also: Recovery biomarker, Doubly labeled water.

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Usual dietary intake

Long-term average daily intake. See also: National Cancer Institute method.

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Usual dietary intake distribution

A distribution that describes usual intakes, including means and percentiles, among a population.

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V

Validity

The degree to which a tool measures what it claims to measure. See also: Validation study.

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Validation study

A study conducted to compare a dietary assessment instrument to a reference instrument that more accurately measures true intake. See also: Calibration study.

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W

What We Eat in America (WWEIA)

The dietary intake interview component of the National Health and Nutrition Examination Survey (NHANES).

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Within-person random error

The difference between assessments of a variable or variables collected from one administration of an instrument compared to a long-term average based on multiple administrations of an instrument.

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Within-person variation

Fluctuations that occur between dietary intakes from one administration of a dietary instrument to the next. See also: Within-person random error.

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