About the Literature Review: Methodology
Overview
Measurement of Validity
Definitions and Abbreviations
Appendix to Methodology
Overview
This review examines studies conducted mainly in industrialized, developed
countries and published in English between 1982 and December 2003 for
pregnant or lactating women, infants, toddlers, and preschoolers. For
school age children and adolescents, our search built on a recent review
of the literature through 2000 (1) and focused primarily on literature
published since that time. Studies were identified in National Library
of Medicine PubMed database and the CABI Publishing Nutrition Abstracts
and Reviews database using a number of comprehensive search strings containing
relevant key words. A number of key word
search strings were developed to search the databases for relevant
documents.
In addition to the key word search strings, author searches were conducted
in both databases to identify relevant publications by key investigators
with expertise in maternal and child nutrition. These searches were supplemented
by cross-referencing from reference lists from reviewed literature. A
Reference Manager Database was developed
to manage the search and review process. In addition, specific criteria
were used to determine whether to include a citation in the literature
review, and a careful process for reviewing
each article was followed.
Measurement of Validity
Validity is the ability of an instrument to measure what it is intended
to measure. Most dietary assessment methods are intended to measure usual
or customary food and/or supplement intake over a defined period of time.
Because true usual diet is difficult if not impossible to measure, investigators
assess relative or criterion validity. Relative validity compares a new
measurement method with one or more established methods believed to have
a greater degree of demonstrated or face validity (2).
The measurement error in the new instrument or method is examined and
calibrated with the reference method. This type of validity assessment
will fail to detect systematic reporting error or bias if both the new
and reference method have correlated error (3).
Alternatively, the new method or instrument can be validated against an
independent, external criterion reference measurement, such as a biomarker
of intake. The doubly labeled water (DLW)
method to measure total energy expenditure is an example of a biomarker
that can be used to independently validate self-reported energy intake.
The error in the DLW method is independent of self-reported intake error,
thus allowing true reporting bias to be detected (3,4).
This review examines studies in which the relative or criterion validity of one diet or supplement assessment method
is evaluated by comparisons with measurements obtained from a reference method. The reference methods vary by study
and target age group and include established dietary assessment methods, biological markers of habitual intake, and
techniques such as direct observation of intake.
Definitions and Abbreviations
Definitions of and abbreviations for dietary assessment methods and reference
methods discussed in this report.(5,6)
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Food records are used to record food intake at the time of
consumption, over a number of days that are not necessarily sequential.
Most studies ask respondents to enter the information on hard copy form,
although tape-recording, bar-coding, and electronic weighing also have been
used to collect descriptive and quantity information.
Weighed FR: The respondent weighs on a small scale all food and beverages
consumed.
Estimated FR: The respondent estimates all food consumed using household
measures or portion size estimating aides.
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Diet History questionnaires are a retrospective assessment
method ascertaining a respondent's "usual" food intake by collecting
descriptive detail and amount information about each food. DHs may include
questions on meal patterns, lists of common foods and groups of generic
food. DH questionnaires are typically administered by a trained interviewer
either in-person or by telephone, but also can be self-reported. |
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The 24HR is a retrospective assessment method in which an
interviewer prompts a respondent to recall and describe all foods and beverages
consumed in the preceding 24 hours or the preceding day. The interview may
be conducted in-person or by telephone and may be paper and pencil or computer
assisted. Portion size estimating aides assist the respondent to recall
amounts consumed. The methodology for conducting the 24HR has evolved during
the last two decades. Among the methods reported are: 3-pass method, 5-pass
method, U.S. Department of Agriculture protocol, University of Minnesota
protocol, Bogalusa Heart Study protocol. |
Food Frequency Questionnaire (FFQ) |
The food frequency questionnaire is a retrospective method
asking respondents to report their usual frequency of consumption of each
food from a list of foods for a specific period (several months or a year).
Food lists vary by the purpose of the study and study population. Frequency
of consumption categories also vary by questionnaire but usually include
per day, week, or month.
Semi quantitative FFQ: In this type of FFQ, portion size information
is collected; portion sizes are specified as standardized portions or choice
(range of portions).
Non-quantitative FFQ: Portion size information not collected.
NCI Health Habits and History Questionnaire (HHHQ): Semi-quantitative
FFQ developed at the National Cancer Institute under the direction of Gladys
Block.
Harvard FFQ (HFFQ): FFQ developed at Harvard University by Walter
Willett and colleagues. Portion size information is included as part of
the food item rather than as a separate listing.
NCI Diet History Questionnaire (DHQ): Semi-quantitative FFQ, using
an embedded question approach, developed at the NCI under the direction
of Amy Subar and Fran Thompson (7,8).
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Comprehensive FFQ-type questionnaire designed to supplement
other dietary assessment method. Information on portion size information
is not collected. May provide information on infrequently consumed foods
(9). |
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Intakes are watched and recorded by trained observers. |
Doubly Labeled Water Method (DLW) for
total energy expenditure (TEE) |
The DLW method is used to measure energy expenditure in free-living
subjects. This method involves the administration of water containing enriched
quantities of the stable isotopes deuterium (2H) and oxygen-18
(18O). The label of "doubly" labeled comes from the
fact that both the hydrogen and oxygen are labeled. The oxygen-18 is eliminated
from the body in the form of carbon dioxide (C18O2)
and water (H218O), and the deuterium is eliminated
in water (2H2O). The difference in elimination rate
between these two isotopes is a measure of carbon dioxide production. Carbon
dioxide production can then be used to calculate energy expenditure by use
of standard equations for indirect calorimetry (3).
The DLW method has been shown to be accurate to 1%, with within-subject
precision of 5 to 8% (10).
Because the method is expensive and analysis requires specialized, expensive
equipment, it cannot be considered routine. However, the method is widely
available and is being applied to dietary assessment validations with sample
sizes ranging from 20 to 500 (10). |
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Dietary assessment method being validated. |
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Method against which the TM is being compared and validated. |
Appendix to Methodology
Review Process
Explains how the Westat master's level registered dietitians carry out
the citation review process.
Citation Relevancy Criteria
The citation review initial screen criteria for title and/or abstract
necessary to order the full text article.
Reference Manager Database
Reference Manager is a commercially available citation management system.
It was used to track all of the references - from identification to article
retrieval to evaluation.
Completed Search Strings
Strings used to search for citations, which are imported into Reference
Manager database.
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