WEEK8 validity Flashcards
What is validity
The degree to which a dietary survey measures what it intends to measure
i.e. the degree to which the method provides a TRUE and ACCURATE measure of HABITUAL or USUAL food intake
A valid diet record is one in which the subject…
Records exactly what they ate during the period of the study and this is what they would have eaten even if the interviewer had not intervened
Reports usual food intake without conscious or subconsious distortion of their usual food consumption patterns
Precision
(Reproducibility)
the extent to which a method is capable of producing the same result when used repeatedly in the same circumstances
Validity vs precision
Just because the method gives the same result on repeat testing (precise) does not always mean that it is valid (accurate)
Validity is concerned with ______ errors
systematic
Size and direction are unknown but they distort data and may lead to estimates of mean intakes that are either too high or too low
Systematic errors are much more difficult to control than random errors, e.g: subjects may eat atypically during a dietary survey
Precision is concerned with ______ errors
Random
Assessing validity: Absolute
where food intake data are compared with an objective measure which is independent of food intake (food biomarker)
Assessing validity: Relative
Adopted by researchers bc of difficulty measuring absolute validity
Measured by comparing the results of the “test” dietary method against another “reference” method selected because of its presumed validity, e.g:
The 7 day weighed dietary record has been widely used as a reference or “gold standard” because it was thought to be more valid and precise than other methods
Biochemical markers
Absolute validity depends on use of biochemical markers
A biochemical marker is defined as “any biochemical index in an easily accessible biological sample e.g: urine or blood, that givens a predictive response to a given dietary component”
Accuracy needs to be well established before use
Con to biochemical markers
for many dietary factors, biomarkers are either inadequate or unavailable
e.g CHO, fat, PUFA
Assessing absolute validity - Urinary Nitrogen
Principles
Based on the principle of the positive correlation between daily nitrogen (protein) intake and daily nitrogen (protein) excretion when dietary intake is constant
Involves the collection of 24hr urine samples and comparison of the amount of nitrogen excreted with the amount ingested
Allowing for incomplete absorption and losses of nitrogen from the GI tract, hair, skin, sweat the amount of nitrogen excreted should be approx 81% of that ingested
What does urinary nitrogen assess?
Protein intake
How many urine collections are necessary for:
individual validation
group validation
individual = x8
group = x1 per subject
What % of nitrogen ingested is excreted?
Where are losses?
81%
incomplete absorption, losses from GI tract, hair, skin, sweat
Limitations of Urinary Nitrogen
Subjects are assumed to be in nitrogen balance
Need to verify that 24 hr collections are complete
How do you verify that a 24hr urine collection is complete?
PABA (Para-amino benzoic acid) - safe and reliable metabolic marker to validate completeness
take 3x80mg doses per day
85% of PABA should be recovered in urine in 24hr collection
if less - sample incomplete- not suitable for analysis
How do you verify that a 24hr urine collection is complete?
PABA (Para-amino benzoic acid) - safe and reliable metabolic marker to validate completeness
take 3x80mg doses per day
85% of PABA should be recovered in urine in 24hr collection
if less - sample incomplete- not suitable for analysis
Doubly Labelled Water
Estimates TEE (Total Energy Expenditure) usually over 10 days
Subjects drink small dose of water containing naturally occurring stable isotopes - Deuterium & Oxygen - 18
These are unreactive and don’t decay so safe for pregnant women and premature infants
2 isotopes disperse in body, are metabolised and then gradually lost over 10-14 days
Difference between disappearance rates to calculate CO2 production (measured by mass spectrometry) which is used to calculate energy expenditure
Pro of Doubly Labelled Water (DLW)
Minimal demand on subjects = free to follow normal routine = average measure of habitual energy expenditure
Con of Doubly Labelled Water
High cost
Sophisticated & complex analysis
Not suitable for large scale studies
Provides marker for energy (MJ/day) only
Can’t identify which foods are under/over-reported
Principle of energy balance equation
metabolisible energy intake = energy expenditure
+
change in body energy stores (+- body fat/lean tissue)
Principle of energy balance equation
metabolisible energy intake = energy expenditure
+
change in body energy stores (+- body fat/lean tissue)
Assessing relative validity
Compare results of “test” dietary method against another “reference” method that is selected bc of its assumed validity
Test and Reference methods
Good agreement doesn’t necessarily indicate validity
Good agreement may indicate similair errors in both methods
Poor agreement = at least one method is invalid
