Introduction to Human Nutrition

Measuring Food Intake 267

sures of repeatability for energy intake obtained with
different dietary methods repeated after an interval
of time.
The different measures of repeatability provide
different information. The correlation coeffi cient is
widely quoted but is not a good measure of repeat-
ability since a good correlation may be obtained even
if one set of measurements has been systematically
biased and has a different mean from the other set.
The mean difference is not a good measure of repeat-
ability in individuals since it depends primarily on
whether the differences are random or systematic.
Measures that refl ect the differences between repeated
measurements within individuals are to be preferred.
The coeffi cient of variation of the differences within
individuals and the coeffi cient of repeatability (which
is simply twice the standard deviation of the differ-
ences and represents the 95% confi dence limits of
agreement) give much better measures of their mag-
nitude. They are also more readily interpreted in
practical terms than either a correlation coeffi cient or
the percentage of individuals classifi ed in the same
quintile, quartile, or tertile. If the standard deviation
of the difference within individuals is of the order of
20–30% of mean intake, one is unlikely to describe
the method as precise or repeatable even if the mean
difference at group level is only 1%.

Validity

Demonstrating that a dietary method measures what

it is intended to measure is even more diffi cult than

demonstrating that a method is repeatable, because

in effect it “requires that the truth be known.”

This is almost always impossible unless it is possi-

ble to observe, surreptitiously, what is consumed over

short periods such as 24 hours or at most a few days.

Observation is usually only feasible in institutional

settings or in situations specially set up to allow unob-

trusive observation of what people eat.

For methods that are designed to obtain informa-

tion on habitual longer-term intake, such as the diet

history or food frequency questionnaires, unobtru-

sive observation is impossible. This is a problem that

has been faced by all investigators of dietary assess-

ment methods and until relatively recently was usually

“solved” by assessing one dietary method in relation

to another dietary method, usually a 7 day weighed

dietary record, which was considered to be the best

available or criterion measure. Comparison with

another dietary method provides at best only a rela-

tive form of validity and at worst information that is

unrelated to validity but refl ects either real differences

or similar errors between the methods. For example,

comparison of data from a single 24 hour recall or a

diet history with data from a 7 day weighed record

for the same individuals does not compare the same

information because the time periods are not con-

current. However, because of the lack of a suitable

external standard against which true validity could be

judged before the 1980s it was usually assumed that

most dietary intake data, and weighed records in par-

ticular, provided valid data. Usually, a method was

judged acceptable if the mean intake, as measured by

both methods, did not differ signifi cantly and if cor-

relations for nutrient intake in individuals exceeded

0.5. The magnitude of the coeffi cient of variation

of the differences within individuals was generally

ignored.

Table 10.6 shows data from three studies that

provide additional information on agreement. All

three studies compared data from a food frequency

questionnaire with multiple days of food intake

records. When different methods are compared the

mean differences tend to be higher (there is greater

bias) than those found in repeatability studies.

However, the range of values obtained for other

Table 10.5 Measures of repeatability for energy intake obtained for
a 3 day food record, a dietary history, and a food frequency question-
naire (FFQ)

Measure of
repeatability

3 day food

record

Dietary

history FFQ

Mean
difference (kJ/day)

156 105 954

Mean difference (% of
overall mean intake)

1.6 1.1 12.5

Coeffi cient of variation
of the differences
within individuals
(%)

16.5 18.6 28.5

Coeffi cient of
repeatability (kJ)

±3266 ±1819 ±4294

Correlation coeffi cient – 0.86a 0.70a
Individuals classifi ed in
the same quartile or
tertile* on both
occasions (%)

56 – 60*

a Intraclass correlation.