266 Introduction to Human Nutrition
defi ciency by adjusting for within-person variation,
they do not enable at-risk individuals to be
identifi ed.
When the purpose of the study is to assess the diet
of specifi c individuals it is necessary to obtain dietary
information over at least a week and preferably longer.
This is best done by obtaining either multiple 24 hour
recalls or 24 hour food records over an extended
period. The minimum number of days needed to
obtain an estimate of nutrient intake with a specifi ed
level of confi dence differs for different nutrients.
Information on energy intake, which tends to show
less day-to-day variation than other nutrients, can be
obtained over a shorter period (days) than informa-
tion on a nutrient for which day-to-day intake is
much more variable, such as vitamin A (weeks).
Precision
In studies of groups, precision is primarily a function
of sample size, while in studies of individuals it is a
function of the number of days of information avail-
able. Precision increases with sample size and with the
number of days for which information is collected,
but so does the cost of the study. Precision therefore
needs to be defi ned in relation to the purpose of the
study.
Usually, what is required of the nutritionist is to be
able to provide the statistician with an estimate of the
level of difference that it is important to be able to
detect (in nutritional, not statistical terms) and an
estimate of the variance or standard deviation for the
measurement(s) in question. For example, when
looking for differences in energy intake between two
groups, would a difference of 500 kJ or 1500 kJ be
regarded as biologically signifi cant?
Since the variance of a dietary measurement
depends not only on the real variation within or
between respondents but also on the error of the mea-
surement, the precision of a dietary estimate can be
improved not only by increasing sample size but also
by reducing measurement error.
Resources
It is inevitable that the resources available, both fi nan-
cial and human, also infl uence the choice of method.
They should not, however, be the primary consider-
ation. The method used should be determined by the
question to be answered. If the method or methods
needed to answer the question are beyond the
resources available it is better either to abandon the
study or to redefi ne the question than to collect inad-
equate data.10.6 Repeatability and validity
This section looks at ways in which it is possible to
assess the repeatability and validity of dietary
methods.Repeatability
Assessing the repeatability (also referred to as the
reproducibility) of a laboratory method is relatively
straightforward because, with care, it is possible to
reproduce both what is measured and the conditions
of measurement. This is almost always impossible in
the case of a dietary intake measurement. Individuals
do not eat exactly the same quantities or the same
foods on different days or weeks.
All measures of repeatability obtained by applying
the same method to the same individuals on more
than one occasion include not only measurement
error but also real day-to-day or week-to-week vari-
ability in intake.
While at fi rst sight it might appear easier to measure
the repeatability of recall methods such as the 24 hour
recall and diet histories, this process also introduces
additional sources of variation since the interviews
have to be conducted at different times and possibly
by different interviewers. Measures of repeatability
for all dietary methods will thus tend to give an over-
estimate of the extent of measurement error because
they will always include an element of variation due
to real differences in what is being measured and in
the conditions under which it is being measured.
Usually, the repeatability of a dietary method is
determined by repeating the same method on the
same individuals on two separate occasions, that is,
by a test–retest study. The interval between tests
depends on the time-frame of the dietary method
being assessed, but should generally be short enough
to avoid the effects of seasonal or other changes in
food habits and long enough to avoid the possibility
of the fi rst interview or recording period infl uencing
the second one.
The difference between the results obtained on
the two occasions can be expressed in a number of
different ways. Table 10.5, which was compiled from
data reported in the literature, shows various mea-