260 Introduction to Human Nutrition
check using a standard list of commonly consumed
foods. The interview usually begins with a review of
the food that was eaten in a specifi c time-frame (e.g.,
yesterday) or on a typical day, and then moves on to
explore the variations in food intake that occur for
each meal over a given period. Information on the
usual size of food portions is obtained with the aid of
food models or photographs in the same way as for a
24 hour recall. The time-frame for a diet history can
range from the previous month to the previous year.
In practical terms it is easier for respondents to
reconstruct the immediate past, but the past year is
often used to capture seasonal variation. Whatever the
time-frame used, it is important for it to be clearly
specifi ed. In the literature the term “diet history”
is sometimes used loosely to describe any form of
diet recall, including the 24 hour recall and self-
administered food frequency questionnaires, as well
as interviewer-administered recalls of habitual or
longer term intake. This broader use can be confusing
and is best avoided. The dependence of the diet
history on both respondent and interviewer skills
may make the results obtained less comparable
between individuals than those obtained from other
methods, and for this reason it is often considered
more appropriate to categorize diet history data (e.g.,
as high, medium, low) rather than to treat them as
intakes expressed in terms of absolute units per day.
The diet history is favored in Scandinavia and the
Netherlands, where a structured interview may be
used. The structured interview is more standardized
but may miss elements specifi c to the individual or
bore the respondents with irrelevant questions. The
open-ended interview allows for tailoring to the indi-
vidual, but risks missing important items.
10.4 Sources of error in dietary studies
The major sources of error in dietary studies have
been reviewed in detail by Bingham (1987). Four pos-
sible sources of error occur to some degree with all
dietary methods, but can be minimized by careful
study design and execution:
● sampling bias
● response bias
● inappropriate coding of foods
● use of food composition tables in place of chemical
analysis.
In contrast, the errors that are associated with spe-
cifi c methods are generally much more dependent on
the nature of the method and the abilities of the
respondents, and therefore less easy to control. Errors
of this type include:
● estimation of portion size
● recall or memory error
● day-to-day variation in intake
● effect of survey method on food intake.Figure 10.6 illustrates the points in the dietary assess-
ment process at which different kinds of error
operate.Sampling bias
Sampling bias arises when the sample studied is not
truly representative of the population of interest. The
importance of minimizing sampling bias depends on
the purpose of the dietary study. Except in method-
ological studies, volunteers are not generally appro-
priate respondents because frequently the objective is
to study a representative sample of a particular group
in order to extrapolate the results to the population
group from which the sample was drawn. For this
purpose it is clearly important that as many as possi-
ble of the sample group originally selected participate
in the study.
The proportion of the sample that agrees to par-
ticipate in the study can vary considerably even with
the same method. It depends not only on the group
being studied but also on the circumstances of the
study. In general, response rates tend to be greater in
studies that use methods such as the 24 hour recall
and food frequency questionnaires (which make
fewer demands on respondents) and lower in studies
such as 7 day weighed records (which require much
more cooperation, effort, and time from the respon-
dents). It is always important to try to maximize
response rates, for example by increasing respondent
motivation, providing specifi c assistance if required,
and by allowing respondents as much fl exibility as
possible in participation within the context of the
study objectives.Response bias
Response bias arises when respondents provide
incomplete or inappropriate responses. The extent of
this problem is not easy to assess, but can be investi-
gated by making measurements that are independent