256 Introduction to Human Nutrition
The multiple-pass approach is thought to assist recall
more effectively than chronological cues and thus
provide more accurate and complete information.
This approach, however, is more time-consuming
than the traditional 24 hour recall and may irritate
respondents by seemingly asking about the food
intake over and over again. Irrespective of the
approach used, it is essential that all interviewers are
thoroughly trained, that the approach is tested in the
target population prior to the study, and that the same
procedure is used by all interviewers with all respon-
dents throughout the study.
A major drawback of the 24 hour recall is that it
provides information for only a single day and there-
fore does not take account of day-to-day variation in
the diet. In large cross-sectional studies in which the
aim is to determine average intakes of a group of
individuals, a single 24 hour recall may be suffi cient.
When the diets of individuals are assessed or when
sample sizes are small, repeated 24 hour recalls are
required. This method is known as multiple 24 hour
recalls. The number of recalls depends on the aim of
the study, the nutrients of interest, and the degree
of precision needed. For example, when diets
consist of a limited variety of foods two 24 hour
recalls may be suffi cient whereas four or more recalls
may be required when diets are complex. Recalls
may also be repeated during different seasons to
take account of seasonal variations. (Note that mul-
tiple 24 hour recalls must not be confused with the
multiple-pass 24 hour recall technique. The multiple-
pass 24 hour recall refers to an interviewing tech-
nique, whereas the multiple 24 hour recall method
refers to repeated 24 hour recalls conducted per
respondent.)
The strengths of the 24 hour recall method are that
it has a low respondent burden in comparison with
food records and thus compliance is high, it does not
require respondents to be literate, it does not alter
usual food intake, and it is relatively quick and inex-
pensive to administer and therefore is cost-effective
when large numbers of respondents are involved. It is
most successful in populations with limited dietary
variety and when respondents are able to accurately
recall and express the types and amounts of foods
consumed and when interviewers are skilled in the
interview technique.
As already stated a major drawback of the 24 hour
recall is that it does not give an accurate refl ection of
habitual dietary intake if only a single 24 hour recall
is conducted. This may be overcome to some extent
by conducting repeated 24 hour recalls. Another dif-
fi culty is that the 24 hour recall relies on the respon-
dent to accurately recall and report the types and
amounts of foods consumed. There is a tendency for
respondents to overestimate low intakes and under-
estimate high intakes. This is known as the fl at slope
syndrome. Respondents may omit certain foods that
are considered “bad” or include foods not consumed
but considered “good” (phantom foods) in order to
impress the interviewer.
Of the methods so far described weighed records
should contain the least error as they report all food
consumed on specifi ed days with weighed portions.
Estimating the size of portions increases error and, if
menu records are quantifi ed with average portions,
then the error at the individual level is further
increased. If food that has already been eaten has to
be recalled then poor memory can introduce an addi-
tional source of error. All methods that report intake
on specifi ed days are also subject, in individuals, to
the error associated with day-to-day variation in
intake, but this error can be reduced by increasing the
number of days studied.Methods for measuring intake over
the longer term
Food frequency questionnaires
Food frequency questionnaires consist of a list of
foods and options to indicate how frequently each
food is consumed. Respondents indicate the frequency
of consumption during a specifi ed period by marking
the appropriate option column. The food lists may
contain only a few food items or may contain up to
200 foods. The type and number of foods included is
determined by the purpose of the study and the target
population. For example, a food frequency question-
naire designed to determine calcium intake would
contain only foods which provide calcium, while a
questionnaire to measure overall dietary adequacy
would need to contain all foods known to be con-
sumed by the target population. Likewise, a food fre-
quency questionnaire designed to assess dietary
intakes of a homogeneous target population with a
diet of limited variety will be shorter than one
designed to assess food intakes of a heterogeneous
population with a variety of food intake patterns.
There are several types of food frequency question-