Measuring Food Intake 247does not mean that we should stop collecting dietary
data but rather that dietary data need independent
validation. Methods need to be developed to assess
the error structure of dietary datasets so that it can be
taken into account in analyzing and evaluating the
data. Basically, there are two types of error: random
error and systematic error.
Random error increases the variance of the esti-
mates obtained and consequently reduces their preci-
sion (see below). The effects of random error can be
reduced by increasing the number of observations.
Day-to-day variation in food intake in individuals is
one example of a random error that can be reduced
by increasing the number of days of observation
(Figure 10.2).
In contrast, the effects of systematic error cannot
be reduced by increasing the number of observations.
Systematic error arises from errors that are not ran-
domly distributed in the group or in the data from a
given individual. Inappropriate nutrient data for
some food items will not affect the food intake data
for all individuals in the same way. For example, inap-
propriate nutrient data will have a greater effect on
nutrient intake data of individuals who consume the
food in large amounts than on the data of those who
consume only small amounts of the food. Systematic
error leads to bias in the estimates of intake
obtained.
Precision/repeatability
In the laboratory the precision of a method is given by
the coeffi cient of variation (CV) of repeated determi-
nations on the same sample made under the same
conditions (see Box 10.2). In the context of dietary
studies we determine whether the same method gives
the same answer when repeated in the same individu-
als, and the terms repeatability and reproducibility are
commonly used to describe the precision of a method.
It is important to note that it is possible for a method
to have high precision (good repeatability) yet not
provide an accurate (valid) estimate of intake.Accuracy/validity
An accurate method is one that measures what the
method intends to measure, i.e., the “truth.” In the
context of dietary studies the truth represents
the actual intake over the period of the study. For
example, a valid diet record is a complete and accurate
record of all the food and drink consumed over the
period that the record is kept. To be a valid record of
habitual intake it also needs to refl ect what would
have been consumed had the individual not been
keeping a record. If the process of recording infl u-
enced what was eaten then the record is not a valid
record of habitual intake, although it may be a true
record of actual intake over the period. Similarly, a
valid 24 hour recall is a complete and accurate account1510
MJ/dayDays10 20 30 40 50 605Figure 10.2 Energy intake of one indi-
vidual from weighed records obtained
for 1 day every sixth day over 1 year. —,
overall mean; , weekly mean; ,
intake on individual days.