Introduction to Human Nutrition

Measuring Food Intake 261

of dietary intake, both during and after dietary study
periods. Measurements suitable for this purpose
will be discussed further in the context of validity.
Response bias can probably best be minimized by
providing the respondents with clear and well-
presented instructions, adequate support, and
appropriate incentives. Such incentives may include
relevant dietary feedback where this is appropriate or
monetary or other rewards provided that these are
within ethical principles.
In dietary studies that involve more than one inter-
viewer, the training of interviewers and the use of
standard procedures for interviewing is one way
of reducing unnecessary random variation (error)
that might otherwise arise because different inter-
viewers conduct interviews in different ways. The use
of standard procedures, however, can also introduce
systematic error; for example, if one interviewer is
assigned to interview all respondents in areas of low
socioeconomic status and another to interview all
respondents in areas of high socioeconomic status. It
is important to recognize also that standard interview

procedures do not necessarily “standardize” respon-

dent response. To date, relatively little work has

focused on the respondent response aspect of dietary

assessment.

Since all dietary methods engage the cognitive

processes of respondents, an appreciation of the

properties of human cognition and its limitations is

fundamental to improving the accuracy of dietary

assessments. Recently, research into the cognitive

aspects of dietary assessment has been undertaken in

an attempt to increase the understanding of how

respondents process dietary intake data (Domel,

1997; Thompson et al., 2002; Vuckovic et al., 2002;

Matt et al., 2006).

Some of the important issues in this area that are

relevant to improving the quality of dietary data

include identifi cation of:

● factors that improve communication between

respondent and investigator

● the most effective cues for recall over different

periods

Inadequate
communication/
descriptions

Behavior and interactions
Messages from researcher
Attitudes to food
Socioeconomic status
Image management

Poor memory Inability to “summarize” dietary patterns

False perception of own diet

Altered food

choice

Report RECORD RECALL HISTORY FFQ

Weighing errors
Poor estimations

Identify food Subject’s choice

from questionnaire

Underreporting
Overreporting

Poor quantification:

• Questionnaire choices


• Subject’s perceptions

Quantify Weighed/ Estimated

estimated

Average portion

or small/medium/large

Frequency As reported Subject’s

estimate

Subject’s choice

from questionnaire

Food table values
differ from actual
composition

Calculate

intake

Food code

x Portion weight

x Nutrient content/g

Researcher’s choice

from food tables

Food code

x Portion weight

x Frequency

x Nutrient content/g

Figure 10.6 The process of dietary assessment showing the different sources of error (shown in italics) and the stages at which they operate
in different dietary methods. FFQ, food frequency questionnaire.