4.5: Summary

This chapter outlines the system­atic and random error that may occur during the collection and recording of food consumption data. In practice, different types of error compound to impact the overall accuracy of reported consumption and estimated intake of nutrients and other dietary components. For example, errors in portion size quantification may counteract or exacerbate the impact of omissions of foods and beverages consumed on nutrient intake estimates.

Quality-control procedures that minimize possible sources of measurement error include training the inter­viewing and coding staff and developing standard inter­viewing techniques and question­naires during the pilot survey. Increasingly, sources of error arising from both respondent and inter­viewer biases and respondent memory lapses can be reduced using computerized probing questions, standardized prompts, and built-in cues during auto­mated dietary inter­views, as well as technology-enabled methods. Nevertheless, misestimation of energy and selective misreporting of certain food types remain important sources of respondent biases. A variety of portion-size measurement aids are now available for use when weighing methods are not possible. These include the use of 2-D graduated food models, photographs, and images and 3-D measurement guides (e.g., household measures) to quantify portions of foods consumed. Training respondents to use these measurement guides to estimate food portion sizes will also improve accuracy. Collection of accurate data on consumer use of dietary supple­ments is also essential; information on brand, dosage, chemical form, and period over which use of the dietary supple­ment has been recorded is required.

Establishing a computerized standard coding system for both foods and eating occasions to avoid coding error is critical, especially for surveillance and cross-country comparisons. Systematic detection of wrongly coded weights of foods is more difficult, although calculation of energy and macronutrient intakes from 24h recall inter­views, while the respondent is still present, allows the correction of any gross error. Finally, care must be taken in the handling of data for mixed dishes and foods eaten in combin­ation. Despite all efforts to minimize sources of random and system­atic error that may occur during the measurement of food and nutrient intakes, some errors remain difficult to predict and to prevent and, as a result, may introduce differential bias in reported food intakes. Ongoing research is needed to investigate the specific type and nature of measurement error, especially in diverse popu­la­tions, so that these can be minimized or corrected statistically. In this way, the analysis and inter­pretation of dietary data can be improved. The existence of dietary measurement error distorts estimates of disease relative risk, and thus has major implications for epidemiological studies of dietary risk factors and disease. Observed diet-disease relationships should be inter­preted cautiously (Freedman et al., 2011), particularly if steps have not been taken to mitigate the error to the extent possible.

Assess­ing the repro­ducibility (Chapter 6) and validity (Chapter 7) of dietary methods used is essential for surveillance, including cross-country comparisons, epidemiologic, and inter­vention research (Buzzard & Sievert, 1994).