Learn More about Season Effect

Dietary intakes may vary with season, although the global marketplace has lessened seasonal influences. Foods most likely to be affected by [glossary term:] season effects are fresh fruits and vegetables. When the season effect is of no intrinsic interest in itself, it is termed a [glossary term:] nuisance effect because seasonal variation can introduce [glossary term:] bias into the results of an analysis.

For [glossary term:] long-term instruments, such as food frequency questionnaires (FFQ), (see the Food Frequency Questionnaire Profile), one issue is whether bias exists in how individuals report for the past year by the season in which they report. Subar et al. found a small difference in reporting frequency over the past year depending on which season the FFQ was administered for 5% of foods queried. The largest seasonal bias was found for citrus fruit.

The second issue concerns how to ask about the frequency of eating foods when the frequency for some foods varies by season. This issue can be addressed to some extent in the construction of the questionnaire. If the study purpose does not require a past year estimate, the period of time queried may be shorter, for example, the past month, to allow more accurate reporting of foods within a particular season. Alternatively, if the study purpose requires estimates for the past year, an FFQ with seasonal food questions (i.e., how often and how much food is consumed in season and not in season) could be used (see the National Cancer Institute's (NCI) Diet History Questionnaire for an example). This approach may be cognitively easier than reporting an estimate that attempts to average consumption across seasons.

Seasonality of foods also can be problematic for [glossary term:] short-term instruments (24-hour dietary recall (24HR) and food records) that capture intake for a given day or a few days at a time (see 24-hour Dietary Recall Profile and Food Record Profile). In this case, seasonal bias can be minimized through study design and analysis procedures. When collecting dietary data through a single administration of the 24HR or food record, respondents can be randomly scheduled such that they are equally proportioned across the seasons. When collecting data through multiple administrations to each respondent, the second and subsequent administrations could be spaced to maximize the seasonal variation potentially experienced by each respondent. For example, if two administrations are planned, spacing them in opposite seasons (i.e. summer and winter; spring and fall) may be optimal.

Regardless of study design, season can be added as a [glossary term:] covariate in the analysis. This is often the most feasible strategy for minimizing its undesired effect.

For More Information

National Cancer Institute, Division of Cancer Control and Population Science, Applied Research Program. Diet History Questionnaire II.

Subar AF, Frey CM, Harlan LC, Kahle L. Differences in reported food frequency by season of questionnaire administration: the 1987 National Health Interview Survey. Epidemiology 1994 Mar;5(2):226-33. [View Abstract]

Subar AF, Thompson FE, Smith AF, Jobe JB, Ziegler RG, Potischman N, Schatzkin A, Hartman A, Swanson C, Kruse L, et al. Improving food frequency questionnaires: a qualitative approach using cognitive interviewing. J Am Diet Assoc 1995 Jul;95(7):781-8; quiz 789-90. [View Abstract]