Do we eat what we buy? Relative validity of grocery purchase data as an indicator of food consumption in the LoCard study

Abstract

The validity of grocery purchase data as an indicator of food consumption is uncertain. This paper investigated 1) the associations between food consumption and grocery purchases using automatically accumulated purchase data, and 2) whether the strength of the associations differed in certain sub-populations. The participants filled in a food frequency questionnaire (FFQ), and a major Finnish retailer issued us with their loyalty-card holders grocery purchase data covering the 1- and 12-month periods preceding the FFQ. We used gamma statistics to study the association between thirds/quarters of FFQ and grocery purchase data (frequency/amount) separately for 18 food groups among the 11,983 participants. Stratified analyses were conducted for subgroups based on gender, family structure, educational level, household income and self-estimated share of purchases from the retailer. We also examined the proportion of participants classified into the same, adjacent, subsequent and opposite categories using the FFQ and purchase data. The gammas ranged from 0.12 (cooked vegetables) to 0.75 (margarines). Single households had stronger gammas than two-adult families, and participants with >60% of purchases from the retailer had stronger gammas. For most food groups, the proportion of participants classified into the same or adjacent category was >70%. Most discrepancies were observed for fresh/cooked vegetables, berries, and vegetable oils. Even though the two methods did not categorize all food groups similarly, we conclude that grocery purchase data are able to describe food consumption in an adult population, and future studies should consider purchase data as a resource-saving and moderately valid measure in large samples.

Food purchase behaviour in a Finnish population: patterns, carbon footprints and expenditures

“…However, our sensitivity analysis showed that the purchase patterns were similar when only those who bought ≥81% of their food from the retailer were included in the factor analysis. This is in line with our previous finding that the proportions of food groups purchased were very similar among customers with high loyalty (22) , and that their purchases reflect the loyalty-card holder's dietary intake (19) . To estimate the similarity of the purchase data with the purchases of the general Finnish population, the average annual expenditure on groceries and non-alcoholic beverages in Finland was 2916 €, and on alcohol and cigarettes 578 € in 2016 (50) .…”

“…As the data covered an extensive time period, they were more accurate regarding habitual consumption and the inclusion of the seasonal variation of food consumption. We have also previously shown that food purchase data are a valid instrument for ranking consumers according to their self-reported food (19) and beer (20) consumption. Our data included detailed information on thousands of product groups, which enabled regrouping based on the principles most appropriate for the purpose.…”

“…participants who made a large proportion of their food purchases from the food retailer) and who made at least 50 kg of purchases during 2018 were included in the analysis. Our previous analyses showed that purchases associated more strongly with dietary intake among the most loyal (degree of loyalty >60 %) customers (19,20) and therefore build a more complete picture of relative food purchases (22) . In this study, we used purchase data aggregated to annual consumption in both volume (kg) and expenditure (€).…”

“…Food retailers' customer loyalty card data provide a unique tool for gaining insights into dietary patterns. We have previously shown that food purchase data are a valid instrument for ranking consumers according to their self-reported food (19) and beer (20) consumption. In this study, the detailed data on purchased product groups over a one-year period enabled an objective assessment of food expenditure and the allocation of carbon footprints for large sets of product groups on a household level.…”

Exploring the Geographic Variation in Fruit and Vegetable Purchasing Behaviour Using Supermarket Transaction Data

“…The degree to which household-level purchases from the retailer represent individual consumption is unknown. Previous validation studies highlight that agreement between purchases and consumption is likely to vary by loyalty status and household composition [ 54 , 55 ], with higher agreement observed for single-person households [ 54 ]. However, accepted adjustment factors remain lacking.…”