During the control period, the median weekly sales of unhealthy snacks amounted to 24 items (IQR: 2.8) per 1000 customers. During the intervention period, the median weekly sales of healthier snacks amounted to 10 (IQR: 4.5) items per 1000 customers (Figure 1). Sales of checkout snacks (unhealthy snacks in the control period vs. healthier snacks in the intervention period) were 2.3 (SE: 1.1, 95% CI: 1.9–2.7; t(8) = 11.0) times lower during the intervention period, as compared to the control period.
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The results of this study indicate that removing the entire assortment of unhealthy snacks from supermarket checkouts resulted in an overall decrease in the sales of checkout snacks. The reduction in the sales of unhealthy snacks was thus not compensated by purchases of healthier snacks.
Study 2 - Placement of healthier snacks at checkouts, combined with a price discount
In the second study, we investigated the impact of placing healthier snacks at checkouts as well as the impact of offering a price discount for the healthier checkout snacks, while keeping the unhealthy checkout snacks available for sale.
Design and setting
The second study was conducted in two supermarkets located in a disadvantaged area in the South-eastern part of Amsterdam, the Netherlands between April and June 2017, using a quasi-experimental real-life design with a control period and an intervention period. Neighbourhood disadvantage was established in the same manner described in the Methods section for the first study. During a two-week control period, the regular unhealthy snacks were offered at the checkout counters as usual. This was followed by a six-week intervention period, during which additional healthier snacks were offered at the checkouts, either with or without a price discount of approximately 15%. The unhealthy snacks remained for sale at the checkouts throughout the entire intervention period. The Medical Ethics Committee of Vrije Universiteit Amsterdam confirmed that this study did not require formal approval by the Medical Research Involving Human Subjects Act (WMO), due to the nature of the measurements (anonymous sales data). The request for approval was therefore waived.
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Healthy Checkout Counter (HCC) Intervention
The intervention consisted of placing three displays at the ends of the conveyor belts in front of the checkout counters, offering single packages of healthier snacks that had already been sold in the two supermarkets before the intervention. There were two types of interventions: 1) a placement intervention, in which the healthier snacks were offered at the checkout counter and 2) the placement + price intervention, in which the healthier checkout snacks were offered at an additional price discount. The placement intervention and the placement + price intervention were alternated between the two supermarkets during the six-week intervention period (see Table 1). During the six-week intervention period, the assortment of healthier snacks was changed every two weeks, resulting in three consecutive two-week periods offering different types of healthier snacks including 1) vegetable snacks (tomatoes and cucumbers), 2) unsalted nuts (five different types) and 3) vegetable snacks (tomatoes and bell peppers) (Figure 3). We conducted separate comparisons of the sales of the three types of healthier snacks during each two-week period to the sales during the two-week control period for the placement intervention and the placement + price interventions. The healthier snacks were selected by the research team and met the guidelines of the Dutch Nutrition Centre (36). The unhealthy snacks remained in their usual place at the checkout counters.
[insert Figure 3]
The main outcome measures were daily sales data for both the unhealthy snacks and the healthier snacks purchased/per 1000 customers. The sales data for the unhealthy snacks refer only to products that were offered exclusively at the checkout counters. The sales data for the healthier snacks refer to products that were placed at the checkout counters, as well as in another place in the supermarket (e.g. in the vegetable department). This was because the supermarket chain was not able to separate the sales data for products that were placed in two (or more) locations in the supermarket. In all, the study addressed 94 unhealthy snacks that were sold exclusively at the checkouts and 9 healthier snacks.
During the intervention period, the researcher assessed the extent to which the intervention was being implemented as intended in the two supermarkets throughout the six-week period by making unannounced weekly visits to the intervention supermarkets and by contacting the managers of the supermarkets by telephone. During each visit, the researcher recorded compliance with the strategies (placement vs. placement + price) and documented the implementation of the intervention with photographs. This resulted in a list of days on which the intervention had been executed correctly and those on which it had not been executed correctly.
Descriptive statistics were used to examine the sales of unhealthy and healthier snacks during the control and intervention periods. Because the sales data were not normally distributed, they are presented as medians with IQR. For the reasons described in the first study, we examined the extent of the effect of the intervention on sales by standardizing the sales data for the snacks included in this study by using their logarithms. The data were analysed using the natural logarithms in order to calculate the proportional changes in sales between the intervention and control periods. Because the sales data included zero values, we added a constant value to the data prior to applying the log transformation (37). We subsequently conducted independent-sample t-tests to investigate the proportional change in sales between the intervention and control periods for the sales of (1) healthier snacks in the placement intervention, (2) healthier snacks in the placement + price intervention and (3) unhealthy snacks. An additional independent-sample t-test was conducted to investigate the proportional change between the intervention and control periods in the sales of the healthier snacks between the placement + price condition and the placement condition. Log-transformed results were back-converted to ratios for presentation and interpretation. The outcome represents the ratio between the geometric means of the intervention period as compared to the control period. The same statistical procedure was applied when analysing the data according to the per-protocol approach, in order to investigate the effect of the intervention excluding sales data for the healthier snacks from days on which the intervention was not executed correctly. Statistical analyses were performed using the statistical software package IBM SPSS Statistics for Windows, version 25.0.
The median sales per 1000 customers per day of the healthier snacks in the placement intervention increased from 4.2 (IQR: 4.6) items in the control period to 7.8 (IQR: 4.6) items in the intervention period (Fig. 3). Sales of the healthier snacks during the placement intervention were 2.1 (SE: 1.3, 95% CI: 1.3–3.3; t(53) = 3.2, p < 0.001) times higher, as compared to the control period (Table 2). In line with these results, sales of the healthier snacks during the placement + price intervention increased from 2.2 (IQR: 4.7) items in the control period to 5.8 (IQR: 2.2) items in the intervention period. Sales of the healthier snacks during the placement + price intervention were 2.7 (SE: 1.2, 95% CI: 2.0–3.6; 110) = 6.9, p < 0.001) times higher, as compared to the control period. No statistically significant difference in effect was found between the placement and the placement + price interventions with regard to the increase in sales of the healthier snacks between the intervention and control periods (ratio: 1.1, SE: 1.3, 95% CI: 0.7–1.9; t(29) = 0.2, p = 0.8). Furthermore, the median sales per 1000 customers per day of the unhealthy snacks increased from 15.4 (IQR: 1.8) items in the control period to 18.1 (IQR: 1.4) items in the intervention period (including both placement and placement + price intervention). Sales of the unhealthy snacks were 1.3 (SE: 1.1) times higher in the intervention period (including both placement and placement + price), as compared to the control period (95% CI: 1.1–1.5; t(40) = 3.8, p < 0.001). The per-protocol analyses revealed similar results (data not shown).
Effect of the healthy checkout counter (HCC) intervention on the sales of unhealthy snacks at checkout counters and the sales of the healthier snacks (placement, placement + price intervention separately) between the intervention period and the control period.
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Placement of healthier snacks
Placement + price of healthier snacks
Placement + price intervention vs. placement intervention
Unhealthy snacks at checkout counters
1Independent-sample t-tests performed on log-transformed data; ratio is the exponent of the log-transformed outcome
*P < 0.05
SE = standard error
CI = confidence interval
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