Department of Statistics

Case study: The Supermarket Healthy Options Project (SHOP)

Are discounts more effective than nutrition education in encouraging supermarket shoppers to buy foods that are better for them?

Among the people who helped answer that question was Dr Yannan Jiang of the Statistical Consulting Centre. She was project statistician and co-investigator on the Supermarket Healthy Options Project (SHOP) led by Associate Professor Cliona Ni Mhurchu of The University of Auckland.

The study tracked the food purchases of 1,104 shoppers at eight Pak’ N Save supermarkets in Wellington, Wanganui and New Plymouth in 2007 and 2008. More than 3000 top-selling food items and non-alcoholic drinks in the supermarkets were classified as healthier or less healthy, using a system similar to the National Heart Foundation’s ‘tick’, which identifies food with lower levels of unhealthy saturated and trans fats, salt and kilojoules.

The participants were randomly allocated to one of four groups to assess the effect of two intervention strategies: culturally-targeted tailored nutrition education, which included shopping lists, recipes and personalised advice; and 12.5% price discounts on healthier food purchases. The main household shoppers, mostly women, recorded their purchases using portable barcode scanners already in the supermarkets.

And the answer to the initial question? Discounts were more effective than nutrition education – they resulted in an 11% increase in the amount of healthier food purchased, most of it fruit and vegetables. Purchases of less healthy food didn’t change. Six months after discounts were withdrawn, there remained a 5% improvement in healthy food purchases compared with baseline data.

However, there was no increase in the amount of healthier food bought by shoppers who received nutrition education alone.

For her analyses, Yannan used SAS and R software packages. The latter is a free statistical computing and graphics software developed in the mid-1990s in the Department of Statistics at The University of Auckland and now used world-wide.

Find more information about R

SHOP was funded by the Health Research Council of New Zealand, and the results were published in 2010 in the American Journal of Clinical Nutrition .