An interesting proposal, but I see following problems with it:<p>- You have both zero control and zero objective input on what subjects are actually eating. They can ignore your coupons, they can lie or omit (by simply forgetting about) what they&#x27;ve eaten. They can buy stuff without using loyalty card (and probably will, in different stores - just how many people shop in one and only one store?).<p>- The data would be very imprecise - while individuals would do the medical tests, unless you&#x27;re targeting singles, their grocery shopping will go towards the <i>household</i>, and you have no idea who ate what.<p>- I&#x27;m not confident that mailing them coupons would change their diet, unless you&#x27;ve got yourself either very poor, or very cooperative subject. The outcome I expect is that people would mostly ignore the coupons if they don&#x27;t feel like eating the thing you want them to.<p>In the end, while interesting, it seems less precise than the current questionnaire&#x2F;journal&#x2F;self-reporting based approach.