So here's a potential IV study for somebody. If people are more likely to buy condoms when these kinds of self-checkouts are introduced, and there's no correlation between self-checkout installation and underlying sexual behaviour trends, you could use the availability of self-checkouts as an instrument for condom use. Ta dah!Pak 'n Save Moorhouse Ave was the first supermarket in New Zealand to have them installed, in 2006.Foodstuffs chief executive Steve Anderson said 20 to 30 per cent of transactions were now through self-checkouts. However, they accounted for a far lower proportion of customer spending.Customer research showed people who used the self-checkouts tended to buy fewer items and were often mothers with children, the technology-minded or introverted."Embarrassing products", such as personal or indulgent items, were more commonly bought at the self-checkout.Anderson said there had been some savings for the business, but "nothing major". It had not saved on employee numbers. "The idea was more about customer choice and convenience," he said. "You still need staff there." [emphasis added]
It would likely be a weak instrument, alas; the correlation isn't likely to be that high.