[keithpeter]

Predictions based on partial data could be problematic.

One example: I pay for fresh food and staples in cash, and buy 'grocery' items on a card. A data collection system that tracked only card purchases would give the impression of a very unhealthy diet.

[andyking]

This is how it is for me, too. I buy fresh vegetables, fresh meat, milk, eggs, bread at a local farm shop and pay cash. I buy other things like pizza, coffee, processed foods at a supermarket and pay with card - and also scan my loyalty card.

I regularly get offers from the supermarket for money off yet more unhealthy food - they are blissfully unaware that I eat quite well in reality!