It is relative easy/simple to calculate this with 'some' accuracy.
If you ask someone how much they smoke per day, (that's pretty consistent) and they respond "2 packs of X brand", then you know the brand, you know the price x365 you can estimate the cost. If you point blank ask me "how much do you spend on pizzas every year", I can't just pull a number out of a hat. I will do the math: €20 per pizza x 1 per month = approx 240.
Are you underestimating how much people lie, even to themselves?
Esp here. “Hey; want to be part of a study where we give you free money to see if it helps you out?”… “Ok, now how much of that money did you use on things that were harmful to you?” You could perform that study with anyone and you’re going to get BS answers and estimations.
If you ask someone how much they smoke per day, (that's pretty consistent) and they respond "2 packs of X brand", then you know the brand, you know the price x365 you can estimate the cost. If you point blank ask me "how much do you spend on pizzas every year", I can't just pull a number out of a hat. I will do the math: €20 per pizza x 1 per month = approx 240.