I was on a call recently working through the details of a complex discrete-choice task. Specifically we were debating how best to apply price prohibitions - restrictions on the design that would prevent certain "monthly" and "one-time" prices from ever appearing together.
Rest assured our thinking was all very logical. We needed to put controls in place because who in their right mind would ever choose an option where a low monthly rate, coupled with a contract, quickly added up to more out of pocket costs than would accrue by skipping the contract and paying a (slightly) higher up-front fee. That's when the client's client chimed in:
"People don't do the math," said the man who spends little to none of his time conducting surveys.