Table 1

Attributes and levels

AttributesLevelsLevels used for the ‘opt-out’ option
Cash reward to start the programme (take-up)US$0, US$10, US$20, US$50US$0
Incentive received conditional on successful discontinuationUS$200, US$400, US$600, US$1500US$0
Half of the incentive received if use is cut in halfYes, NoNo
Incentive formatCertain cash amount, lottery with a 1 in 10 chance of winningCertain cash amount