BACKGROUND AND AIMS: The same information may be perceived differently, depending on how it is described. The risk information given on many gambling warning labels tends to accentuate what a gambler might expect to win, e.g. 'This game has an average percentage payout of 90%' (return-to-player), rather than what a gambler might expect to lose, e.g. 'This game keeps 10% of all money bet on average' (house-edge). We compared gamblers' perceived chances of winning and levels of warning label understanding under factually equivalent return-to-player and house-edge formats.

DESIGN: Online surveys: experiment 1 was designed to test how gamblers' perceived chances of winning would vary under equivalent warning labels, and experiment 2 explored how often equivalent warning labels were correctly understood by gamblers.

SETTING: United Kingdom.
PARTICIPANTS: UK nationals, aged 18 years and over and with experience of virtual on-line gambling games, such as on-line roulette, were recruited from an on-line crowd-sourcing panel (experiment 1, n = 399; experiment 2, n = 407).
MEASUREMENTS: The main dependent variables were a gambler's perceived chances of winning on a seven-point Likert scale (experiment 1) and a multiple-choice measure of warning label understanding (experiment 2).

FINDINGS: The house-edge label led to lower perceived chances of winning in experiment 1, F  = 19.03, P < 0.001. In experiment 2, the house-edge warning label was understood by more gamblers [66.5, 95% confidence interval (CI) = 60.0%, 73.0%] than the return-to-player warning label (45.6%, 95% CI = 38.8%, 52.4%, z = 4.22, P < 0.001).

CONCLUSIONS: House-edge warning labels on electronic gambling machines and on-line casino games, which explain what a gambler might expect to lose, could help gamblers to pay greater attention to product risk and would be better understood by gamblers than equivalent return-to-player labels.