First, the total number of active scuba divers is unknown; estimates range from 1.5 to 3.5 million in the United States alone. Therefore, valid estimates of risk using traditional methods are not possible. Fatality estimates range from a low of 2-3 per 100,000 to 6-9 per 100,000, depending on the number of fatalities and estimations of the number of active divers in a given year.

Second, most studies of diver fatalities define a diver as someone certified as a diver. This is problematic because some individuals (a) scuba dive, but have not been certified, (b) are certified and never dive, and (c) may hold as many as 25 advanced level certifications with the result that such a diver would be treated statistically as 25 divers.

Third, risk estimates in this activity have not considered the fact that someone who dives once in a given year is treated statistically in the same way as a diver who makes several hundred dives.