Size matters -- patient organisations exaggerate prevalence numbers.

Patient organisations serve their members with information and support concerning a specific disease. In many cases they also contribute to research funding and lobby to improve the situation for their members. The larger group of patients an organisation claims to represent, the bigger their potential influence. Our hypothesis is that patient organisations exaggerate the number of persons affected with a specific disease. Prevalence figures from patient organisations in Sweden were collected via their own web sites. About 93 patient organisations were identified, 29 of which presented the estimates of disease occurrence used in this study. We calculated the probability for a person to have at least one disease and the proportion of the population not having any of the diseases listed. About 60% of the Swedish population have at least one disease covered by our sample of patient organisations. Nine tenths (87%) of the population would be ill if one assumes that an individual could only have one disease. Our rough estimates suggest that patient organisations exaggerate the number of ill persons. To render other messages on their agenda more trustworthy, we propose that some patient organisations moderate their prevalence and/or incidence figures.