Understanding risk: a randomized controlled trial of communicating contraceptive effectiveness

To determine which of three different approaches increased women's understanding of risk of pregnancy associated with different contraceptive methods.

We randomly assigned 461 reproductive-age women to one of three tables presenting pregnancy risk (Food and Drug Administration table with numbers, World Health Organization table with numbers and categories, or table with categories). We evaluated participant knowledge before and after being shown the assigned table.

The most important reason for choosing a contraceptive was how well it works (53%), followed by ease of use (13%), and protection against sexually transmitted disease or human immunodeficiency virus (11%). Before looking at the tables, about half the participants knew that hormone shots are more effective than pills (48%) and that pills are more effective than condoms (57%). For these two key comparisons, the category table compared to the Food and Drug Administration table with numbers improved knowledge significantly more (37% versus 20% and 27% versus 14%; both P <.05). Compared with those assigned to the Food and Drug Administration table with numbers, significantly fewer participants assigned to the category table said the table was difficult to read (6% versus 19%; P <.01). Most participants in all three groups said their assigned table provided enough information to choose a contraceptive method.

The table with categories communicated relative contraceptive effectiveness better than the tables with numbers. However, without being presented with numbers, participants grossly overestimated the absolute risk of pregnancy using contraceptives. A combination of categories and a general range of risk for each category may provide the most accurate understanding of both relative and absolute pregnancy risk.