**Single-event probabilities**

Single-event probabilities can lead to miscommunication because people tend to fill in different reference
classes. This type of miscommunication happens frequently with mundane statements
such as those made in weather reports:

Example: ‘There is a 20 percent chance that it will rain tomorrow’.

Some people think that it will rain 20 percent of the time, others that it
will rain in 20 percent of the area, and still others that it will rain on
20 percent of the days that are like tomorrow. Although the third option is
the intended message, approximately two thirds of the people will interpret
this statement incorrectly.

**Conditional probabilities**

Information in the form of conditional probabilities is often misinterpreted.
Sometimes it is difficult to make inferences on the basis of probabilities.

Example: The statement ‘If a woman has breast cancer, the probability
that she will test positive on a screening mammogram is 90%’ is often
confused with the statement ‘If a woman tests positive on a screening
mammogram, the probability that she has breast cancer is 90%’.

**Relative risks **

One of the most common means of describing clinical benefits in the world
of medicine and public health is the relative risk reduction. Since relative
risks are larger numbers than absolute risks, results presented in this manner
appear to be greater than the same results presented as absolute risk reductions.

Presenting benefits as absolute as absolute benefits or in terms of the number
needed to treat to save one life are two simple examples of ways to make results
more understandable.

**Next : Representation of Probabilities **

**Previous : Certainty**