Table 1.

Examples of confusing statistical information, with alternatives that foster insight

Type of information	Examples	How to foster insight
Single event probabilities	“You have a 30% chance of a side effect from this drug”	Use frequency statements: “Three out of every 10 patients have a side effect from this drug”
Conditional probabilities	The probability of a positive test result if the patient has the disease (sensitivity)	Use natural frequencies, alone or together with conditional probabilities (see box)
	The probability of a negative test result if the patient does not have the disease (specificity)
	The probability of the disease if the patient has a positive test result (positive predictive value)
Relative risks	If four out of every 1000 women (age. 40 or older) who do not undergo mammography screening die of breast cancer, compared with three out of every 1000 who are screened, the benefit is often presented as a relative risk: “Mammography reduces breast cancer mortality by 25%.”	Use absolute risks, alone or together with relative risks: “In every 1000 women who undergo screening one will be saved from dying of breast cancer.”
		Use the number needed to treat or harm: “To prevent one death from breast cancer, 1000 women need to undergo screening for 10 years.”