What do all these numbers mean?

This is a first draft of Chapter 6 of "Statistical Evidence."

6.0 Introduction

You will see a lot of statistics being tossed around in medical journals. You don't have to know how to compute these numbers but you do need to know how to interpret them. Here's a simple guide to some of the numbers you are likely to encounter in a medical journal article.

6.1 Hypothesis testing

6.1.1 P-value

6.1.2 Confidence interval

6.1.3 Why you should prefer the confidence interval?

6.2 Measures of risk

The material in this section is based on the publication: Understanding the odds ratio and the relative risk. Simon SD. J Androl 2001: 22(4); 533-6.

Consider the following data on survival of passengers on the Titanic. There were 462 female passengers: 308 survived and 154 died. There were 851 male passengers: 142 survived and 709 died (see table below).

	Alive	Dead	Total
Female	308	154	462
Male	142	709	851
Total	450	863	1,313

Clearly, a male passenger on the Titanic was more likely to die than a female passenger. But how much more likely? You can compute the odds ratio or the relative risk to answer this question.

6.2.1 Odds ratio

The odds ratio compares the relative odds of death in each group. For females, the odds were exactly 2 to 1 against dying (154/308=0.5). For males, the odds were almost 5 to 1 in favor of death (709/142=4.993). The odds ratio is 9.986 (4.993/0.5). There is a ten fold greater odds of death for males than for females.

6.2.2. Relative risk

The relative risk (sometimes called the risk ratio) compares the probability of death in each group rather than the odds. For females, the probability of death is 33% (154/462=0.3333). For males the probability is 83% (709/851=0.8331). The relative risk of death is 2.5 (0.8331/0.3333). There is a 2.5 greater probability of death for males than for females.

6.2.3 Number needed to treat

6.2.4 Which measure is best?

6.3 Diagnostic testing

6.3.1 Sensitivity

6.3.2 Specificity

6.3.3 Positive predictive value

6.3.4 Negative predictive value

6.3.5 Likelihood ratio

This webpage was written by Steve Simon on (unknown date), edited by Steve Simon, and was last modified on 2008-07-08. Send feedback to ssimon at cmh dot edu or click on the email link at the top of the page. Category: Statistical evidence