# Making Sense of Medical Statistics

When researchers conduct a study into a clinical topic such as a screening test or disease treatment, the raw experimental results are interpreted using statistical analysis. Statistics is the branch of mathematics involved in collecting, analyzing, and presenting data, and making predictions and recommendations using those data.

Statistics are an essential part of evaluating clinical research. Recently, an article in Psychological Science for the Public Interest (Gigerenzer et al., 2008) explored the implication of ignorance of statistics both in the general public and among media and health professionals. Lack of knowledge of this field has a negative impact on public health. This series presents basic information about statistics and explains examples taken from the article.

## Basic Principles of Statistics

Given a set of data, a number of measures can be derived that describe the set. These are called descriptive statistics and include values such as the mean and median, which show an average of the data, and the standard deviation, which describes how much variability is in the data.

In a clinical study, the results are never clear-cut. Theoretically, there is always a possibility that the observed data are the result of random chance. Using statistical methods, the researchers calculate the likelihood that the results are the result of chance. They determine in advance a threshold that this probability, abbreviated p, must reach, typically 5%, 1%, or 0.5% (expressed as p = .05, p =. 01, or p = .005, respectively). If p equals, say, 0.04, that means there is only a 4% probability that the results were due to chance, and therefore a 96% probability that a real correlation has been observed. A p value less than the threshold value is called a statistically significant result.

This type of analysis is called applied statistics. The pieces of information being analyzed may be raw data (such as blood test values) or they may be values derived from descriptive statistics.