**absolute risk increase (ARI)**

The increase in risk with a new therapy compared with the risk without the new therapy.

**absolute risk reduction (ARR)**

The reduction in risk with a new therapy compared with the risk without the new therapy; it is the absolute value of the difference between the experimental event rate and the control event rate (|EER – CER|).

**absolute value**

The positive value of a number, regardless of whether the number is positive or negative. The absolute value of *a* is symbolized |*a*|.

**actuarial analysis**

See *life table analysis.*

**addition rule**

The rule which states the probability that two or more mutually exclusive events all occur is the sum of the probabilities of each individual event.

**adjusted rate**

A rate adjusted so that it is independent of the distribution of a possible confounding variable. For example, age-adjusted rates are independent of the age distribution in the population to which they apply.

**age-specific mortality rate**

The mortality rate in a specific age group.

**alpha (α) error**

See *type I error.*

**alpha (α) value**

The level of alpha (α) selected in a hypothesis test.

**alternative hypothesis**

The opposite of the null hypothesis. It is the conclusion when the null hypothesis is rejected.

**analysis of covariance (ANCOVA)**

A special type of analysis of variance or regression used to control for the effect of a possible confounding factor.

**analysis of residuals**

In regression, an analysis of the differences between *Y* and *Y′* to evaluate assumptions and provide guidance on how well the equation fits the data.

**analysis of variance (ANOVA)**

A statistical procedure that determines whether any differences exist among two or more groups of subjects on one or more factors. The *F* test is used in ANOVA.

**backward elimination**

A method to select variables in multiple regression that enters all variables into the regression equation and then eliminates the variable that adds the least to the prediction, followed by the other variables one at a time that decrease the multiple *R* by the least amount until all statistically significant variables are removed from the equation.

**bar chart or bar graph**

A chart or graph used with nominal characteristics to display the numbers or percentages of observations with the characteristic of interest.

**Bayes’ theorem**

A formula for calculating the conditional probability of one event, *P*(*A*|*B*), from the conditional probability of the other event, *P*(*B*|*A*).

**bell-shaped distribution**

A term used to describe the shape of the normal (Gaussian) distribution.

**beta (β) error**

See *type II error.*

**bias**

The error related to the ways the targeted and sampled populations differ; also called measurement error, it threatens the validity of a study.

**binary observation**

A nominal measure that has only two outcomes (examples are gender: male or female; survival: yes or no).

**binomial distribution**

The probability distribution that describes the number of successes *X* observed in *n* independent trials, each with the same probability of ...