Glossary

C

carryover effect

In a crossover trial, there is a possibility of treatments having an (unwanted) effect in the period following the one in which they are administered. One way to deal with carryover effects is to allow sufficient time between periods, or washout, for the carryover effect to become negligible. If, however, this is impractical or unethical, you can add the carryover effect to the model.

categorical

A variable with a discrete number of values; an ordinal or nominal variable. Categorical variables are often used as grouping variables or factors.

cell

A cell is the cross-classification of levels from one or more factors. For example, if you have customer factors for geographic region, marital status, and educational level, then married college graduates in your northern sales territory constitute a cell.

Covariate

A scale variable that has been added to a model. In a predictive model, changes in the value of a covariate should be associated with changes in the value of the target (dependent) variable.

crossover trial

This is a study in which each subject is observed for multiple periods and receives a different level of treatment at each period. The advantages of such a study are an economy of time and subjects and that "within-patient" comparisons of treatments are possible. A potential problem with this design is the possibility of carryover effects.

Cumulative Proportion Surviving at End of Interval

The proportion of cases surviving from the start of the table to the end of the interval.

Cumulative Proportion Surviving at the Time

The proportion of cases surviving from the start of the table until this time.

F

Factor

An independent variable defining groups of cases.

H

Hazard Rate

An estimate of the risk of experiencing the terminal event during the interval, conditional upon surviving to the start of the interval.

I

Interval Start Time

The time period that marks the beginning of the interval. An interval extends from the start time up to, but not including, the start time of the next interval.

L

level

The values of a factor are referred to as levels of the factor, or factor levels.

N

N of Cumulative Events

The number of cases that have experienced the terminal event from the start of the table until this time.

N of Remaining Cases

The number of cases that, at this time, have yet to experience the terminal event or be censored.

Number Entering Interval

The number of surviving cases at the beginning of the interval.

Number Exposed to Risk

The number of surviving cases minus one half the censored cases. This is intended to account for the effect of the censored cases.

Number of Terminal Events

The number of cases that experience the terminal event in this interval.

Number Withdrawing during Interval

The number of censored cases in this interval.

P

Probability Density

An estimate of the probability of experiencing the terminal event during the interval.

Proportion Surviving

One minus the proportion terminating.

Proportion Terminating

The ratio of terminal events to the number exposed to risk.

S

Scale

A variable can be treated as scale (continuous) when its values represent ordered categories with a meaningful metric, so that distance comparisons between values are appropriate. Examples of scale variables include age in years and income in thousands of dollars.