The research glossary defines terms used in conducting social science and policy research, for example those describing methods, measurements, statistical procedures, and other aspects of research; the child care glossary defines terms used to describe aspects of child care and early education practice and policy.
Principal Component Analysis
Principal component analysis is a procedure that is used to reduce a set of variables to a smaller set of variables, which are called principal factors. The principal factors retain most of the important information that is found in the larger set and are much easier to analyze and interpret.
Probability
A description of the likely occurrence of a particular event. Probability is conventionally expressed on a scale from 0 to 1; a rare event has a probability close to 0, a very common event has a probability close to 1.
Probability of Selection
In probability samples, the probability of selection is the probability that a member of the population will be selected to participate in the study sample.
Probability Sampling
A random sample of a population, which ensures that each member of the population has a chance of being selected for the sample.
Probit Models
A probit model is a type of regression where the dependent variable can only have two values. For example, a child from a low income family is either enrolled in a Head Start program or not.
Product Moment Correlation Coefficient
See Pearson's Correlation Coefficient.
Program Evaluation
Research that is conducted in order to determine the effectiveness of an intervention program.
Projection
Estimates of the future size and other demographic characteristics of a population, based on an assessment of past trends and assumptions about the future course of demographic behavior.
Propensity Score Matching
Propensity score matching is a statistical matching technique that is used to estimate the effect of a treatment or intervention when data come from a nonrandomized (observational) design. It uses a set of observable characteristics to predict the probability that participants will be assigned the treatment. Its purpose is to eliminate or reduce systematic differences between those who received the treatment and those who did not; thus, mimicking a randomized controlled trial design.
Proxy Variable
A variable used to "stand in" for another variable. Proxy variables are used when the variable of interest is not available in the data, either because it was not collected in the data or because it was too difficult to measure in a survey or interview.