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Paired Difference Experiment - Experiment in which observations are paired and differences are analyzed.

Parameters - Calculations made when dealing with a population. A parameter is always descriptive, because data is being collected on every element in the set, and therefore no inferences can be drawn.

Parametric Tests - Class of statistical tests used when the variable is measured on at least an interval scale.

Pearson's Correlation (Coefficient of Correlation) - Measures the degree and direction of the linear relationship between two variables. Takes on values between -1 and 1. A perfect (strongest) correlation is indicated by -1 or 1, and 0 indicates no correlation (weakest).

Percentile Ranking - The rank or percentile rank of a particular score is defined as the percentage of individuals in the distribution with scores at or below the particular value. When a score is identified by its percentile rank, the score is called a percentile.

Performance of Objective Tasks - Method of assessing attitudes that rests on the presumption that a subject's performance of a specific assigned task will depend on the person's attitude.

Personal Interview - Direct, face-to-face conversation between a representative of the research organization (the interviewer) and a respondent, or interviewee.

Population - Refers to the total group of elements that conforms to some designated specifications.
Positively Skewed Distribution - Refers to a distribution where the mean has the largest value, followed by the median, then the mode. The graph of this type of distribution will show most of the scores piled up on the left hand side of the distribution, with a tail tapering to the right.

Power - Function associated with a statistical test indicating the probability of correctly rejecting a false null hypothesis.

Pragmatic Validity - Also called predictive validity or criterion-related validity. Approach to validation of a measure based on the usefulness of the measuring instrument as a predictor of some other characteristic or behavior of the individual.

Precision - Desired size of the estimating interval when the problem is one of estimating a population parameter.
Pretest - Use of a questionnaire on a trial basis in a small pilot study to determine how well the questionnaire works.

Primary Data - Information collected specifically for the purpose of the investigation at hand.

Primary Source - Originating source of secondary data.

Probability Sample - Sample in which each population element has a known, nonzero chance of being included in the sample.

Proportionate Stratified Sampling - Stratified sample in which the number of observations in the total sample is allocated among the strata in proportion to the relative number of elements in each stratum in the population.