Glossary Items Starting with "P"

p value: The probability of obtaining the statistic (e.g., t or F) or a larger statistic by chance if the null hypothesis is true. Statistical analysis programs routinely compute p values in addition to the test statistic.
panel design: See longitudinal design.
parallel distributed processing (PDP): See connectionist models.
parametric statistics: Inferential statistical procedures that rely on sample statistics to draw inferences about such population parameters as the mean and variance.
parsimony: The guiding principle in science that a simple theory is preferred over a more complex theory if both theories explain the data equally well.
partial correlation: A correlation between two variables in which the effects of a third variable are statistically removed from one of the two original variables before computing the correlation.
partial counterbalancing: Control procedure in which the order of presentation of conditions is randomly selected for each participant.
participant assignment: Procedure of assigning participants to a group or condition. Participant assignment may be made randomly (experimental research) or on the basis of preexisting variables (differential research).
participant effects: See subject effects.
participant observer: Any researcher gathering data in a setting in which the researcher is an active part. Participant observation tends to be less obtrusive than other observational procedures. However, the possibility for experimenter reactivity is high.
participant selection: The procedures by which potential participants for a research study are identified. Participant selection affects external validity. Participant selection may include random sampling, stratified random sampling, or designation of an ad hoc sample.
participant variable: Synonymous with organismic variable.
participants at risk: Participants involved in a research project that poses some potential risk to them. When participants are at risk, the researcher is responsible for informing them of the risks and minimizing those risks.
participants' rights: Guarantees of proper treatment that participants can justly expect in research.
participants subsection: That section of a research report in which the participants and the methods of participant selection are described.
partitioned: In an ANOVA calculation, the total sum of squares is separated (partitioned) into the between-groups sum of squares and the within-groups sum of squares.
path analysis: A procedure that seeks to unravel causal links between variables from correlational data by hypothesizing detailed causal models and factoring the correlation matrix to see how closely the pattern of observed relationships fits the hypothesized causal model.
Pearson product-moment correlation: Index of the degree of linear relationship between two variables in which each variable represents score data.
percent agreement: A measure of interrater reliability in which the percentage of times the raters agree is computed.
percentile: Normative score that converts the raw score earned by a participant into a number from 0 to 100. This number reflects the percentage of participants who score lower.
percentile rank: See percentile.
perfect correlation: Correlation of a +1.00 or a -1.00. When two variables are perfectly correlated, knowing the score on one variable permits perfect prediction of the score on the other. In a scatter plot, a perfect correlation is shown by all points falling on a straight line (but not a horizontal or vertical line).
personal computer: A self-contained computer dedicated to serving the needs of a single user. Most personal computers are either desktop models, with a keyboard, screen, and a box housing the computer and accessories, or notebook models, in which all critical elements are built into a small, portable package. Personal computers are often contrasted with mainframe computers.
phases of research: The stages of a research project. These phases are idea-generating, problem-definition, procedures-design, observation, data-analysis, interpretation, and communication.
Phi: A measure of relationship between two nominal variables.
phylogenic continuity: An evolutionary concept about the continuity of structure and function between humans and other animals.
pilot testing: Evaluating for feasibility prior to using a measure or procedure in your research project.
placebo: An inert or innocuous control treatment or medication that appears to the participant to be exactly like the experimental treatment.
placebo effect: Any observed improvement due to a sham treatment. Placebo effects are probably the result of participants' expectations for treatment effectiveness.
planned comparison: Sometimes called a contrast; a specific comparison of mean performance between groups in a research study. Planned comparisons must be planned before data collection and should be based on theoretical considerations.
population: Any clearly defined set of objects or events (people, occurrences, animals, etc.). Populations usually represent all events in a particular class.
population parameters: Any summary statistic computed on the entire population.
positive correlation: Relationship between two variables in which one variable increases as the other variable increases.
positive practice effects: Enhancement of performance on a dependent measure that results from previous exposure to the measurement procedure.
positively skewed: Distribution in which scores are concentrated near the bottom of the scale.
post hoc comparison: Secondary analyses that evaluate relationships between variables not specifically hypothesized by the researcher prior to the study.
post hoc test: See post hoc comparison.
power: See power of a statistical test.
power analysis: Procedures that determine the power of a statistical test or research procedure to detect group differences if those differences exist.
power of a statistical test: Ability of an inferential statistical procedure to detect differences between groups when such differences actually exist.
practical significance: Often contrasted with statistical significance. Practical significance refers to whether the observed difference between groups or conditions is large enough to have a meaningful impact on the participant.
practice effects: Any change in performance on a dependent measure that results from previous exposure to the measurement procedure.
precision versus relevance problem: The concern that higher-constraint laboratory research may be less relevant than lower-constraint naturalistic research and, conversely, that lower-constraint research may be unacceptably imprecise.
pre-data check: See research design checklist.
predictor: See predictor measure.
predictor measure: The variable in regression that is used to predict the scores on the criterion measure. For example, a test score (the predictor measure) might be used to predict future performance in a job.
preexisting variable: Any characteristic of the participants that existed prior to the research study. If preexisting variables are not controlled, they can confound the results of a study. Preexisting variables are particularly problematic in differential research.
pretest-posttest design: Set of research designs in which participants are tested at two points in time, before and after the administration of the independent variable.
pretest-posttest, natural control-group design: Nonexperimental research design in which preexisting groups are measured before and after the manipulation of an independent variable. These naturally occurring groups are assigned to different levels of the independent variable.
principle of initial equivalence: The necessity of having experimental groups equal on the dependent measure before any manipulation occurs.
probability: The ratio of specific events to the total number of possible events. For example, the probability of rolling a 5 on each role of a die is 1/6.
probability sampling: A sampling procedure in which all participants have an equal probability of being selected and the selection of any participant does not change the probability of selecting any other participant. Often contrasted with nonprobability sampling.
probe: Refers to the process of comparing the mean performance among groups of participants in a research study to see which groups are statistically different from one another.
problem-definition phase of research: Research phase in which vague and general research ideas are converted into precise questions to be studied.
procedures-design phase of research: Research phase in which the specific procedures to be used in the gathering and analyzing the data are developed.
procedures subsection: The section of a research report that describes how the study was carried out.
process of inquiry: The perspective taken by this text that views research as a dynamic process focused on formulating questions and systematically answering those questions through carefully controlled studies.
program evaluation research: Specific area of field research for evaluating the effectiveness of a program in meeting its stated goals.
properties of the abstract number system: See abstract number system.
pseudoscience: Popular distortions of scientific knowledge and procedures that appear on the surface to be scientific but lack critical scientific procedures. Some fields, such as astrology, extra-sensory perception, the study of alien abductions, and medical quackery, have traditionally relied on pseudoscience to make them appear legitimate.
psychoanalysis: The psychological treatment approach that is based on the psychodynamic theories of Freud and his followers.
psychodynamic theory: Freud’s theory that behavior is shaped by a complex interaction of internal forces and the constraints of the external environment.
psychology: Scientific study of the behavior of organisms.
psychophysics: One of the earliest approaches to the study of behavior. Psychophysics involves the presentation of precise stimuli to participants under controlled conditions and the recording of participants' responses.
pure research: Another term for basic or fundamental research (see basic research).