*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*).