- a posteriori comparison
- See
*post hoc comparison.*

- a priori comparison
- See
*planned comparison.*

- ABA design
- See
*reversal design.*

- abscissa
- The
*x*-axis on a graph.

- abstract
- A brief description of a research study that appears at the
beginning of the paper and is included in abstract journals,
such as
*Psychological Abstracts*.

- abstract number system
- The commonly used number system with its well-defined rules
and characteristics, including identity, magnitude, equal
intervals, and true zero.

- accessible population
- That subset of a target population that is available to the
researcher and from which the sample is drawn.

- ad hoc sample
- Sample of participants drawn from an accessible population.
Characteristics of the ad hoc sample must be described to define
the limits of generalizability.

- all-or-none bias
- The tendency to see statements as either true or false when
they are actually probabilistic. Virtually all scientific
theories are probabilistic in that they correctly predict what
will happen a percentage of the time.

- alpha level
- Level of Type I error (the probability of rejecting the null
hypothesis when the null hypotheses is true).

- Analysis of Covariance (ANCOVA)
- Statistical procedure similar to analysis of variance, used
to evaluate whether two or more groups have different population
means. Analysis of covariance statistically removes the effects
of extraneous variables on the dependent variable and hence
increases the power of the statistical test.

- analysis of variance (ANOVA)
- Statistical procedure used to analyze for mean differences
between two or more groups. ANOVAs compare the variability
between groups with the variability within groups. Many
variations of analysis of variance are possible, including
repeated measures ANOVAs and factorial ANOVAs.

- ANOVA summary table
- Table that organizes the results of an analysis of variance
computation. For each source of variation, the appropriate
degrees of freedom, sums of squares, mean squares, and
*F*-ratios are listed. (Examples are given in Chapters 10 to 12.)

- apparatus subsection
- That section of a research report in which physical aspects
of the study (apparatus, measuring instruments, etc.) are
described.

- applied psychology
- Any use of psychological principles, theories, or
technologies to deal with existing problems or concerns. Applied
psychology refers to research that is specifically aimed at
understanding and correcting problems faced by people.

- applied research
- Research to provide solutions to practical problems. Applied
research is contrasted with
*basic research*.

- archival records
- Any source of data (such as census data) for events that
have already occurred.

- artifact
- Any apparent effect of a major conceptual variable that is
actually the result of an uncontrolled confounding variable.
Artifacts threaten the validity of research.

- artificial intelligence
- Machines that are designed to evaluate and respond to
situations. Most artificial intelligence machines are
computer-based, and many of them have achieved remarkable levels
of performance in specific areas.

- assent
- Agreeing to participate when one is unable to give legal
consent.

- association
- Relationship or correlation.

- assumptions (of science)
- Basic tenets that form the basis for more complex scientific
theory and research.

- attrition
- Potential confounding variable in research. Attrition is the
loss of participants before or during the research. The
participants who remain may not be representative of the
population. Hence, conclusions drawn may not generalize to the
entire population.

- authority
- A way of acquiring knowledge. New ideas are accepted as
valid because some respected authority has declared the idea to
be true.

- automation
- Use of equipment to conduct most or all aspects of
presenting stimuli and recording participants' responses.
Automation reduces the work, increases precision in data
gathering, and minimizes experimenter bias.

- average deviation
- The sum of the deviations from the mean divided by the
number of scores.