
Case study
an observation technique in which one person is studied in depth in the hope of revealing universal principles.

Cohortsequential study
A research method in which a cross section of the population is chosen and then each cohort is followed for a short period of time.

Control group
in an experiment, the group that is not exposed to the treatment; contrasts with the experimental group and serves as a comparison for evaluation the effect of the treatment.

Correlation
a measure of the extent to which two factors vary together, and thus of how well either factor predicts the other.

Correlational study
A type of research that is mainly statistical in nature. Correlation studies determine the relationship (or correlation) between two variables.

Crosssectional study
a study in which people of different ages are compare with one another.

Dependent variable
the outcome factor; the variable that may change in response to manipulations of the independent variable

Doubleblind study
an experimental procedure in which both the research participants and the research staff are ignorant about whether the research participants have received the treatment or a placebo. Commonly used in drugevaluation studies.

Expectancy bias
The researcher allowing his or her expectations to affect the outcome of a study.

Experimental group
in an experiment, the group that is exposed to the treatment, that is, to one version of the independent variable

Experimental method
An approach to research that relies on sensory experience and observation as research data

Frequency distribution
A summary chart, showing how frequently each of the various scores in a set of data occurs.

Histogram
A bar graph depicting a frequency distribution. The height of the bars indicates the frequency of a group of scores.

Hypothesis
a testable prediction, often implied by a theory.

Illusory correlation
the perception of a relationship where none exists

Independent variable
the experimental factor that is manipulated; the variable whose effect is being studied.

Inferential statistics
Statistical techniques(based on probability theory) used to assess whether the results of a study are reliable or whether they might be simply the result of chance. Inferential statistics are often used to determine whether two or more groups are essentially the same or different.

Longitudinal study
research in which the same people are restudied and retested over a long period.

Mean
the arithmetic average of a distribution, obtained by adding the scores and then dividing by the number of scores.

Median
the middle score in a distribution; half the scores are above it and half are below it.

Mode
the most frequently occurring score(s) in a distribution

Naturalistic observation
observing and recording behavior in naturally occurring situations without trying to manipulate and control the situation.

Normal distribution
a symmetrical, bellshaped curve that describes the distribution of many types of data; most scores fall near the mean and fewer and fewer near the extremes.

Operational definition
a statement of the procedures used to define research variables. For example: human intelligence may be operationally defined as what an intelligence test measures.

Placebo
effect experimental results caused by expectations alone; any effect on behavior caused by the administration of an inert substance or condition, which the recipient assumes is an active agent.

Population
all the cases in a group being studied, from which samples may be drawn

Random assignment
assigning participants to experimental and control groups by chance, thus minimizing preexisting differences between those assigned to the different groups.

Random sample
a sample that fairly represents a population because each member has an equal chance of inclusion

Range
the difference between the highest and lowest scores in a distribution

Replication
repeating the essence of a research study, usually with different participants in different situations, to see whether the basic finding extends to other participants and circumstances.

Representative sample
A sample obtained in such a way that it reflects the distribution of important variables in the larger population in which the researchers are interested  variables such as age, income level, ethnicity, and geographic distribution

Scientific Method
A fivestep process for empirical investigation of a hypothesis under conditions designed to control biases and subjective judgments.

Significant difference
Psychologists accept a difference between the groups as "real", or significant, when the probability that it might be due to an atypical sample drawn by chance is less than 5 in 100 (indicated by the notation p < .05)

Standard deviation
a computed measure of how much scores vary around the mean score.

Survey research
a technique for ascertaining the selfreported attitudes or behaviors of a particular group, usually by questioning a representative, random sample of the group.

Theory
 an explanation using an integrated set of principles that organizes observations and predicts behaviors or events.

