Department of Psychology

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Additive rule
The sum of the probabilities of separate (mutually exclusive) events determine the probability that any event will occur

Alpha
The set (and acceptable) probability that the null hypothesis is true. Psychologists typically set their alpha at .05. An alpha of less than .05 can be interpreted to mean that the differences are not due to sampling error.

Alternative hypothesis
The assumption that there is a difference between groups, because the statistical difference is great enough to offset any chance variation due to sampling errors. Can be directional or non-directional

Analysis of variance
a test that measures the difference between the means of two or more groups

Before and after
Designs with a pretest and posttest but no separate control group--the subject is its own control based on previous behavior

Bimodal
A frequency curve in which there are two modes, resulting in a "dip" in the middle of the graph. The mean and median fall between the two modes

Case study
An intensive study of one subject

Category
An aspect of a variable--e.g. "male" and "female" if the variable is gender

Causal relationship
a relationship between two variables in which one variable causes the other. This relationship is hard to discover with absolute certainty

Central limit theorem
states: The means of samples drawn from a population will be distributed normally

Central tendency
The term used to describe what the centerpoint of the data is

Chi Square one-way
a test that determines whether there is a meaningful relationship between two nominal variables

Chi Square two-way
a test that determines whether there is a meaningful relationship between two nominal variables that each have more than one level

Cluster sampling
The researcher subdivides an expansive area into smaller units and then a subset of the identified clusters is randomly selected. Subjects within clusters are related in some meaningful way.

Concurrent validity
"The extent to which an independent measure of an already measured trait shows similar results to the first/former test. " The extent to which an independent measure of an already measured trait shows similar results to the first/former test.

Conditional probability
The probability that one event will occur given that some other event has occurred.

Confidence interval
The range under which a specified percentage of cases fall. For example, if a mean has a standard deviation of 1, there is 68% confidence that a score will fall within one standard deviation of the mean.

Construct validity
The construct is a collection of related behaviors that are associated in a meaningful way. A judgemental-empirical approach to testing a previously hypothesized relationship.

Contamination
When the comparison group is in some way affected by, or affects, the treatment group, causing an increase of efforts. Also known as compensatory rivalry or the John Henry effect.

Content validity
The extent to which the scope of a test is broad enough to encompass all aspects of the definiton of what you're measuring--the definition contains the concept

Criterion validity
The extent to which the test "agrees" with previous tests that measure the same concept

Cumulative frequency
How many data points are in and below a given score interval

Dependent variable
The output or response that is the result of changes to the independent variable.Generally speaking, the tester has no control over this

Deviation
How far a score is from the mean

Direct relationship
High values on one variable tend to occur with high values on the other variable
Directional
A hypothesis which predicts that one group will have a higher or lower result than another group

Discriminant validity
The extent to which the test does not measure anything that is not in its intended concept--for example, some argue that an IQ test measures achievement, not potential, thus it measures the wrong concept

Ethnographic research
An intensive (case) study of a group.

Experimenter bias
Expectations of an outcome may inadvertently influence participant or cause the experimenter to view data in a different way.

Face validity
When an instrument appears to measure what it is supposed to measure.

Frequency
how often a score occurs in a set of data

Frequency polygon
The resulting shape from finding the midpoints in a frequency distribution (histogram) and then connecting the points with a line.

Hawthorne effect
When members of the treatment group change in terms of the dependent variable because their participation in the study makes them feel special—so they act differently, regardless of the treatment.

Histogram
a graph of the frequency of a data set

Historical research
An examination of data to understand the past.

History
Outside events that may occur during the course of the experiment or between repeated measures of the dependent variable. A threat to Internal Validity.

Independent variable
The input or stimulus that is manipulated or observed. It is controlled by the tester

Instrumentation
The reliability of the instrument may change in calibration (if using a measuring device) or from change in human ability to measure differences (due to fatigue, experience, etc). A threat to internal validity

Interaction
Two or more threats to internal validity can interact (selection-maturation, selection-history, etc.).

Interaction effect
What is seen when different levels of the IV and DV interact to give different results for different groups

Interobserver reliability
There are consistent results among different testers or coders who are rating the same information.

Inter-quartile range
The middle 50% of the scores, as determined by the median. A good measure of variability if there are many outliers

Interval measurement
uses a set scale: the researcher knows the order of the data points both relative to each other and in absolute terms. The zero has no meaning, but there is a meaningful average

Inverse relationship
High values on one variable tend to occur with low values on the other variable

Level
An aspect of a variable--e.g. "male" and "female" if the variable is gender 

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