Psychological Statistics Terms and Definitions
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Population
An entire set of objects, individuals, or objects that are being studied.

Sample
A subset or a portion of a population.

Variable
 A characteristic or attribute about each subject of a population or sample that can assume different values.
 (For example, if we measured the reaction time of 10 subjects, then reaction time will be a variable.)

Data
The valuesâ€”measurements or observationsâ€”that the variables can assume. With a singular data point we are referring to the value of the variable associated with one subject from our sample.

Data set
A collection of data values for a variable from each of the subjects or participants in a sample.

Parameter
A measurement that describes a characteristic of a population. A parameter is a value that describes the entire population.

Statistic
A statistic describes a characteristic of a sample. It is a numerical value that summarizes the sample data.

Descriptive Statistics
Procedures for organizing and summarizing data so that the important characteristics can be described and communicated.

Inferential Statistics
Procedures for determining whether sample data represent a particular relationship in the population.

Design
The way in which a study is laid out.

Independent Variable
In an experiment, a variable that is changed or manipulated by the experimenter; a variable hypothesized to cause a change in the dependent variable.

Random Sampling
A method of selecting samples so that all members of the population have the same chance of being selected for a sample.

Measure of Central Tendency
Statistics that summarize the location of a distribution on a variable by indicating where the center of the distribution tends to be located.

Mean
The score located at the mathematical center of a distribution.

Statistical Hypotheses
Two statements that describe the population parameters the sample statistics will represent if the predicted relationship exists or does not exist.

Alternative Hypothesis (Ha)
The hypothesis describing the population parameters that the sample data represent if the predicted relationship does exist.

Null Hypothesis (H0)
The hypothesis describing the population parameters that the sample data represent if the predicted relationship does not exist.

criterion
The probability that defines whether a sample is unlikely to have occurred by chance and thus is unrepresentative of a particular population.

region of rejection
That portion of a sampling distribution containing values considered too unlikely to occur by chance, found in the tail or tails of the distribution.

critical value
The score that marks the inner edge of the region of rejection in a sampling distribution; values that fall beyond it lie in the region of rejection.

zdistribution
The distribution of zscores produced by transforming all raw scores in a distribution into zscores.