BI 2.1.3

Variance

A parameter of variability of data about the mean score, which is the square of the standard deviation.

Standard Deviation

A parameter of variability of data about the mean score.

Degrees of Freedom

A parameter that is equal to the number of observations or groups in a study minus some value(s) that limit the observations' or groups' freedom to vary.

Causal Relationship

A relationship in which one factor can be said to be the cause of another. This relationship can be tested with controlled experiments.

Retrospective Cohort Study

A research study in which the medical records of groups of individuals who are alike in many ways but differ by a certain characteristic (for example, female nurses who smoke and those who do not smoke) are compared for a particular outcome (such as lung cancer). This is also called a historic cohort study.

Prospective Cohort Study

A research study that follows over time groups of individuals who are alike in many ways but differ by a certain characteristic (for example, female nurses who smoke and those who do not smoke) and compares them for a particular outcome (such as lung cancer).

Cross Sectional Study

A research study that involves a single observation (such as an interview or questionnaire), which may be useful to determine how variables affect each other at the same time and period.

'Student's' t-test

A statistical test to determine whether there are significant differences between two independent groups' means being tested on the same dependent variable.

Experimental Study

A type of study in which all variable measurements and manipulations are under research control. The goal is to control the environment in such a way that manipulation of the independent variable yields a direct, corresponding change in the dependent variable.

Observational Study

A type of study in which individuals are observed or certain outcomes are measured. No attempt is made to affect the outcome (for example, no treatment is given).

Confounding Variable

A variable that is not accounted for in the experimental design, varies systematically with the dependent variable, and prevents a clear interpretation of the effect of the independent variable on the dependent variable.

Statistically Nonsignificant

Describes a mathematical measure of difference between groups. The difference is said to be statistically nonsignificant if it is not greater than what might be expected to happen by chance alone. This means that there is not enough evidence to reject the null hypothesis, and the results of the experiment are attributed to chance.

Statistically Significant

Describes a mathematical measure of difference between groups. The difference is said to be statistically significant if it is greater than what might be expected to happen by chance alone. This means that there is enough evidence to reject the null hypothesis and the results of the experiment are not attributed to chance.

Dependent Variable

In an experiment, a measure expected to vary across different levels of the independent variable. It is also called the response variable.

Independent Variable

In an experiment, the variable that the experimenter manipulated. It may also be called the treatment variable.

Double-Blind Experiment

Neither the participant nor the treatment allocator knows whether the participant has been allocated to a treatment or control group.

Triple-Blind Experiment

Neither the participant, the treatment allocator, nor the response gatherer knows whether the participant has been allocated to a treatment or control group.

Mean

The arithmetical average of a group of scores.

Median

The center of a distribution of score, such that half of the scores are above that number and half of the score in the distribution are below that number.

Correlation

The degree of relationship between two variables.

Validity

The extent to which the result cannot be attributed to error in sampling or measurement.

Reliability

The likelihood that the same experiment will yield identical results if performed many times over, by the same team or by other teams.

Mode

The most frequently occurring score.

Controlled, Randomized Experiment

The most powerful experimental design because it allows for the inference of causation. The participants are randomly chosen from a population and randomly assigned to one of two (or more) groups, usually an experimental group that receives some treatment and a control group that receives a placebo.

Single-Blind Experiment

The participant does not know whether he or she has been allocated to a treatment or control group.

P Value

The probability of results of the experiment being attributed to chance.

Replication

The repeated ability to duplicate the results of a scientific experiment by different experimenters, which helps establish a hypothesis's usefulness (or non-usefulness in cases where findings cannot be replicated).

Null Hypothesis

The starting point in scientific research where the experimenter assumes there is no effect of the treatment or no relationship between the two variables.

Alternative Hypothesis

What the experimenter thinks may be true or wishes to be true before he or she begins an experiment. Also called the research hypothesis.

Directional Alternative Hypothesis

Where the alternative hypothesis is specifically stated beforehand; for example, Group 1's mean is greater than Group 2's mean. This is also called a one-tailed test of significance.

Nondirectional Alternative Hypothesis

Where the null hypothesis will be rejected if either Group 1's mean exceeds Group 2's mean, or vice versa, or where the null hypothesis will be rejected if a relationship exists, regardless of its nature. This is also called a two-tailed test of significance.