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| Abscissa | The x-axis, or horizontal axis, of a graph. |
| Central tendency | A number that best represents a group of numbers. |
| Class interval | Arbitrarily selected portions of a measurement scale; usually equal. |
| Clinical case history | Records or data from therapeutic situations; used to identify behaviors and suggest problems that need to be studied. |
| Contingency | The concept of causality or dependency; indicates that there is a meaningful relationship between two variables. |
| Control group | In a scientific experiment, the subjects whose responses are used as the basis for comparison; the experimenter compares the responses of the control group with the responses of experimental groups. |
| Correlation | The tendency of two or more variables to vary together (concomitantly); the numerical representation of the relationship between these two variables. |
| Debriefing | At the end of an experiment with humans, informing the subjects about what has happened in a complete and accurate manner. |
| Dependent variable | The measured response in an experiment. |
| Descriptive statistics | Measures or techniques that allow a summary portrayal of collected data. |
| Directional hypothesis | A prediction that a specific change in the conditions of an experiment will result in a particular change in the outcome of the experiment. |
| Double-blind control | An experimental situation in which both the subjects and those who administer the experiment are unaware of how or when the variables are manipulated by the experimenter. |
| Empirical | Refers to anything that is directly observable or measurable. |
| Experimental group | In a scientific experiment, the subjects who respond to an independent variable that is "specially" manipulated by the experimenter; the responses of the experimental group can then be compared with the responses of the control group. |
| Experimental method | A technique involving the controlled comparison of conditions to determine whether the variable investigated affects the results obtained. |
| Extraneous variable | A condition that may affect the outcome of an experiment but is irrelevant to the purposes of the experiment. |
| Frequency distribution | A graphic or tabular representation of the number of cases found in each class interval of a distribution. |
| Frequency polygon | A line graph representing a frequency distribution. |
| Histogram | A bar graph representing a frequency distribution. |
| Hypothesis | A tentative explanation of a relationship or a proposition that a relationship exists; an experiment should be designed so that a hypothesis can be either confirmed or disconfirmed. |
| Independent variable | A condition manipulated by the experimenter; the experimenter manipulates the independent variable to determine the effect of such manipulations on the dependent variable. |
| Inferential statistics | Measures or techniques that allow the analysis or evaluation of relationships existing within a sample of data or between samples of data; can be used to make predictions. |
| Informed consent | In experiments with humans, providing accurate and sufficient information about and obtaining consent to participate in the experiment. |
| Matched sampling | A technique for selecting subjects in which the experimenter makes sure that each group in the experiment contains the same number of subjects who possess certain characteristics which might influence the outcome. |
| Mean (M or barred X) | The average score in a distribution of scores; calculated by summing all the scores and dividing that sum by the number of scores. |
| Median | The middle score in a distribution of scores; the number of scores above and below it are equal, and the score is at the fiftieth percentile. |
| Mode | The score that occurs most frequently in a distribution; there may be more than one mode in a distribution. |
| Naturalistic observation | The careful observation of events not manipulated by the observer. |
| Normal curve | The graphic representation of the normal probability distribution. |
| Normal probability distribution | An idealized distribution based on data collected from large samples; most scores are at or near the mean, with a few scores at the extremes; often used for making statistical inferences. |
| Null hypothesis | A prediction that a specific change in the conditions of an experiment will not result in a change in the outcome. |
| Objectivity | A condition that exists when judgments are free from bias or the influence of personal feeling. |
| Operational definition | Defining a variable by the procedure used to produce or measure it. |
| Ordinate | They y-axis, or vertical axis, of a graph. |
| Percentile | The point below which a given percentage of the total number of scores in a distribution falls. |
| Placebo | A chemically inert material that has the same appearance as an active drug; by analogy, the "placebo effect" occurs in any situation in which subjects believe they are experiencing a manipulation by the experimenter when in fact they are not. |
| Population | The entire group from which samples may be chosen. |
| Random sampling | A technique for selecting subjects in such a way that every potential subject in the population has an equal chance of being chosen. |
| Range | A measure of variability calculated by determining the difference between the highest and lowest scores in a distribution. |
| Regression | Use of the knowledge of a previously obtained correlation and the value of one variable to predict the value of another variable. |
| Sample | A group selected from a population; an attempt should be made to make the sample as representative of the population as possible. |
| Scattergram | The pictorial representation of a correlation. |
| Single-blind control | An experimental situation in which the subjects are unaware of how or when the variables are manipulated by the experimenter. |
| Skew | Occurs when the scores of a distribution occur with greater frequency at one end of a distribution. |
| Standard deviation (SD or lower case sigma) | A measure of variability based on the differences of each score from the mean. |
| Statistics | The discipline that deals with the collection, analysis, interpretation, and presentation of numerical data. |
| Stratified sampling | A technique for selecting subjects in such a way that significant subgroups within the population are reflected accurately in the composition of each group in the experiment. |
| Subjectivity | A condition that exists when judgments are affected by bias, prejudice, or personal feeling. |
| Variability | The extent to which scores are dispersed in a distribution. |
| Variable | A characteristic of anything that can have two or more values. |
