A research method in which an investigator manipulates one or more factors (independent variables) to observe the effect on some behavior or mental process (dependent variable). Experiments aim to establish cause-and-effect relationships.
Independent Variable
The factor that is manipulated by the experimenter. Its effect on the dependent variable is the focus of the study.
Dependent Variable
The factor that is measured; the variable that may change when the independent variable is manipulated.
Confounding Variable
A factor other than the independent variable that might produce an effect in an experiment, thus confusing the interpretation of the results.
Random Assignment
Assigning participants to experimental and control groups by chance, thus minimizing preexisting differences between the groups.
Case Study
An in-depth analysis of a single individual, group, event, or community. Case studies are often used in exploratory research.
Correlation
A measure of the extent to which two variables are related, indicating how well one variable predicts the other. It does not imply causation.
Meta-Analysis
A statistical procedure that combines data from multiple studies to arrive at a single conclusion. It provides a more comprehensive analysis of a research topic.
Naturalistic Observation
Observing and recording behavior in naturally occurring situations without trying to manipulate or control the situation.
Survey Technique
A method of gathering data by asking participants questions, typically through questionnaires or interviews.
Hypothesis
A testable prediction, often implied by a theory. It specifies a relationship between variables.
Falsifiability
The principle that a scientific theory must be stated in a way that it can be disproven. A theory is not useful if it cannot be tested and potentially shown to be false.
Operational Definitions
A carefully worded statement of the exact procedures used in a research study. This allows for replication and clear understanding of the variables.
Replication
Repeating the essence of a research study, usually with different participants in different situations, to see whether the basic finding extends to other circumstances.
Participants
Individuals taking part in a research study.
Sample
A subset of the population selected for study.
Population
The entire group of individuals from which the sample is drawn.
Variation
The extent to which individual scores differ in a data set.
Representative Sample
A sample that accurately reflects the characteristics of the population as a whole.
Random Sampling
A method of selecting participants for a study in which each member of the population has an equal chance of being included.
Convenience Sampling
A method of selecting participants for a study based on their accessibility and availability.
Sampling Bias
A flaw in the sampling process that results in a sample that is not representative of the population.
Generalizability
The extent to which the findings of a study can be applied to other populations, settings, or conditions.
Experimental Group
In an experiment, the group exposed to the treatment, that is, to one version of the independent variable.
Control Group
In an experiment, the group not exposed to the treatment; serves as a comparison for evaluating the effect of the treatment.
Placebo
A substance or treatment with no active therapeutic effect, given to a control group to control for the effects of expectation.
Single-Blind Procedures
An experimental procedure in which either the participants or the experimenters (but not both) are unaware of which participants are in the experimental and control groups.
Double-Blind Procedures
An experimental procedure in which both the participants and the experimenters are unaware of which participants are in the experimental and control groups. This minimizes experimenter bias.
Confirmation Bias
The tendency to search for information that supports one's preconceptions and to ignore or distort contradictory evidence.
Hindsight Bias
The tendency to believe, after learning an outcome, that one would have foreseen it. Also known as the "I-knew-it-all-along" phenomenon.
Overconfidence
The tendency to be more confident than correct – to overestimate the accuracy of one's beliefs and judgments.
Experimenter Bias
The influence of the experimenter's expectations on the outcome of the study.
Social Desirability Bias
The tendency of participants to respond in a way that they believe will be viewed favorably by others.
Qualitative
Data in the form of words, descriptions, or narratives.
Quantitative
Data in the form of numbers.
Structured Interviews
An interview in which a set of standardized questions are asked in a specific order.
Likert Scales
A type of rating scale that measures attitudes or opinions, typically with a range of responses (e.g., strongly agree to strongly disagree).
Peer Review
A process in which scientific work is evaluated by other experts in the field.
Directionality Problem
A situation in which it is unclear which variable in a correlation came first, thus making it difficult to infer causation. Does A cause B, or does B cause A?
Third Variable Problem
A situation in which a third, unmeasured variable is responsible for the correlation between two other variables.
Self-Report Bias
Inaccuracies or distortions in participants' reports of their own thoughts, feelings, or behaviors.
Institutional Review
A committee that reviews research proposals to ensure that they comply with ethical guidelines.
Informed Consent
An ethical principle requiring that research participants be told enough to enable them to choose whether they wish to participate.
Informed Assent
Agreement to participate in research by a person who is not legally able to give informed consent (e.g., a child).
Protect from Harm
An ethical principle requiring researchers to minimize the risks of physical or psychological harm to participants.
Confidentiality
The assurance that participants' personal information will not be disclosed to others.
Anonymity
The assurance that participants' identities will not be known to the researchers.
Deception
Misleading participants about the true purpose of a study or the events that will actually transpire.
Confederate
An actor who participates in a research study under the direction of the investigator, without the participants' knowledge.
Debriefing
The post-experimental explanation of a study, including its purpose and any deceptions, to its participants.
Measures of Central Tendency
A single number that represents the typical or average value in a data set. Examples include mean, median, and mode.
Measures of Variation
Statistics that describe the degree of dispersion in a data set. Examples include range and standard deviation.
Percentile Rank
The percentage of scores in a distribution that are equal to or lower than a given score.
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.
Range
The difference between the highest and lowest scores in a distribution.
Standard Deviation
A computed measure of how much scores vary around the mean score. Represented by the symbol σ.
Formula: σ=N∑(x<em>i−μ)2, where σ is the standard deviation, x</em>i represents each value in the population, μ is the mean of the population, and N is the number of values in the population.
Normal Curve
A symmetrical, bell-shaped curve that describes the distribution of many types of data; most scores fall near the mean (68 percent fall within one standard deviation of it) and fewer and fewer near the extremes. Also commonly referred to as a normal distribution.
Normal Curve Score Distribution
A distribution of scores that follows a normal curve pattern. Scores are symmetrically distributed around the mean.
Skewness
A measure of the asymmetry of a distribution. A distribution can be positively skewed (tail to the right) or negatively skewed (tail to the left).
Bimodal Distribution
A distribution with two distinct peaks, indicating two common scores or ranges of scores.
Regression Toward the Mean
The tendency for extreme or unusual scores to fall back (regress) toward their average.
Scatterplot
A graphed cluster of dots, each of which represents the values of two variables. The slope of the points suggests the direction of the relationship between the two variables. The amount of scatter suggests the strength of the correlation (little scatter indicates high correlation).
Correlation Coefficient
A statistical index of the relationship between two variables (from -1 to +1). Represented by the symbol r.
Effect Size
A measure of the strength of the relationship between two variables or the magnitude of an experimental effect.
Statistical Significance
A statistical statement of how likely it is that an obtained result occurred by chance. Typically, a result is considered statistically significant if the probability of it occurring by chance is less than 5% (p < 0.05).