Experiment? Correlation? What’s the Difference?

Rob McEntarffer taught English, psychology, and philosophy for thirteen years at Lincoln Southeast High School in Lincoln, NE. He has degrees in educational measurement and teaching, learning, and teacher education from the University of Nebraska, Lincoln and currently works as an assessment/evaluation specialist. He is a
coauthor of the Student Study and Retrieval Guide for Psychology in Your Life, Fourth High School Edition.

Rob McEntarffer
Image Credit: Rob McEntarffer

As we teach students about psychological research, one of the most important aspects we want them to understand is that psychological researchers make deliberate decisions at the beginning of any study. For example, which research method will they use to gather data about a research question? Psychological researchers use a wide variety of methods to investigate thinking and behavior. It is essential that students understand the crucial differences between experimental and correlational methods. These are two of the most common and important methods psychologists use in research studies, but they use them for very different purposes, and each method helps researchers develop different conclusions.

When students read about a research study, one of the first critical thinking questions they should ask is: is this an experiment or a correlational study? There are other possibilities (naturalistic observation, case study, etc.), but many (probably most!) of the studies students will encounter are either experimental or correlational.

A table like the one below can help students differentiate between experimental and correlational research:

Question: Does the study involve at least two different groups of people who make up at least one “experimental” and one “control” group?Question: Are the researchers measuring two things about one group of people (usually a large group of people)?
If the answer is yes, this is probably an experiment!If the answer is yes, this is probably a correlational study!
Explanation: In any experiment, the researcher is trying to figure out whether changing one variable (the independent variable) impacts another variable (the dependent variable). Researchers try to make sure that the ONLY difference between the experimental and control group is the independent variable, so that they can be sure that changing the independent variable causes the change in the dependent variable.Explanation: The goal of a correlational study is to measure two factors to determine how they are related. Usually correlational studies involve a large group of people. Researchers measure the two factors about each person and analyze the results. If the two factors are correlated, they will notice that one factor is high when the other factor is high (positive correlation), or one factor is low when the other factor is high (negative correlation), or that they don’t see a relationship at all (no correlation).
Example: A researcher wants to figure out if students who use a specific studying technique learn more. The researcher randomly divides a large group of students into two groups. One group uses the targeted study technique and the other group doesn’t.Example: A researcher wants to find out if students who get good grades study for a longer amount of time. The researcher surveys all the students in the school, measuring their final grades and the amount of time they spent studying.

After students understand the conceptual differences between experimental and correlational studies, a useful next step is to discuss the “powers” of each research method. Since experimental and correlational studies are used for different purposes, researchers use them to come up with different kinds of conclusions based on their data. Students can think of these conclusions as different “superpowers” of each research method:

Experimental method superpower: Cause and effect!Correlational method superpower: Prediction!
Explanation: Since researchers carefully change only one variable (the independent variable) between the experimental and the control group, if there is a difference in the outcome variable (the dependent variable) between the experimental and the control group, they can conclude that the independent variable CAUSES a change in the dependent variable.Explanation: Once researchers gather enough data about their two factors, they can be confident about the patterns they see. If they find a correlation, they can accurately predict what one factor will be based on the level of the other factor.

Students can use their new knowledge of these “superpowers” to think critically about claims made based on research studies. This is why we teach students that “Correlation doesn’t equal causation.” Students need to make sure they don’t mix up the kinds of conclusions that can be made based on correlations (predictive relationships between variables) with the inferences that can be made from experiments (causal relationships between variables). As students read psychological research, they should note that researchers are careful when they discuss what their studies mean: when they discuss experiments, they will use words like “cause,” “change,” or “impact” when they talk about the difference between the experimental and control group. When researchers talk about correlational studies, they will use words like “relationship,” “predict,” or “associated with” as they talk about the two factors that are correlated.

There are plenty of other details we could teach students about the different variations on the typical two group experimental methods. There are ways to use only one group of people and include the independent variable at specific points and then remove it (within subjects design), and technically true experiments have to involve random assignment to groups (experiments that don’t involve random assignment are called “quasi-experiments”). There are also more complex correlational studies; often survey studies involve multiple different correlations within the different factors measured in longer surveys.

But as introductory psychology teachers, we probably need to focus on helping students understand and apply these basic differences between experimental and correlational studies, and the “superpowers” of each kind of research method. This focus provides our students with the basic tools they need to think critically about experimental and correlational research studies.

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