Experiment in AP Psychology

In AP Psychology, an experiment is a research method in which the researcher manipulates an independent variable, randomly assigns participants to conditions, and measures a dependent variable. It is the only method that can establish a cause-and-effect relationship.

What is Experiment?

An experiment is the research design where the researcher actively changes something and watches what happens. The thing the researcher changes is the independent variable (IV). The thing being measured is the dependent variable (DV). Participants who get the treatment form the experimental group. Participants who don't get it (or get a placebo) form the control group, which gives you a baseline to compare against.

The secret ingredient is random assignment. When every participant has an equal chance of landing in either group, pre-existing differences (motivation, mood, ability) get spread out evenly. That way, if the groups end up different on the DV, the IV is the most likely reason. Think of it as a fair coin flip that protects your conclusion. Without manipulation and random assignment, you don't have a true experiment, and you can't claim cause and effect.

Why Experiment matters in AP® Psychology

Experiments live in Science Practice 2 (Research Methods and Design), a course-wide skill that shows up no matter which unit a question is about. It directly supports AP Psych Science Practice 2.A (determine the type of research design used in a study) and AP Psych Science Practice 2.B (evaluate the appropriate use of research design elements in experimental methodology). It also anchors AP Psych Science Practice 2.C, because you can only judge non-experimental methods well if you know what they lack compared to an experiment. And AP Psych Science Practice 2.D applies too, since experiments that use deception or confederates still have to meet ethical standards. Because research scenarios can be about memory, sleep, social behavior, or anything else, knowing experiments well pays off across the whole exam.

How Experiment connects across the course

Random Assignment and Control Groups (Science Practice 2)

These are what turn a study into a true experiment. The control group tells you what "normal" looks like, and random assignment makes sure the two groups started out equal. If either piece is missing, a difference between groups could come from something other than the IV.

Correlational Studies (Science Practice 2)

A correlational study measures two variables as they naturally occur. An experiment reaches in and changes one. That single move, manipulation plus random assignment, is why experiments can show causation and correlations can't. A survey linking stress to school absences, like the 2018 SAQ scenario, is correlational because nobody assigned anyone a stress level.

Deception, Confederates, and Debriefing (Science Practice 2)

Many classic social psychology experiments only work if participants don't know the real purpose. Researchers may use a confederate, someone secretly working with the experimenter, to create the IV. Ethics rules allow deception only when it's justified, and participants must be debriefed afterward so they learn the truth.

Case Studies (Science Practice 2)

A case study gives deep detail about one person or a small group but controls nothing. It's great for generating hypotheses, which an experiment can then test with many participants and real controls.

Is Experiment on the AP® Psychology exam?

Experiments are a constant on both sections. On MCQs, expect a short study description followed by questions like "Which of the following is the independent variable?", "What is the purpose of the control group?", or "Why can't the researcher conclude causation?" On the free-response section, released SAQs repeatedly hand you a research scenario and ask you to identify or evaluate the design. Examples include 2017 SAQ Q1, 2018 SAQ Q2 (a health survey about stress and absences), 2019 SAQ Q1 (a psychologist observing children who arrived alone or in groups during a door-to-door candy activity), and 2021 SAQ Q2 (a teacher whose group 1 was simply the students who happened to arrive early). The 2021 setup shows a favorite exam move. When groups form by chance circumstances instead of random assignment, you should point out that the groups may differ in other ways, so cause and effect is weakened. Your job is to name the IV and DV operationally, spot whether random assignment happened, identify the control condition, flag confounding variables, and say whether a causal conclusion is justified. Tie every answer to the specific scenario. Generic definitions alone usually don't earn the point.

Experiment vs Correlational Studies

Both can involve two variables and both can produce numbers. The difference is who's in control. In an experiment, the researcher manipulates the IV and randomly assigns participants, so cause and effect can be claimed. In a correlational study, the researcher only measures variables as they already exist, so the result shows a relationship but not causation. A quick test is to ask whether the researcher decided which participants got which condition. If yes, and that decision was random, it's an experiment. If the groups formed on their own, it's not.

Key things to remember about Experiment

• An experiment is the only research method that can establish cause and effect, because the researcher manipulates the independent variable and controls other factors.

• The independent variable is what the researcher changes, and the dependent variable is what the researcher measures to see the effect.

• Random assignment spreads participant differences evenly across groups, and without it you cannot confidently say the IV caused the results.

• The control group provides a baseline, so any difference in the experimental group can be traced back to the treatment.

• Groups that form on their own, like students who happen to arrive early, are not randomly assigned and can introduce confounding variables.

• Experiments that use deception or confederates must still follow ethical guidelines, including informed consent where possible and debriefing afterward.

Frequently asked questions about Experiment

What is an experiment in AP Psychology?

An experiment is a research method where the researcher manipulates an independent variable, randomly assigns participants to groups, and measures a dependent variable. It falls under Science Practice 2 and is the only design that supports cause-and-effect conclusions.

Is a study an experiment if it has two groups?

No, not automatically. A true experiment needs the researcher to manipulate the IV and randomly assign participants. In 2021 SAQ Q2, group 1 was made of students who happened to arrive early, so the groups weren't randomly assigned and a causal conclusion is weakened.

What's the difference between an experiment and a correlational study?

An experiment manipulates a variable and uses random assignment, so it can show causation. A correlational study only measures variables as they naturally occur, like a survey linking stress to absences, so it shows a relationship but not cause and effect.

Why is random assignment so important in an experiment?

Random assignment gives every participant an equal chance of being in any group, which balances out differences like personality or ability. That way, a difference in the dependent variable can be credited to the independent variable instead of to who happened to be in each group.

Is the experiment on the AP Psych exam?

Yes, constantly. Research design is a course-wide science practice, and released SAQs from 2017, 2018, 2019, and 2021 all built questions around research scenarios where you identify the design, variables, or flaws.