Source: General Psychology lecture notes, The Ohio State University
Difficulty: Introductory | Prerequisites: Naturalistic Observation and Correlation notes
Tags: experimental design, independent variable, dependent variable, control group, experimental group, random assignment, double-blind, within-subject, between-groups, placebo effect, PSYCH 1100
The experiment is the only research method that can establish cause and effect, which is why it sits at the centre of the research methods unit. If you have already covered naturalistic observation and correlation, you know those methods describe relationships but cannot prove one thing causes another. The experiment solves that problem by manipulating one variable, controlling everything else, and randomly assigning participants to conditions. This topic introduces the vocabulary (independent variable, dependent variable, control group) and the design features (random assignment, blinding) that make causal conclusions possible.
An experiment manipulates one factor (the independent variable) while holding everything else constant, then measures the outcome (the dependent variable). Random assignment and blinding (single or double) protect against bias. The two main designs are between-groups (different people in each condition) and within-subject (same people tested under every condition).
Experiment
A research method in which the researcher manipulates one or more variables, controls extraneous variables, and measures the effect on an outcome variable, in order to establish a cause-and-effect relationship.
Think of it as: the researcher deliberately changes something and watches what happens, while keeping everything else the same.
Independent variable (IV)
The variable the researcher deliberately manipulates or varies across conditions. It is the presumed cause.
In simple terms, this means: the thing the researcher changes on purpose.
Dependent variable (DV)
The variable the researcher measures as the outcome. It is the presumed effect.
In simple terms, this means: the thing the researcher measures to see if the manipulation made a difference.
Experimental group
The group of participants that receives the treatment or manipulation (the active condition of the independent variable).
Think of it as: the group that gets the "thing" you are testing.
Control group
The group of participants that does not receive the treatment. It serves as a baseline for comparison.
Think of it as: the group left alone so you have something to compare against.
Random assignment
The process of placing participants into experimental or control groups using chance (e.g., a coin flip), so that each person has an equal probability of ending up in either condition.
In simple terms, this means: every participant has a fair, random shot at being in any group, which helps ensure the groups start out equivalent.
Double-blind procedure
An experimental setup in which neither the participants nor the researchers who interact with them know which condition each participant is in.
Think of it as: nobody in the room knows who is getting the real treatment, which prevents expectations from tainting the results.
Placebo effect
A measurable change in behaviour or physiology caused by the participant's belief that they are receiving treatment, rather than by the treatment itself.
In simple terms, this means: you feel better (or perform differently) just because you think you got the real thing.
Between-groups design (between-subjects)
An experimental design in which different participants are assigned to each condition of the independent variable.
Think of it as: Group A does one thing, Group B does another, and you compare the two groups.
Within-subject design (repeated measures)
An experimental design in which the same participants experience every condition of the independent variable.
Think of it as: every participant goes through all versions of the experiment, and you compare their performance across conditions.
The basic idea: manipulate the factor you think matters (the IV), control everything else, and measure the outcome (the DV).
If the groups differ only in the IV and you see a difference in the DV, you can attribute that difference to the IV. This is why experiments are the only method that supports causal claims.
Independent variable (IV): what the researcher deliberately varies between conditions.
Dependent variable (DV): what the researcher measures.
Memory trick: the IV is what I Vary; the DV is the Data I collect (what Depends on the IV).
Different participants are placed in different conditions.
Requires at least two groups:
Experimental group: receives the treatment or manipulation.
Control group: does not receive the treatment; serves as the comparison baseline.
Example from the lecture: students were given text in either a plain font (control) or a decorative/hard-to-read font (experimental), then tested on memory. The IV was font type; the DV was the number of correct answers on the memory test.
The same participants serve in every condition.
Advantage: eliminates individual differences between groups, because each person is compared against themselves.
Disadvantage: order effects (fatigue, practice) can contaminate results, so researchers often counterbalance the order of conditions.
Random assignment
Each participant has an equal chance of being placed in any condition.
Purpose: makes the groups equivalent at the start, so any difference at the end can be attributed to the IV rather than to pre-existing differences between participants.
Double-blind procedure
Neither the participants nor the experimenters who interact with them know which condition a participant is in.
Prevents the placebo effect (participants' expectations influencing their behaviour) and experimenter bias (researchers unconsciously treating groups differently).
Single-blind: only the participant is unaware; double-blind: both parties are unaware.
Randomised controlled trials (RCTs) in medicine follow exactly this structure: patients are randomly assigned to receive either a new drug (experimental group) or a sugar pill (control group), and neither the patient nor the administering doctor knows which is which (double-blind). This is the gold standard for testing whether a treatment works.
The font-readability experiment described in the lecture notes mirrors real cognitive psychology research on "desirable difficulty," where harder-to-process material can sometimes improve long-term retention.
Students often confuse random assignment with random sampling. Random assignment is about placing participants into groups; random sampling is about selecting participants from a population. They solve different problems.
The control group does not mean "nothing happens." It means the group does not receive the specific manipulation being tested. A control group may still complete a task, take a pill (a placebo), or read a passage, just not the version being tested.
Double-blind does not mean the researcher never finds out which group was which. It means they do not know during the experiment, while data is being collected. The groupings are revealed afterwards for analysis.
Students sometimes think the dependent variable is the one the researcher "depends on." The name means it depends on (is affected by) the independent variable, not that the researcher is relying on it.
⚠️ Be able to identify the IV and DV in any described experiment. This is one of the most common question formats in introductory psych exams.
⚠️ Know why random assignment matters: it makes the groups equivalent at the start, ruling out pre-existing differences as an explanation for results.
⚠️ Understand the purpose of the double-blind: it controls for both participant expectations (placebo effect) and experimenter bias.
⚠️ Be able to distinguish between-groups and within-subject designs and name one advantage and one disadvantage of each.
⚠️ If a question asks which research method can establish causation, the answer is the experiment (not correlation, not naturalistic observation).
Fill in the blank: The variable the researcher manipulates is the __________ variable.
True or false: In a double-blind experiment, the participants know which group they are in but the researcher does not.
Fill in the blank: __________ assignment gives each participant an equal chance of being placed in any condition.
True or false: A within-subject design uses different participants in each condition.
Fill in the blank: The group that does not receive the treatment is called the __________ group.
Answers: 1. Independent. 2. False (neither the participants nor the researcher knows). 3. Random. 4. False (it uses the same participants in every condition). 5. Control.
Q: A researcher wants to test whether background music improves reading comprehension. She randomly assigns half the participants to read in silence and the other half to read with classical music playing. She then gives everyone a comprehension quiz. Identify the IV, DV, experimental group, and control group.
A: IV: background condition (music vs silence). DV: reading comprehension score on the quiz. Experimental group: participants who read with classical music. Control group: participants who read in silence.
Q: Why is random assignment important in an experiment?
A: Random assignment ensures that each participant has an equal chance of being in any condition, which makes the groups roughly equivalent at the start. This means any difference in the DV at the end can be attributed to the IV, rather than to pre-existing differences between the people in each group.
Q: What is the difference between a single-blind and a double-blind procedure?
A: In a single-blind procedure, the participants do not know which condition they are in, but the researcher does. In a double-blind procedure, neither the participants nor the researchers who interact with them know which condition each participant is in. The double-blind controls for both the placebo effect and experimenter bias.
Q: In the font experiment from class, students read material in either a plain font or a decorative font, then took a memory test. Was this a between-groups or within-subject design? Explain.
A: Between-groups. Different students were assigned to the plain-font condition and the decorative-font condition. Each student experienced only one condition.
Q: Give one advantage and one disadvantage of a within-subject design compared to a between-groups design.
A: Advantage: individual differences are eliminated because the same person serves in every condition, so differences in outcomes are more likely due to the IV. Disadvantage: order effects (practice, fatigue, boredom) can contaminate results, because participants go through multiple conditions in sequence.
This connects directly to the naturalistic observation and correlation notes. The whole reason you learn those methods first is so you can see the gap they leave: they describe, but they cannot explain cause and effect. The experiment fills that gap.
The placebo effect reappears in the health psychology and psychopharmacology sections of most introductory courses. Understanding why double-blinding matters here will help you evaluate drug studies and therapy outcome research later.
Random assignment connects to the statistics unit. The logic behind why randomisation works is the same logic behind sampling distributions and hypothesis testing.
Experiment, experimental method, independent variable, IV, dependent variable, DV, control group, experimental group, random assignment, randomisation, double-blind, single-blind, placebo, placebo effect, between-groups design, between-subjects, within-subject design, repeated measures, counterbalancing, order effects, confounding variable, experimenter bias, demand characteristics, causal inference, research methods, PSYCH 1100, General Psychology