Distinguish descriptive, correlational, and experimental methods, including variables, controls, sampling, operational definitions, and ethics.
A lesson of AP Psychology, in Scientific Foundations of Psychology, about 10 minutes.
Scientific Foundations: Research Methods in Psychology
By the end of this lesson, you will be able to design and evaluate psychological research studies, analyze threats to validity, and interpret statistical findings from psychological research.
Why Research Methods Matter for AP Psychology
Research methods questions appear in BOTH sections of the AP Psychology exam:
Multiple choice: ~10-15 questions testing research concepts
FRQ Question 2: almost always involves experimental design or interpreting results
More importantly, understanding how psychological research works helps you evaluate the claims you encounter throughout the course — and throughout your life.
The Research Process
Hypothesis Formation
A hypothesis is a specific, testable prediction about the relationship between variables.
Good hypothesis characteristics:
Specific (names the variables being studied)
Testable (can be supported or refuted with evidence)
Falsifiable (there's some possible result that would disprove it)
Operational definition: Defining a variable in terms of how it will be measured. This is essential for replication.
"Anxiety" is too vague. "Anxiety, measured by a score on the Beck Anxiety Inventory and heart rate at rest" is an operational definition.
Variables
Independent variable (IV): The variable the researcher manipulates — the presumed "cause."
Dependent variable (DV): The variable the researcher measures — the presumed "effect."
Control variable: Variables held constant to prevent them from influencing the results.
Extraneous variable / Confound: Uncontrolled variables that could explain changes in the DV and threaten the validity of conclusions.
Types of Research Methods
Case Study
Description: An in-depth investigation of a single individual, event, or group.
Classic examples:
Phineas Gage: railway worker with a metal rod through his skull; survived but personality changed dramatically; revealed frontal lobe's role in personality
H.M. (Henry Molaison): hippocampus removed to control seizures; couldn't form new explicit memories; revealed hippocampus's role in memory formation
Genie: feral child raised in isolation; informed theories about critical periods for language acquisition
Strengths: Rich detail; useful for rare conditions; can generate hypotheses.
Weaknesses: Cannot generalize; researcher bias; participant may not be representative.
Naturalistic Observation
Description: Observing behavior in natural settings without manipulation.
Examples: Jane Goodall observing chimpanzee behavior; developmental psychologists observing children on playgrounds; ethologists recording animal mating rituals.
Strengths: High ecological validity (behavior is "real"); no demand characteristics (participants behave naturally).
Weaknesses: Observer effect (behavior changes when observed); cannot control variables; cannot establish causation; time-consuming.
Observer reliability: The degree to which two independent observers agree — high inter-rater reliability is essential.
Survey Research
Description: Participants answer questions about their behavior, attitudes, or experiences.
Strengths: Can collect data from large samples efficiently; can study things that can't be observed.
Weaknesses:
Social desirability bias: People answer in socially acceptable ways, not necessarily honestly
Self-report limitations: People may not have accurate insight into their own behavior
Sampling bias: If the sample isn't representative, results won't generalize
Description: Measures the statistical relationship between two or more variables WITHOUT manipulating either.
Correlation coefficient (r): Ranges from -1 to +1; indicates direction and strength of relationship.
Third variable problem: A correlation between A and B may be explained by C causing both. Ice cream sales correlate with drowning deaths — hot weather causes both.
Directionality problem: Does A cause B, or does B cause A?
CORRELATION DOES NOT IMPLY CAUSATION — this is arguably the most important methodological principle in psychology.
Experimental Research
Description: Researcher manipulates the IV and measures the DV while controlling other variables.
Key elements:
Random assignment to experimental/control groups (eliminates confounds)
Control group (no treatment; provides baseline)
Experimental group (receives treatment/manipulation)
Double-blind procedure (neither participants nor researchers know group assignment)
Why random assignment matters: It distributes participant characteristics (age, intelligence, personality) equally across groups by chance, ensuring any difference in DV scores is likely due to the IV.
Threat to internal validity: Anything that could explain the DV results OTHER than the IV.
Standard deviation: Measures spread — how far scores typically deviate from the mean. A small SD means scores cluster tightly; a large SD means they spread widely.
The Normal Distribution
Most naturally occurring psychological measures (IQ, height, personality scores) follow a normal distribution — bell-shaped, symmetric around the mean.
68-95-99.7 Rule:
68% of scores fall within 1 SD of the mean
95% fall within 2 SD
99.7% fall within 3 SD
Z-score: Standardized score telling how many SDs above or below the mean a given score is.
Inferential Statistics
Statistical significance: A result is significant if it is unlikely to have occurred by chance alone.
p-value: The probability that the observed result is due to chance.
p < .05: Significant — less than 5% chance it's random. We reject the null hypothesis.
p = .01: Very significant — 1% chance it's random.
Effect size: How large the observed difference is, independent of sample size. A result can be statistically significant with a tiny effect size (if the sample is very large). Practical significance matters.
Replication: The true test of scientific validity. A single study, even with p < .05, is not conclusive — results must be replicated by independent researchers.
Replication crisis in psychology: Many classic psychology findings failed to replicate when retested. Priming effects, ego depletion, and some social psychology findings have been substantially weakened by replication failures.
Evaluation Frameworks
Internal Validity
Does the IV explain the DV results? Is the conclusion that the IV caused the DV change justified?
Control with: random sampling, field experiments, replication with different populations.
Reliability
Are the results consistent? Would you get the same results if you measured again?
Test-retest reliability: Consistent results at different time points
Inter-rater reliability: Consistent results when different observers rate the same behavior
Validity of Measurement
Are you measuring what you think you're measuring?
Face validity: The measure looks like it measures the construct
Construct validity: The measure actually measures the theoretical construct
Predictive validity: The measure predicts related outcomes it should predict
Summary
The scientific method: Observation → Hypothesis → Design → Collect data → Analyze → Conclude → Replicate.
Research types: Case study (depth, no generalization), naturalistic observation (ecological validity, no control), survey (scale, self-report limitations), correlational (relationships, not causation), experimental (causation, with controls).
Experimental design: IV (manipulate) → DV (measure); control group; random assignment; double-blind.
Statistics: Mean/median/mode; SD; normal distribution; p < .05 = significant; effect size ≠ significance.
Validity and reliability: Internal validity (confound control), external validity (generalization), reliability (consistency), measurement validity (measuring what you think).
AP Exam Tip: For every research methods question, ask: "Is this experimental or correlational? If experimental, what are the IV and DV? If correlational, can we conclude causation?" These two questions will guide you to the correct answer in most cases.