AP Psychology · Exam guide
AP Psychology Statistics in Context: Interpreting the Mean, Correlation, Effect Size and p-Value
One row of the Article Analysis Question asks you to say what a statistic means for the study. Graders want an interpretation, not a definition. This guide shows what to say for each statistic you are likely to see. The numbers are made up for practice.
What each statistic tells you
| Statistic | What it tells you | A full-credit sentence |
|---|---|---|
| Mean | The average of the scores. | The mean score of the group that practiced recall was 78, higher than the 70 of the rereading group. |
| Median | The middle score. It is less affected by extreme scores than the mean. | Half of the participants scored above 74. |
| Standard deviation | How spread out the scores are around the mean. | The scores varied from the mean by about 6 points, a small spread compared with the group difference. |
| Correlation coefficient (r) | The direction and strength of a relationship, from -1 to +1. | An r of +0.60 shows a fairly strong positive relationship: students who slept more tended to score higher. |
| Effect size | How big the difference or relationship is, apart from sample size. | The difference of 8 points was large compared with the spread of the scores. |
| p-value | How likely results this extreme would be if there were no real effect. | A p-value of 0.01 means results this large would be unlikely to happen by chance alone, so the difference is statistically significant. |
Worked example
In a made-up study, the mean exam score for students who slept eight hours was 85 and for those who slept five hours was 74. The reported p-value was 0.02.
It defines the statistic loosely and never interprets this study's result.
It interprets the number for this study and states what it means for the finding.
Correlation is not cause
An r near +1 or -1 shows a strong relationship, but it does not show that one variable causes the other. A third variable, or the reverse direction, could explain it. Only an experiment with random assignment supports a cause-and-effect claim.
Common mistakes
- Defining the statistic instead of interpreting it for this study.
- Reading the sign of r as strength. A negative r can be strong. The sign gives only the direction.
- Treating statistical significance as proof of importance. A tiny effect can be significant in a large sample.
- Calling a correlation a cause.
- Giving the number without units or without saying which group or variable it describes.
Frequently asked questions
What p-value counts as statistically significant?
Usually a p-value below 0.05.
What does a negative correlation mean?
As one variable rises the other tends to fall. A coefficient of -0.80 is stronger than +0.30.
What is the difference between statistical significance and effect size?
Significance says a result is unlikely to be chance. Effect size says how large it is.
Do I need to calculate statistics on the exam?
The summary gives you the statistic. Your task is to interpret it in the context of the study.
Where can I find the official scoring rules?
Exam formats change a little from year to year, so check the current course and exam description on the College Board site before test day.
Keep practicing
The AP Psychology course has five Article Analysis prompts and five Evidence-Based prompts, each with the research summaries, a timed writing space that scores against the official rubric, and a 7 out of 7 model response. These free lessons also build the skills the free-response section tests:
See the AP Psychology course Free lessons and lesson list
Other free-response guides:
- AP Psychology FRQ Guide: The Article Analysis and Evidence-Based Questions
- AP Psychology Article Analysis Question (AAQ) Explained: How to Earn 7 Points
- AP Psychology Evidence-Based Question (EBQ) Explained: How to Earn 7 Points
- AP Psychology Research Methods and Ethics Explained: Naming the Method and the Guideline
- AP Psychology Multiple-Choice Guide: The Three Question Styles and How to Answer Them
- AP Psychology Evidence-Based Question: Concepts to Pair with Evidence, Unit by Unit
AP is a registered trademark of the College Board, which is not affiliated with and does not endorse this site. This guide is our own explanation of the question types and scoring. For the official scoring guidelines, see the College Board.