AP Biology / AP Statistics · Unit 1
Unit 1: Exploring One-Variable Data and Collecting Data
Unit 1 is 20–30% of the AP Statistics exam — the largest single unit. In the 2026–27 redesign, the College Board merged the old Units 1 and 3 so that describing distributions, sampling, and experimental design are taught and tested together.
Weight and scope
Exam weight: 20–30%. Expect roughly 9–13 MCQs and heavy overlap with FRQ 1 (Practices 1 and 2) and FRQ 4.
Topic list (2026–27 CED)
- Introducing statistics: what can we learn from data?
- Categorical variables: frequency and relative-frequency tables, bar graphs
- Representing quantitative variables: dotplots, stemplots, histograms
- Describing distributions: shape, center, spread, unusual features
- Summary statistics: mean, median, standard deviation, IQR, five-number summary
- Boxplots and the 1.5 × IQR outlier rule
- The Empirical Rule and normal distribution as a model
- Comparing distributions of a quantitative variable across groups
- Planning a study: population vs. sample, parameter vs. statistic
- Random sampling methods: SRS, stratified, cluster, systematic
- Sources of bias: undercoverage, nonresponse, response bias, wording
- Experimental design: treatments, control, random assignment, replication, blocking
- Scope of inference: when you can generalize, when you can conclude causation
Vocabulary you must be precise about
| Term | Meaning |
|---|---|
| Parameter | A number describing a population (e.g., μ, p, σ) |
| Statistic | A number describing a sample (e.g., x̄, p̂, s) |
| Random sampling | How subjects are selected — enables generalization to the population |
| Random assignment | How subjects are placed into treatments — enables cause-and-effect |
| Confounding variable | A variable that changes with the explanatory variable, obscuring its effect |
| Resistant measure | A statistic not strongly affected by outliers (median, IQR) |
| Bias | A systematic tendency for the sample to differ from the population |
What actually gets tested
- Describe a distribution. Always address shape, center, spread, and unusual features (SOCS) — all four, in context. Missing one loses a rubric point.
- Compare two distributions. You must explicitly compare the same feature (e.g., "Group A's median is higher than Group B's"). Describing each separately without comparing is a common point loss.
- Identify the sampling method and its bias. Name the method (stratified, cluster, convenience) and explain a specific bias in context.
- Design an experiment. State the treatments, how subjects are randomly assigned, how many subjects, and what response is measured.
- Scope of inference. If subjects were randomly sampled → you can generalize. If subjects were randomly assigned → you can conclude cause-and-effect. Both → you can do both. Neither → you can do neither.
Calculator moves
- 1-Var Stats for a quick five-number summary, mean, and standard deviation.
- Boxplot with outliers — the modified boxplot flags outliers using the 1.5 × IQR rule automatically.
- Normal CDF and invNorm for the Empirical Rule and z-score questions.
- Desmos in Bluebook — the 2027 digital exam includes a built-in Desmos calculator that computes 1-Var Stats and normal probabilities directly.
Common mistakes
- Confusing random sampling with random assignment.
- Writing "outlier" instead of using the 1.5 × IQR rule to justify it.
- Describing shape as "normal" when you mean "roughly symmetric."
- Forgetting context in every conclusion — always name the units and population.
FAQ
How much of the AP Statistics exam is Unit 1?
Unit 1 is 20–30% of the exam — the largest single unit. It combines exploring one-variable data with collecting data (sampling and experimental design), which the 2026–27 CED merged into a single unit.
What is the difference between an observational study and an experiment?
In an experiment, the researcher imposes a treatment on subjects and can therefore establish cause-and-effect. In an observational study, the researcher observes existing conditions without intervening, so only association — not causation — can be established.
When should I use mean vs. median?
Use the mean and standard deviation for roughly symmetric distributions with no strong outliers. Use the median and IQR for skewed distributions or distributions with outliers — those are resistant measures.
Keep going
Practice Unit 1 with real rubric feedback.
Cramapple's Unit 1 practice grades your 'describe' and 'compare' answers criterion by criterion — the exact rubric points AP graders check for.
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