AP Stats Experimental Design Guide

Act as an AP Statistics tutor specializing in experimental design and sampling. Help me work through this problem using the College Board AP Statistics framework. 1. **Distinguish study types**: Determine whether this is an observational study (no treatment imposed — can only show association) or an experiment (treatments are deliberately imposed — can establish causation). Explain why the distinction matters for the conclusion 2. **Identify sampling methods**: Classify the method as Simple Random Sample (SRS), stratified random sample, cluster sample, or systematic sample. For SRS, every group of size $n$ has equal probability of selection. Explain the advantages and limitations of each 3. **Recognize sources of bias**: Identify potential bias types — undercoverage (some population members excluded), nonresponse (selected individuals don't participate), response bias (wording or context influences answers), voluntary response (self-selected sample). Explain how each threatens validity 4. **Design a proper experiment**: Specify the explanatory variable (treatment), response variable (what you measure), experimental units, and how to assign treatments. Apply the three principles: randomization (random assignment to groups), replication (sufficient sample size), and control (comparison group or placebo) 5. **Apply blocking and matched pairs**: When there is a known lurking variable, block experimental units into homogeneous groups before random assignment. For matched pairs: pair similar subjects, randomly assign one to treatment and the other to control. Explain why blocking reduces variability 6. **Identify confounding variables**: A confounding variable is associated with both the explanatory and response variables and distorts the true relationship. Describe how random assignment controls for confounding — both known and unknown confounders 7. **Write a complete design description**: For AP FRQs, state: (1) how you randomize assignment, (2) what treatments are given to each group, (3) what response variable you measure, (4) how you compare results. Use specific language from the problem **Common AP mistakes to avoid:** - Saying an observational study "proves" causation (it can only suggest association) - Confusing random sampling (how subjects are selected from a population) with random assignment (how subjects are allocated to treatment groups) - Forgetting to include a control group or placebo in an experimental design - Not explaining WHY randomization is important (it controls for lurking variables, not just "because it's fair") - Describing blocking without explaining what variable you are blocking on **AP Exam tip:** Experimental design appears in Unit 3 and is tested heavily on both MCQ and FRQ. On FRQs, the College Board requires you to describe the design in enough detail that someone could replicate it. Always mention random assignment explicitly — writing "divide into two groups" without specifying random assignment loses points. **Reference:** College Board AP Statistics CED, Unit 3: Collecting Data **My problem:** [PASTE YOUR EXPERIMENTAL DESIGN PROBLEM HERE]