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Unit 3: Collecting Data

7 dot points across 7 inquiry questions. Click any dot point for a focused answer with worked past exam questions where available.

How do random selection and random assignment together decide what conclusions a study can support?

Topic 3.7 Inference and Experiments: use the presence or absence of random selection and random assignment to determine the scope of inference, that is, whether results generalize to a population and whether a causal conclusion is justified.
A focused answer to AP Statistics Topic 3.7, on the scope of inference, using random selection (generalization) and random assignment (causation) to decide what conclusions are valid, with a worked four-quadrant analysis.
10 min answer →

Why does how we collect data decide whether the data can tell us the truth?

Topic 3.1 Introducing Statistics: Do the Data We Collected Tell the Truth? Recognize that the method of data collection determines the kinds of conclusions that can be drawn, and that poorly collected data cannot be fixed by analysis.
A focused answer to AP Statistics Topic 3.1, on why the data-collection method determines what conclusions are valid, the difference between random error and bias, and why analysis cannot rescue badly collected data.
9 min answer →

What are the principles of a well-designed experiment, and what does each one protect against?

Topic 3.5 Introduction to Experimental Design: identify the components of an experiment (units, treatments, response) and apply the principles of comparison, random assignment, replication, and control, including blinding and the placebo effect.
A focused answer to AP Statistics Topic 3.5, on experimental units, treatments and factors, and the principles of comparison, random assignment, replication, and control, plus blinding and the placebo effect.
10 min answer →

How do we decide between an observational study and an experiment when planning to answer a question?

Topic 3.2 Introduction to Planning a Study: distinguish observational studies from experiments, identify explanatory and response variables, and recognize that only an experiment with imposed treatments can support a causal conclusion.
A focused answer to AP Statistics Topic 3.2, distinguishing observational studies from experiments, identifying explanatory and response variables and confounding, and explaining why imposing treatments is what enables causal claims.
9 min answer →

What kinds of bias can creep into a sample, and why does none of them shrink with sample size?

Topic 3.4 Potential Problems with Sampling: identify undercoverage, voluntary response, convenience, nonresponse, and response bias, explain how each distorts results, and recognize that bias is not reduced by a larger sample.
A focused answer to AP Statistics Topic 3.4, identifying undercoverage, voluntary response, convenience, nonresponse, and response bias, the direction each pushes results, and why bias persists no matter how large the sample.
9 min answer →

What sampling methods use chance to select a sample, and how do they differ?

Topic 3.3 Random Sampling and Data Collection: describe and distinguish simple random, stratified, cluster, and systematic random sampling, and explain why random selection supports generalization to a population.
A focused answer to AP Statistics Topic 3.3, describing simple random, stratified, cluster, and systematic random sampling, how each uses chance, their trade-offs, and why random selection allows generalization, with a worked SRS selection.
10 min answer →

When should we block or pair subjects instead of using a completely randomised design?

Topic 3.6 Selecting an Experimental Design: compare completely randomised, randomised block, and matched pairs designs, and explain how blocking and pairing control a known source of variation to make treatment effects clearer.
A focused answer to AP Statistics Topic 3.6, comparing completely randomised, randomised block, and matched pairs designs, and explaining how blocking and pairing remove a known source of variation to sharpen the comparison.
10 min answer →