The Investigative Question Revisited and Data Collection
An investigative question names the variables to measure, the analysis that will be applied, and the population the answer should describe, and the data collection design follows from it. A census records every unit in the population; a sample survey asks a standard set of questions of a sample; an observational study records variables without intervening (prospectively or retrospectively); an experiment assigns treatments to experimental units and measures a response. The reach of the conclusion is set by where randomness entered: random selection supports generalizing to the sampled population, and random assignment supports a cause-and-effect claim.
The traps here are about scope. One is announcing a sample statistic as if it were the population parameter, or generalizing a convenience or volunteer sample to everyone. The other is citing the wrong randomness: claiming cause because the sample was random, or claiming the results generalize because assignment was random. And in any observational comparison, a confounding variable, one associated with both the explanatory and the response variable, stands ready to explain the association without any causation at all.
The work
Lesson live · diagnostic and drills coming soon
Lesson
The Investigative Question Revisited and Data Collection
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Turns an investigative question into a data collection plan, sorts census, survey, observational study, and experiment, and drills the distinction the exam cares most about: selection licenses generalization, assignment licenses cause.
Diagnostic
10-item topic check
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Ten items on the reach of a conclusion: statistic versus parameter, which design was used, which randomness supports which claim, and how a confounding variable is actually named. Take it cold to find your failure mode, or after the lesson to confirm it is gone.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears it for now and moves you to the next. Two in a row is a checkpoint, not proof: if the error resurfaces later, the misconception comes back.