The Investigative Question Revisited and Data Collection
▶︎ Watch it animatedinteractive step-through · ~3 min · optionalAn 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
3 ways in · any order
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.