Cogito
Probability and Statistics · Chapter 7 · Lesson 2
Experiments and Observational Studies
Only one of them can prove a cause.
12 problems · about 21 minutes · TEKS P.S.4.B, S-IC.B.3
What this lesson teaches
The student distinguishes experiments from observational studies and identifies the elements of good design.
- An experiment assigns treatments; an observational study only records.
- Only randomised experiments can establish causation.
- Control, randomisation, blinding and replication are the marks of good design.
Warm Up
Straightforward practice. Get the method working first.
5 problemsSubjects are randomly assigned to two diets. Experiment or observational? 1 or 2.
Answer 1
Why An experiment.
Why does random assignment allow a causal conclusion?
Answer It balances out other differences between the groups.
Why Other differences even out across groups.
Which design can establish causation? 1 experiment, 2 observational.
Answer 1
Why Random assignment.
A group receiving a fake treatment is called what? 1 control, 2 treatment.
Answer 1
Why The comparison group.
Neither subjects nor measurers know the groups. Double blind? 1 yes, 0 no.
Answer 1
Why Both sides blinded.
Build It Up
The same ideas with more to keep track of.
3 problemsSubjects improve from believing they were treated. Placebo effect? 1 yes, 0 no.
Answer 1
Why Belief alone.
Students choose their own study method. Experiment or observational? 1 or 2.
Answer 2
Why No assignment.
What balances out unknown differences between groups? 1 random assignment, 2 a large sample.
Answer 1
Why Size alone does not balance groups.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
4 problemsSort each feature by which design it belongs to.
Answer Experiment: Random assignment to groups, A placebo control group · Observational study: Recording choices people already made, Following existing habits over time
Why Assignment is the dividing line.
Can an observational study alone prove cause? 1 yes, 0 no.
Answer 0
Why Lurking variables remain.
The Design: Researchers randomly assign a drug or a placebo. Experiment or observational? 1 experiment, 2 observational.
Answer 1
Why An experiment.
The Watch: Researchers record existing exercise habits and compare health. Which is it? 1 or 2.
Answer 2
Why Observational.