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Statistics

Observational vs Experimental Studies

Lesson

Studies come in two big flavors. The difference matters because only one of them supports a causal conclusion.

Observational study

The researcher measures but does not intervene. People do what they would normally do; you just record what happens.

Examples: survey, census, tracking, looking at existing records.

Experiment

The researcher imposes a treatment — usually assigning subjects to different conditions (ideally at random) — and measures the response.

Examples: clinical trials, lab tests, A/B testing.

Why it matters

Only experiments can establish CAUSE. Observational studies show correlation but always carry the risk of lurking variables. People who exercise might also eat better; you can’t pin a health outcome on just one.

Spot the type

  • “Researchers measured / surveyed / tracked” → observational.
  • “Researchers assigned / gave / imposed” → experimental.

How to type your answer

Type 1 for observational\text{observational} and 2 for experimental\text{experimental}.

Practice

Work through these. Stuck? Click Get a hint.

Warm-Up

Quick problems to get going.

Problem 1

Researchers give half the participants a drug, half a placebo.\text{Researchers give half the participants a drug, half a placebo.}

Problem 2

Survey people about their exercise habits.\text{Survey people about their exercise habits.}

Problem 3

Watch shoppers and record their purchases.\text{Watch shoppers and record their purchases.}

Problem 4

Randomly assign students to two teaching methods.\text{Randomly assign students to two teaching methods.}

Practice

Standard problems matching the lesson.

Problem 5

Doctors track patients’ diets and outcomes over time.\text{Doctors track patients' diets and outcomes over time.}

Problem 6

Researchers give participants different doses and measure response.\text{Researchers give participants different doses and measure response.}

Problem 7

A study compares fish populations in two lakes.\text{A study compares fish populations in two lakes.}

Problem 8

Half the gardens get fertilizer A, half get fertilizer B.\text{Half the gardens get fertilizer A, half get fertilizer B.}

Problem 9

Sociologists observe communities without intervention.\text{Sociologists observe communities without intervention.}

Problem 10

A teacher splits students into "tutoring" vs "no tutoring".\text{A teacher splits students into "tutoring" vs "no tutoring".}

Problem 11

A poll measures voting preferences.\text{A poll measures voting preferences.}

Problem 12

A factory randomly assigns workers to two shift schedules.\text{A factory randomly assigns workers to two shift schedules.}

Problem 13

Birds are counted in different habitats.\text{Birds are counted in different habitats.}

Problem 14

Plants are exposed to different light levels by researchers.\text{Plants are exposed to different light levels by researchers.}

Problem 15

Test scores compared across schools that already exist.\text{Test scores compared across schools that already exist.}

Problem 16

Subjects are told to consume 0, 1, or 2 cups of coffee in a study.\text{Subjects are told to consume 0, 1, or 2 cups of coffee in a study.}

Problem 17

Surveying employees about job satisfaction.\text{Surveying employees about job satisfaction.}

Problem 18

Assigning patients to drug A vs drug B in a trial.\text{Assigning patients to drug A vs drug B in a trial.}

Challenge

Harder problems — edge cases, trickier numbers, multiple steps.

Problem 19

A natural disaster’s effects are studied after the fact.\text{A natural disaster's effects are studied after the fact.}

Problem 20

Students are randomly given different study guides.\text{Students are randomly given different study guides.}

Problem 21

Scientists watch eclipses to time them.\text{Scientists watch eclipses to time them.}

Problem 22

Researchers split mice into 3 diet groups.\text{Researchers split mice into 3 diet groups.}

Problem 23

Studying air pollution levels in cities.\text{Studying air pollution levels in cities.}

Problem 24

Athletes randomly assigned shoe brands; performance measured.\text{Athletes randomly assigned shoe brands; performance measured.}

Problem 25

Tracking ice cream sales vs temperature over time.\text{Tracking ice cream sales vs temperature over time.}

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