Researchers record the weekly running distance and resting heart rate of 200 adults who run as much as they choose. What kind of study is this?

A school compares two math apps by flipping a coin for each student to assign App A or App B, then comparing quiz scores. What kind of study is this?

Nutrition researchers compare test scores for students who already eat breakfast at home with scores for students who already skip breakfast. What kind of study is this?

A factory randomly assigns 40 machines to use a new lubricant or the current lubricant, then compares repair counts after one month. What kind of study is this?

Transportation researchers ask 300 commuters how they already get to work and record each commuter's arrival delay in minutes. What kind of study is this?

An athletic trainer randomly assigns injured athletes to an ice routine or a heat routine, then compares days until return to practice. What kind of study is this?

Public health researchers review records for students whose parents already chose a flu shot or no flu shot, then compare flu absences. What kind of study is this?

A teacher randomly chooses half of her class periods to use a review game and the other half to use worksheets, then compares unit test scores. What kind of study is this?

Crop scientists record the fertilizer brand each farmer already uses and the wheat yield in kilograms per field. What kind of study is this?

A website randomly shows visitors either a blue checkout button or a green checkout button and compares purchase rates. What kind of study is this?

Researchers invite adults to report the amount of caffeine they already drink each day and then measure anxiety scores. What kind of study is this?

A greenhouse labels 60 tomato plants with random numbers and gives each plant 0, 50, or 100 mL of fertilizer solution, then compares growth. What kind of study is this?

An insurance company compares crash rates for drivers who chose to install a tracking app with rates for drivers who did not install it. What kind of study is this?

A principal flips a coin for each homeroom to decide whether it receives text reminders about homework, then compares homework completion. What kind of study is this?

A clinic compares recovery times for patients who chose physical therapy with times for patients who chose rest at home. Why can this comparison not show that therapy caused faster recovery?

A school finds that students who sign up for tutoring have higher exam scores than students who do not. Why can this not prove that tutoring caused higher scores?

Researchers compare allergy symptoms for households that already own air purifiers with symptoms for households that do not. Why is a causal claim about purifiers not justified?

A sleep study compares teenagers who already use screens more than 3 hours per night with those who use less. Why can it not show that screen time causes shorter sleep?

A restaurant study finds lower cholesterol among diners who usually choose vegetarian meals than among diners who choose meat meals. Why can this not prove the meal choice caused the cholesterol difference?

Employees who requested standing desks report fewer backaches than employees who kept regular desks. Why can the company not conclude that standing desks caused fewer backaches?

A city sees higher cycling rates in neighborhoods that already have bike lanes than in neighborhoods without them. Why can this comparison not prove bike lanes caused the higher cycling rate?

A survey finds that teens who already use a meditation app report less stress than teens who do not use it. Why should the study avoid claiming the app caused less stress?

A farm currently records milk yield for cows whose owners choose either a new feed or the old feed. What change would make the study an experiment?

Researchers now compare grades for students who download a calculator app on their own with grades for students who do not. What change would make this an experiment?

A beach study compares sunburn rates for people who already bought Brand A sunscreen with rates for people who bought Brand B. What change would make it an experiment?

A study now compares homework accuracy for students who choose fast music, slow music, or no music while studying. What change would make it an experiment?

A company compares sales for workers who volunteered for a training course with sales for workers who did not volunteer. What change would make this an experiment?

A running store compares injury rates for runners who already chose cushioned shoes with rates for runners who chose minimalist shoes. What change would make this an experiment?

A greenhouse currently records growth for plants watered at amounts chosen by different gardeners. What change would make the study an experiment?

Researchers compare sleep for students whose bedrooms already have blackout curtains with sleep for students whose rooms do not. What change would make this an experiment?

A website compares purchase rates for users who choose dark mode with rates for users who choose light mode. What change would make this an experiment?

A hospital compares error rates on shifts where nurses volunteered to use a new checklist with shifts where they did not. What change would make the study an experiment?

In a study of screen time and sleep, 400 teenagers report the screen time they already use, and researchers measure sleep duration. Who set each teenager's screen-time level?

In a plant study, researchers use random numbers to give each seedling 2, 4, or 6 hours of extra light daily. Who set the extra-light amount?

A medical study records each patient's blood pressure and the medication dose that the patient's regular doctor had already prescribed. Who set the medication dose?

Researchers compare test scores at schools whose principals already chose early or late start times. Who set the start-time category for each school?

A tutoring study uses a lottery to assign students to 0, 1, or 2 tutoring hours per week, then compares quiz scores. Who set the tutoring hours?

A cyclist safety study records crash reports and the helmet type each cyclist had already bought. Who set the helmet type in the study?

In a lab study, researchers draw slips from a box to assign seedlings to red, blue, or white light, then measure growth. Who set the light color?

A cafeteria study records whether each student already chooses salad, sandwiches, or pizza for lunch, then measures afternoon focus. Who set the lunch type?