Across 40 towns, towns with more public libraries also reported more traffic crashes. Town records show larger towns had more residents, more libraries, and more roads. What lurking variable best explains the association?

In 30 high schools, schools offering more AP classes also sold more cafeteria lunches. Enrollment records show larger schools had more students, more classes, and more lunches sold. What lurking variable best explains this association?

At 18 city clinics, clinics with more doctors reported more flu diagnoses. Clinic logs show high-volume clinics served many more patients each week. What lurking variable best explains why both counts were higher?

Across 24 farms, farms with more tractors had higher crop sales in dollars. Farm records show farms with more hectares planted used more tractors and harvested more crops. What lurking variable best explains the association?

In downtown neighborhoods, more coffee shops were associated with more phone theft reports. City foot-traffic counters showed busier neighborhoods had more visitors, more shops, and more theft opportunities. What lurking variable best explains the pattern?

At 12 beach towns, weekly sunscreen sales rose with weekly lifeguard rescues. Attendance counts show hotter weekends brought more beach visitors, more sunscreen purchases, and more rescues. What lurking variable best explains this association?

Among 28 high school athletic programs, programs with more coaches also reported more injuries. Rosters show larger programs had many more athletes, coaches, practices, and injury reports. What lurking variable best explains the association?

A survey of 300 households found homes with more smoke alarms reported more kitchen fires. Store receipts show most extra alarms were bought in the month after a fire. What direction does the timing support?

In a school survey, students using tutors had lower recent quiz scores. Sign-up records show most began tutoring after a quiz score below 70. What direction does the timing support?

At a factory, weeks with more quality inspections also had more defective products. Work logs show managers ordered extra inspections after a defect spike. What direction does the timing support?

In 16 neighborhoods, blocks with more security cameras had more theft reports. Installation records show cameras were usually added after several thefts. What direction does the timing support?

A newspaper compared 25 high schools and found schools where more students reported eating breakfast also had higher average test scores. The data came from a one-week survey with no random assignment. What conclusion about the association is justified?

A survey of 200 tenth graders found students reporting more daily screen time also reported fewer hours of sleep. Researchers did not assign screen times. What does this association support?

During 20 rainy-season weeks in one city, weeks with higher umbrella sales also had more car crashes, and weather records show those weeks had more rain. What does the association support?

At one school dance, a teacher recorded 12 songs and found louder songs had more students on the floor. The DJ picked songs informally, and the sample was one night. What conclusion is most reasonable?

In one algebra class of 22 students, students wearing blue shoes averaged higher test scores than others. Shoe color was not assigned, and the class was the only group studied. What conclusion is most reasonable?

In a park survey, riders with expensive bikes were more likely to wear helmets. Observers only recorded bike price and helmet use one afternoon. What does this association support?

Researchers observe that students who use a study app earn higher algebra scores, and they want evidence that the app causes higher scores. What design would best support the causal claim?

A principal observes that classrooms with indoor plants have fewer student headaches, and wants evidence that plants cause the reduction. What design would best support the causal claim?

A city observes that cyclists wearing reflective vests get more passing space from cars, and wants evidence that vests cause wider passing distance. What design would best support the causal claim?