Coffee stations in offices often just ask users to leave money in a tray to pay for their coffee, but many people cheat. Researchers at Newcastle University replaced the picture of flowers on the wall behind the coffee station with a picture of staring eyes. They found that the average contribution increased significantly above the well-established standard when people felt they were being watched, even though the eyes were patently not real.
a) Was this a survey, an observational study, or an experiment? How can we tell?
b) Identify the variables.
c) What does "increased significantly" mean in a statistical sense?