There are two basic ways to collect data about whether one thing affects another.

• In an observational study you measure what is already happening and do not change anything. You might record how long students sleep and what grades they get.
• In an experiment you assign a treatment to the units and then measure what happens. You might tell some students to sleep eight hours and others to sleep six.

Both can find patterns. Only an experiment, done well, can say much about cause and effect. Here is why.