Caro has plotted this scatter diagram to show the numbers of umbrellas she sold against the amount of rain that fell each day. Caro can see from the scatter diagram that the greater the rainfall, the greater the number of umbrellas sold. Tess says that if Caro sells more umbrellas, there will be greater rainfall. Give a reason why Tess is not correct.

The scatter diagram shows a positive correlation between daily rainfall and the number of umbrellas sold. This means that as one variable increases, the other tends to increase as well. However, this correlation does not mean that one variable *causes* the change in the other. In this situation, the logical cause-and-effect relationship is that an increase in rainfall (the cause) leads to an increase in people buying umbrellas (the effect). Tess's statement reverses this. She suggests that selling more umbrellas (the effect) would cause more rainfall (the cause). This is illogical. This is a classic example of confusing correlation with causation.