A bank branch has found that since it began sending a text message the day before an instalment falls due, the share of instalments paid on time has risen. Before the messages are extended to every branch, the head office wants to know whether the messages are what produced the change. Which of the following would be of most use to find out?

The head office needs to know whether anything besides the messages changed at the branch in the same months. A parallel telephone drive would account for the improvement by itself and would show that extending the messages elsewhere may achieve nothing. Cost matters only once the messages are known to work, and instalment size and other bank accounts do not bear on what caused the change.