For Exercises 12.1 to 12.3, see page 597; and for 12.4 and 12.5, see page 599.
12.4 Special causes.
Rachel participates in bicycle road races. She regularly rides 25 kilometers over the same course in training. Her time varies a bit from day to day but is generally stable. Give several examples of special causes that might raise or lower Rachel's time on a particular day.