An insurance company with a number of branch offices across the country has designed a diary system for salespeople, which is designed to increase their pro ductivity. The diary system is colour-coded: green is for time spent with clients making sales calls, blue is for time spent supporting existing clients, and red is down time (lunch, travelling, etc.). The idea is that the visual cue of the coded diary will focus the salesperson's attention on sales. A number of poor performers were selected and introduced to the diary system. Average sales increased by 11%. Can we conclude that the diary system caused the increase?