With this revised set of activities we were able to extend the study to all of the operators. In addition to giving the operators written descriptions of the activity categories, we explained and answered questions until we were sure that everyone had a clear understanding of the activity categories. The activities listed in the table may appear cryptic, but they should be understood as mnemonic codes for very carefully defined activities. Operators also received simple data cards and Divilbiss Electronics Random Reminders. Random Reminders are pocket-sized devices that provide signals either a beep or a silent vibration at genuinely random times. Most of our operators preferred to use the vibrating signal, especially where a beep might not be heard. Recording data was very simple. Each time they were signaled, operators wrote down the numeric activity code for the activity in which they were engaged at the exact instant of the signal. At some future time we may use a bar code data entry system, but for the present, simply writing down the code number work very well.