Our staffing levels are based on the figures reported to our logging system. It is easy to look at a year's work, average it out to the number of working days, and decide that each person has an acceptable workload. I think for our team it works out at fewer than three calls per day per engineer. Sadly, in the real world, this hides the fact that some days I drive for six hours and work for one, and on others I work six hours and drive one.
On the days when I drive for hundreds of miles and do only one job, the other jobs that arrive have to wait until I am back in my own area. Fate has a way of arranging for calls to arrive in clusters, not evenly spaced as the bean counter's simulation would have you believe.
There are a couple of things that need to be taken into consideration when deciding on staff cover levels. One is that you can arrange the statistics to form the conclusion needed. The other is that there is no such thing as an average. All the calculations for budgets, manning, and resources are based on averages, but real life is different. An example of this was a batch of print heads that were supplied for our machines. They were not up to standard, so we were replacing them frequently until the rogue ones were all replaced.
The trouble was that the bean counters looked at the totals and continued to order the part, even when we no longer needed it. It is a nightmare job to keep stock levels at workable levels, and I do not envy anyone who has that task, although I feel that it takes more than a review of records to manage this kind of task. Reviewing part usage is only part of the task, checking with the frontline workers to see that the circumstances that prevailed during the period of maximum usage still prevail is another part.
If I had the skill to accurately predict the future, I would have retired from work years ago. Sadly I don't, so I keep going back every Monday morning, chasing around, doing the best I can. If anyone ever comes up with a mathematical formula that can calculate staff coverage, parts ordering, and the best route between calls, I will be very grateful.
There is a reality gap between real life and the ordered, predictable world of the bean counter, which is easier to understand so companies use their data to model their requirements. Up at the sharp end of the business life isn't so easy to keep in order. Every call has a back story to it and real people who are trying to make their living. My biggest frustration is that people prefer to believe the spreadsheet over real life.