Calculation of the required number of FTEs to support the CIS/IT service?

Question

ITIL

Hello!

Can anyone tell me if there are any universal ways to estimate the required number of FTEs to support CIS or IT services?

There is such a key:
An IT service is being supported, at the moment 4 people are assigned to it for full time.
There are delays in the service on appeals, as employees are overloaded, and also if someone goes on vacation or sick leave, the number of delays and waiting times for issuing a decision increases even more.

To understand the approximate labor costs, the following was done:
1. The types of operations that are performed within the framework of maintenance tasks were identified
2. Their number
3. The average time to complete these operations
4. Labor intensity was calculated -> FTE

The output turned out to be ~ 6 FTE (including discounts for sick leave / vacations).
Now it is not clear how to link all this with the required SLA level of 95%.
The methodologists suggest using Erlang, but when using it, the output is extremely inadequate data (from the 0.4 FTE series at 80% load per month), since the error is too large, because it is realistic for them to evaluate only the call center or the 1st line of the technical pod.

Does anyone have any suggestions?

Answer 1

The calculation does not indicate the work schedule of people: 7x24, 8x5. 6 FTE for 24x7 not sewn on a mare's tail, shift picking is done at the rate of 5 sh / e per one constantly working engineer.
We believe that at your SLA level, unavailability should be 36 hours a month. This is the first KPI. We start the requirement - the maximum time of a single accident, for example, 4 hours. In total, no more than 9 accidents per month, that is, no more than two accidents per week. If you do only this, it will cover the 5 described w / e.
We classify all tasks for the duty shift: critical, non-critical, normal, for each we make a regime and standard processing times (we have already set for the critical one). The share of processed tasks in terms of tasks is the second KPI.
We add preventive work, the timeliness of their implementation is the third KPI.
I can wind up a lot more, but in the end you will get a KPI formula that you will optimize.