In other words, if you take traffic (1 Erl – is 3600 second) and this amount of traffic you divide number of drops (4 drops on the one cell), than the equation will just the average time between the drop occurs (3600/4=900). And “call minutes between drop” is just 15 minutes.
I hope that now it is much clearer.
The meaning of this indicator is rather complicated to understand, because it has no real physical meaning ! But in a simpler way, it gives a ratio traffic / call drop rate. The lower is the ratio, the more critical is the cell.
If it goes below 40s, then it indicates a probable hardware failure : every 40s of “cumulated calls”, there is a call drop.