In the planning cycle, you start off with a load of assumptions.
Then observations are made of existing systems, and you might be tempted to say that your assumptions were wrong... but are they?
Does experience of system A automatically mean that the assumption for system B is wrong?
Alternatively, you might have made an assumption of how system C might behave in future, but then be told that it behaved in a certain way last year.... does that mean your expectations for the future HAVE to change?
As Capacity Planners, we are frequently expected to make predictions about systems or services. Often, the developers have very little idea how the system will behave, so how are we expected to know? An assumption is as good a place to start as any other.
There is always a sense of nervousness about planning assumptions. Are they valid? Are they so wrong that BIG mistakes are going to be made provisioning the infrastructure? Will many thousands of pounds be spent unnecessarily, or will not enough be provisioned and an expensive upgrade be needed within months of “Go Live”?
Surely altering planning assumptions to match historical observations is no better a way of planning than extrapolating historical activity into the future? It is the old adage of steering a ship by its wake. It'll not warn you of the rocks ahead.
But if we don't learn from our history, then we are bound to repeat our mistakes. I propose that forecasts should have a "range". A "worst case"/"best case" view. If history is repeated, then the future will unfold in one direction. However, if the planning assumption is more applicable to the future activity, then the future will unfold in a different direction again.
The following chart shows how this could be reported. The labeling on the axis is irrelevant. It could be resource utilisation, Quality of service, or £s to be invested. The main thing is that you have a range of possibilities that allow you to evaluate the impact of the planning assumptions coming from design.
