Why are Assumptions important and what should happen to the project plan if an assumption changes?
Most projects create a list of Project Assumptions at some point in time. These typically are documented at the start of the project and filed in a safe place. Aside from helping to pad out the project charter, they are usually ignored.
A good starting point may be a dictionary definition.
"The act of taking something for granted, or something taken for granted."
In other words, it is something that we cannot establish as being true at this point in time, but is likely to be true.
Why make an Assumption
In order to move forward without absolute information, we need to make assumptions. In a project, or in fact in life, we rarely have absolute information. We need to either 'assume', or we might as well stay in bed each morning.
We assume that because the weather forecast is for a sunny day, and there is not a cloud in the sky, it will not rain so we will not take an umbrella with us to work. We assume that the washing machine will not break down and overflow before we set a load of washing as we walk out the door. We assume the bus will come on time when we go to the bus stop at a certain time.
In a project, there is always a high degree of unknown. If we wait until all information was available, we would probably never start. Typically we assume that resources will be available and that business users know their business. We assume that funding will not be withdrawn.
The Danger with Assumptions
There is a natural tendency for an Assumption to become accepted as the truth. I read somewhere that a culture is a group of shared Assumptions. People share a common Assumption and believe it to be the truth. For example in the US, people assume it is fine to have your own gun regardless of your need to actually use it for any reason other than protect yourself from other people with guns.
In Australia, we assume the opposite. We believe that unless you are a country dweller, who may need a gun to carry out your farming duties, you should not have a gun.
Without getting into the rights and wrongs of the situation, here are two cultures that have many similarities, but share opposite Assumptions. If you talk to people from each country, they would not look on the assumption (to have or not have guns) as an Assumption. They would see it as a normal part of their life. The way things are. Something they take for granted, and certainly not something they question daily (or most of them anyway).
A similar thing can happen with projects. People assume something, and as the project progresses, they forget to challenge the Assumption. For example, they assume ...
The 1800+ word solution provides very good material about the basis of assumptions. It includes the danger of assumptions, the risk of assumptions, and provides a rating of assumptions in terms of confidence, impact and priority. Two examples are provides for managing assumptions in a project.