His assumption about the results turned out to be wrong.
His assumption about the weather proved to be wrong.
On what data is this assumption based?
We are aware of making too many assumptions.
We need to gather more data to verify our assumptions.
It would be wise to test whether this assumption is correct.
If our assumptions are wrong, the entire plan could fall apart.
Every assumption should be documented so that it can be verified later.
I made a plan based on incorrect assumptions in a project and had to revise it later.
