Assumptions are the backbone of all strategic initiatives, indeed, most day to day activity. Often these assumptions are implicit, they  make sense, we do not even question the assumption, by default they become as good as a fact.

Really good strategic and design initiatives turn assumptions into facts by finding supporting data, and in the absence of the data to make an assumption a fact, it remains an assumption, subject to continual scrutiny.

A really useful first step in any planning process is simply to ensure that any assumptions are exposed for what they are, assumptions, subject to change in the face of new information .