My master list of project phases
I talked about the importance of 'de-risking' in the early stages of product development.
You do this by traditional Lean UX methods such as prototyping and usability testing.
But the goal should be to increase confidence in your assumptions in Alpha. Assumptions which pass this phase should go into BETA where you build the real thing. At beta stage you'll have de-risked enough to build confidence that it's worth investing in a beta or MVP.