I'm a bit confused about what the CMMI requires for estimation. Is it valid for us to define our process as simple as guessing? Could we use something similar to Planning Poker or other similar agile techniques? The problem is that we've been told we need to have a quantitative and repeatable process for estimating in order to meet Project Planning (PP) SP 1.4 "Estimate the project effort and cost for the work products and tasks based on estimation rational." But in reality we don't have historical data and many of us think estimating is subjective anyway.
When you start implementing Maturity Level 2 there is no expectation that you have any historical data for estimating. The best you can do at that point is guess, SWAG, or use engineering judgment. One of the characteristics of being a Maturity Level 2 organization is that you are learning. You are learning what it means to estimate a project. You are learning how to improve your estimates so that over time you will be able to produce accurate estimates to build the foundation to allow the organization to move to Maturity Level 3. So, by the time you are ready for a Maturity Level 2 SCAMPI A appraisal, the expectation is that you have a repeatable estimation process (repeatability is at the core of Maturity Level 2) that is based on historical project data. Estimation is not difficult, it just takes a little bit of brain power and being systematic in your approach. One of the purposes of achieving Maturity Level 2 is moving from being subjective in estimation to being objective. If you have people that believe estimation is subjective, then I am afraid that you are still operating as a Maturity Level 1 organization.