Estimation Rationale

I guess this seems little silly, but I needed more clarity on it. According to PP 1.2, attributes need to be estimated. Can anyone give me examples of attributes for project? I guess, when we say small, medium , large type projects they form the characteristics of project and not the attibutes. For sure, once we are clear with attributes we can surely ensure that our effort estimates (PP 1.4) are based on estimation rationale. How will project attibutes map up to estimation rationale?

What PP SP 1.2 is addressing is the Basis of Estimate for the project tasks, deliverables, etc. Basically this practice is looking for the sizing attributes that you use to ultimately determine your effort estimates. For a document, an attribute you might estimate is the number of pages for the document that have to be added, modified, or deleted. For hardware, it might be the number of drawings you have to maintain. For software, it might be the number of lines of code, function points, etc. Just using small, medium, or large doesn’t provide a Basis for Estimate. The Basis of Estimate is then your estimation model that is derived from the historical data from prior projects going through this same process. When you first begin to use the Basis of Estimate approach you may not have very good data, or no data at all. So the best you can do is make a guess as to the estimate (SWAG or engineering judgment). But over time as you collect the project data from each project, you will be able to refine this approach and have much better estimates. Then you take these sizing parameters or attributes and apply a productivity factor, again based on your historical data, to arrive at the effort and cost estimates in SP 1.4.