To determine what's comparable, I'll look at square footage, bedroom count, lot size, age and style (e.g., single story vs. two-story). I try to find homes that have similar features as the subject property (e.g., swimming pool, garage size). Once I have selected the homes that I want to use for the CMA (ideally 3 active listings, 3 pending listings and 3 sold listings), I'll take a look at all the features and I make the necessary adjustments to indicate whether a comparable property is superior or inferior to the subject and then I choose the one that are most comparable to the subject. As you can see, preparing a CMA is time consuming if it's done right and having access to data that provides you with the necessary detail is crucial. I hope this information helps.
I agree with Ute that the information and process used to build a CMA varies by the property and area. The one thing I would add is the information about the sold, pending and active listing prices. An active listing is what the seller WANTS for their property, a pending is what the buyer THINKS the property is worth (pending inspections and other suprises that may change the terms of the transaction) and RECENT SOLD (I reiterate recent) is what both buyer and seller have actually agreed the property is TRULY WORTH. What your neighbor got for their property 2 years ago in a sellers market has much less validity than the reality of the past 3-6 months.
The C in CMA means information that is as current and comparable as you can find. The A means you really need a professional analyzing the data as it is only as good as those reading and adjusting the numbers. In accounting they say GIGO (garbage in, garbage out). In a perfect world we would find the 3, 3 and 3 as Ute outlined below all in the same neighborhood with the same amenities. That rarely is the case and more often we the closest we can, adjust for all factors involved and give a high, medium and low range of pricing for that particular property. That is why market knowledge and experience are so crucial.
One last thought is to throw out those properties that look suspect, both high and low and go for the mid range. A few years ago there were a plethora of 100% loans with closing costs and other things (like furnishings or additional appliances) thrown into the loan and pricing of houses. If all the other comparable homes in a neighborhood were in the $150-160 thousand dollar range and you have one that's $175 and another that's $135, it's a red flag for mitigating factors and they probably should be deleted from your mix.
Hope this helps and good luck with your home buying.
Trisha Lee, REMAX Boone Realty, Columbia, MO