I have estimated the cost of a product by COCOMO, then I estimated it by FP too. the results were so different from each other! The cost estimated by FP was about two times more than the estimated cost by Cocomo. which one is more reliable as this product belongs to a large project. shoud I skip the FP result?
Which one is more reliable in a large project, COCOMO or FP?
cost estimation;cocomo;fpa
In general I would trust FP over Cocomo.  Cocomo offers a lot of twiddle factors, and in general people tend to twiddle twiddle factors in a way that gives them the answers that they want to hear.Even were that not true, when it comes to a large project I would almost always pick the larger number on general principle.  And then double it.  Large software projects have a very strong tendency to overrun both schedule and budget.  And not by small factors.  I remember one study that looked at projects which were projected cost more than $1 million.  Of those that completed, on average at delivery they took more than twice as long as what was scheduled, cost more than twice what was estimated, and were delivered with less than half the originally specified features.  And those were the successes - a substantial fraction never got delivered at all.There are many, many reasons for that.  Read Software Estimation for an excellent description, and details on how to do better.