Software Estimation: Demystifying the Black Art: The Black Art Demystified (Best Practices (Microsoft))
Steve McConnell

£21.94

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This is another great book by Steve McConnell.

His empahsis is on large projects with thourough development processes; he gives many examples of rigorous studes that back up his advice.

If you follow all the advice in this book - you'll be able to make very accurate estimates with a clear idea of the risks involved.

Even if you don't need so much detail - the book provides many tips to greatly improve the accuracy of even rough calculations. 

Tip #1  Distinguish between estimates, targets and commitments.

Tip #31 Look for something you can count that is a meaningful measure of the scope of work in your environment.