Onboarding is a real thing but Brook's calculations are absurd by today's standards. 1 Month to onboard someone? We have developers committing code in their first week. Sometimes day 1 or 2.
Read McConnell's essay. It's v. good.
That doesn't mean they're fully on-boarded until 6 months in.
Brooks exact numbers might be come from a different time, but the principle holds unless you have an uncommonly decoupled set of problems.
If you're doing anything complex (which can just be: if your codebase is more than a year or so old) than the quality and velocity of code someone can contribute on day 1 is vastly lower than they can a month later.
I see the Mythical Man Month tossed around as an excuse for not trying sometimes - by people who haven't even tried to split up and spread out chunks of work in parallel - but onboarding cost is definitely real.
You gotta evaluate "can I get enough overall throughput at this point in time to make up for the total efficiency lost" and also the rest of the business priorities outside of this one project.
Hmm... not sure I agree with this. A few points come to mind:
- Some portions of a project become hotspots at different times and have a natural code concurrency limit (it may not be 1, but certainly 10 people can't make changes to the same class or other critical code path concurrently)
- Onboarding is a real thing. It requires current staff to train newcomers, at the very least. At worst, they have to account for the newcomer's coding/architectural misses
- Communication and consensus building with more folks takes exponentially more time (or is it combinatorial, I can't keep that straight).