One of the more informal metrics (if you can call it that) is the "Truck Factor" of the team. The Truck Factor measures the amount of spread of knowledge within a team.
Formally, the truck factor is the number of people that need to be hit by a truck before the project is in serious trouble. Of course, they don't actually need to be hit by a truck, they could leave the company, fall ill, or take a vacation.
The idea is that if only one or two people know the critical components of a system, then the project is in serious trouble should they leave. A Truck Factor of one is the worst, with most of the critical knowledge with just one person. Ideally everyone in the team should know every part of the system, but thats not very practical. An achievable goal is that at least 25% to 50% of the team knows any one component. Hopefully it won't be the same 50% who know every component :).
Small teams of under 10 people usually target a truck factor of 4-5 for most parts of the system (thats around 50% of the team). Larger teams will probably target a truck factor of around 8 (which would probably be around 25% of the team). This means that should a couple of critical people go on vacation or leave the company, there are enough people in the team who can cover for them.
Ironically, smaller teams usually have larger truck factors compared to larger teams. In teams of 5-10 members, there is a high chance that most of them know most parts of the system, with only a couple of modules having a low truck factor. In teams with above 20 people, chances are that a few people at the top (designer, architect, lead) know the system thoroughly and the rest only know about the couple of modules that they worked on. Should these people leave at around the same time, the project could be in serious trouble.
One of the things that we have been working on is increasing the truck factor. Every once in a while, we target areas with a truck factor of one and that person then gives a seminar to the rest of the team. Not only does it help with knowledge sharing, but it also means that the person can take a vacation without worry.
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