This is not really an answer to you're question, but I feel I must butt in and say it since I get the vague feeling you're trying to put a number beside how good a sys admin is.
Measurements - This one by Joel is a good one
Sins of Commission - Very good article by Joel... read it!!!
Essentially it comes down to, when you try to measure something like how good someone is at their job, whether you provide an incentive, or just want to know how good they are, you always loose in what you're not measuring.
I'll provide an example, at work (large Canadian telecom) we have an outage database and track weighted outages throughout the year, and have an objective in our "performance reviews". We also have an outage notification system where internal people can see essentially what's broken right now. Now because this is tied to performance and bonuses, I've seen people outright lie on the outages so that it doesn't weigh as heavily against them.
Now I discovered our brand new untested firewall was dropping packets, causing tunnels to drop intermittently, and some other minor stuff. I opened the outage ticket as low or no impact, but said customers could see intermittent drops or throughput issues, but it would be rare.
A week later and someone doing performance testing from another group and I were chatting, and he was telling me about sometimes we're getting bad throughput compared to the other carriers, and I simply said well that's likely due to the packet drop issue we have. He had no idea it was going on, and sure enough when I checked the ticket my supervisor had closed it the next day as not an issue. We went around him and notified upper management, but it makes me wonder how often this has happened. I know employee's who literally won't touch stuff because they're afraid of being chewed out by their manager the next day if they break something.
Once a year or a statistic is not the way to monitor you're employee's. Up time may need to be measured due to SLA's, but honesty trumps all and you need to reward good behaviour, and the only way to do that is to do you're own job well, work with you're employee's on root cause analysis and outage notifications, comment immediatly on positive or negative feedback you have, don't leave anything waiting for the bi-annual review.
More to what you may be looking for is to talk to those people who you're admin interacts with and see if they respond kindly and curteously, or if you're the group everyone hates dealing with. If there's an outage, instead of measuring the uptime, get the sysadmin to explain what happened and why, and what can be done to ensure it never happens again, and follow up to see that the fix is done.