311 people is the population
966 sick days will be called a line of inquiry
0.3215 in comparison to 5.383 can simply be said through comparison of population vs sick day contribution.....there is no fancy words to use, and it might even confuse people if it got too technical anyway. something like "the average amount of sick days per person is ........, and joe bloggs has had 16 times more than the average"
As for whether it is fair...that's a moral judgement down to you whether you want to make an example of someone or not!
One thing i do know, its a lot of sick days for 1 person!! I wouldn't want them employed, but then maybe it requires treading carefully if they have good ground..