Every situation is different, but the very first criteria is whether or not it's needed. We've been blessed in that for many years now we've had lots of regulars who were smart and trustworthy and more than qualified for the position, but we've only had a need for a few at any given time, so lots of good candidates never get asked simply because we're still way ahead of the spam to this point.
If one were needed, I would look at tenure and general temperament. In some cases, other factors like schedule would come into play. For example, a bit of spam was getting through overnight because most of the mods and myself were asleep, and I was determined to add one who could cover more of the night. I assumed this would be someone from overseas in a different time zone, but Destiny keeps later hours than most of us, so I asked her to help us out, and she's done a great job.
So, each situation is different, but the criteria that never changes is picking someone considerate who can exercise solid judgment, someone who doesn't mind talking decisions out with the rest of us, and someone who's been around long enough to demonstrate that they have a stake in this place.
I don't think we'll need another mod for awhile yet, but our post totals have been quite high this year, so if it continues, we'll certainly have to revisit the issue at some point.
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