I've done this both ways - using the same core group of people vs. mixing and matching.
1 & 2 are valid points, however here's my complete list
Small Group Does all Interviews:
- Group norms quickly and meetings to come to a general set of hiring objectives is fast
- People learn each other's questions and usually buffer each other - one guy won't ask the other guy's favorite question.
- Answers produced by the group are nicely consistent after a while, and you get it in terms of "not as good as the other guy..."
- Small group gets burned out very quickly. Engineers start to hide from interviews.
- Productivity on regular work starts to diminish as people in small group are constantly tapped for interviews, which really breaks their flow if they are coders.
- Productivity on rest of team may also diminish because you generally choose the smart, sensible, articulate people for the interview panel... these are also the people you rely on to keep the team humming and to clarify inter-engineer problems, you only find out how much things have gone nuts when you are done interviewing and the folks on your panel get a bunch of unpleasant surprises.
- People who are not on the panel are completely left out of the process - what if they had good viewpoints?
- Booking becomes a nightmare, as a very limited group of people have to be scheduled.
Wider Group does Interviews:
- The whole team builds a consensus on what a new hire should look like - which can also help people understand each other better.
- People get slightly less burned out - they have to interview less.
- The team as a whole doesn't loose it's key players to endless interviews.
- More junior/less articulate people get more practice talking to people
- No elite group - feels more fair.
- More flexibility in booking for a candidate's schedule.
Some people are really lousy interviewers, you have to suffer through that and have management judgement to balance out and coach this skill set.
It takes longer to get consensus as the focus changes with who is interviewing ...this time.
Process has to be more codified - there needs to be a normal system of meet and greet, hand off between interviewers, and goodbye and walk to the door process. And some emergency "what to do if there's a problem" things (example - someone has a crisis and can't make their slot). I once lost a candidate in the building because we lost a conference room, relocated and no one told me! :)
You can end up with some bad mashups. Both in how you show team personality to the candidate, and how you review and redact the candidate's responses - you want enough talented interviewers and charismatic people to make this a good use of everyone's time and leave a positive impression with candidate. Random selection won't get you that. You can also have a biased view point - for example if you have a mix of Strong Talent A and Strong Talent B, and only Strong Talent A covers the pool today, they will give you a slanted view point of the candidate w/ respect to A and give you no info on B.
Getting the feedback is more work as you have to remember who was on what interview round.
My take is that interviewing is a community event (even when it's done one on one) - if you have a good team, you want to make sure that you get someone the team can accept - not someone a few people on the team can accept.
That doesn't necessarily mean a free for all.
My strategy for a small team is:
- Manager + Some one near the team + 2-3 people on the team.
- Careful mashups - get a pool that will give you some diverse opinions while balancing out the odd ducks vs. charismatic people - for my 2-3 on the team slot, I often give the organizer a set of rules - one from column A, two from Column B, and never ever both Bob and Mary in one set. (note, sometimes that's because I find both Bob and Mary to be super chatty, and I know the interview will never end on time... other times it's because without Bob or Mary around to help the team, the progress of the team will sink into the sea)
- Reasonably clear process - everyone knows how to greet, interview, handoff and say good bye. Everyone knows what I'll ask for for feedback and how the feedback process will work.
- Be aware and adjust the load as you go - you may find someone is a hidden gem of an interviewer and want them more often... you may find someone can't get anything done while interviewing and need to decrease the load.
If you're working in the tech market, this is a marathon, not a sprint these days. If you have 1 open position and you fill it in a week... let me know where you live so I can recruit there!
Many interview rounds can go on for 3+ months. Think about the drain on the team needed for that level of effort and plan accordingly.
Also realize that with a job that can take 3 months to fill - you will likely have to turn down a first pool of candidates while waiting to see what else you get. Comparing and contrasting is a myth - you can only compare against a long term set of experiences, you can't pick people like you are picking produce - they won't wait quietly for you to decide if you are also waiting to see what else comes in.