A surprising proportion of applicants, with plausible-looking CVs, cannot write code to calculate the mean of a bunch of numbers. They spend most of the interview on what is meant to be a 30-seconds-long relax, have a laugh and move on question. Many apply through agencies, and the agents know I ask this question and I know they give the candidates a preparation pack and this does not help.
I've been lucky enough not to have seen that happen, though I have seen at least one candidate tragically fall apart on a more complex question, and that can be painful for everyone.
Why do you keep using these agencies that send you unqualified candidates?
Also, I think the better way to structure coding challenges is as a series of increasingly difficult problems that build on each other. If the candidate gets stuck on the easier problems, you can just quietly ignore the later ones. It feels very risky to start an interview with something like "Okay, hah, here's a real simple one to get us started," because if they fall apart there because of nerves or something, they will not recover in time. What do you think?
> Why do you keep using these agencies that send you unqualified candidates?
Corporate politics.
> as a series of increasingly difficult problems that build on each other.
...I mean, that's exactly how I structure these. When a candidate is stuck on the very first one of the series, though, and spends the entire interview on it... I think that's a fail. What do you think?
Asking someone to calculate the mean of a group of numbers is so basic that it's admitting that your prescreening process to that point is broken. I hope we're talking about Zoom interviews and you're not actually bringing people into a physical location before asking that question.
Assuming your process isn't broken, there's a psychological risk to asking a qualified person a question that is excruciatingly basic. If they know it's a question only a fool or liar would get wrong, then a momentary blank can lead to an anxiety spiral. I think even the first question should be plausibly challenging.
I also don't understand why you're letting someone spin out for an entire interview on a simple question. Are we talking like half an hour or more? Maybe just let them try for a couple minutes and then pull back, give them a breather if they look stressed, and gently confirm that they've done actual hands-on programming in the past. Maybe their bootcamp misled them, or their previous job was a project manager sort of thing. Bad fit, sorry we didn't catch this earlier, best of luck to you.
I have no explanation for this phenomenon.