Neil Mitchell's Blog (Haskell etc)

Interviewing usually involves some level of subjectivity. I once struggled to decide about a candidate, and after some period of reflection, the only cause I can see is that I was biased against the candidate. That wasn't a happy realisation, but even so, it's one I think worth sharing.

Over my years, I've interviewed hundreds of candidates for software engineering jobs (I reckon somewhere in the 500-1000 mark). I've interviewed for many companies, for teams I was managing, for teams I worked in, and for other teams at the same company. In most places, I've been free to set the majority of the interview. I have a standard pattern, with a standard technical question, to which I have heard a lot of answers. The quality of the answers fall into one of three categories:

• About 40% give excellent, quick, effortless answers. These candidates pass the technical portion.
• About 50% are confused and make nearly no progress even with lots of hints. These candidates fail.
• About 10% struggle a bit but get to the answer.

Candidates in the final bucket are by far the hardest to make a decision on. Not answering a question effortlessly doesn't mean you aren't a good candidate - it might mean it's not something you are used to, you got interview nerves or a million other factors that go into someone's performance. It makes the process far more subjective.

Many years ago, I interviewed one candidate over the phone. It was their first interview with the company, so I had to decide whether we should take the step of transporting them to the office for an in-person interview, which has some level of cost associated with it. Arranging an in-person interview would also mean holding a job open for them, which would mean pausing further recruitment. The candidate had a fairly strong accent, but a perfect grasp of English. Their performance fell squarely into the final bucket.

For all candidates, I make a decision, and write down a paragraph or so explaining how they performed. My initial decision was to not go any further in interviewing the candidate. But after writing down the paragraph, I found it hard to justify my decision. I'd written other paragraphs that weren't too dissimilar, but had a decision to continue onwards. I wondered about changing my decision, but felt rather hesitant - I had a sneaking suspicion that this candidate "just wouldn't work out". Had I spotted something subtle I had forgotten to write down? Had their answers about their motivation given me a subconscious red-flag? I didn't know, but for the first time I can remember, decided to wait on sending my internal interview report overnight.

One day later, I still had a feeling of unease. But still didn't have anything to pin it on. In the absence of a reason to reject them, I decided the only fair thing to do was get them onsite for further interviews. Their onsite interviews went fine, I went on to hire them, they worked for me for over a year, and were a model employee. If I saw red-flags, they were false-flags, but more likely, I saw nothing.

However, I still wonder what caused me to decide "no" initially. Unfortunately, the only thing I can hypothesise is that their accent was the cause. I had previously worked alongside someone with a similar accent, who turned out to be thoroughly incompetent. I seem to have projected some aspects of that behaviour onto an entirely unrelated candidate. That's a pretty depressing realisation to make.

To try and reduce the chance of this situation repeating, I now write down the interview description first, and then the decision last. I also remember this story, and how my biases nearly caused me to screw up someone's career.

Summary: In previous companies I had a lot of freedom to design an interview. This article describes what I came up with.

Over the years, I've interviewed hundreds of candidates for software engineering jobs (at least 500, probably quite a bit more). I've interviewed for many companies, for teams I was setting up, for teams I was managing, for teams I worked in, and for different teams at the same company. In most places, I've been free to set the majority of the interview. This post describes why and how I designed my interview process. I'm making this post now because where I currently work has a pre-existing interview process, so I won't be following the process below anymore.

I have always run my interviews as a complete assessment of a candidate, aiming to form a complete answer. Sometimes I did that as a phone screen, and sometimes as part of a set of interviews, but I never relied on other people to cover different aspects of a candidate. (Well, I did once, and it went badly...)

When interviewing, there are three questions I want to answer for myself, in order of importance.

Will they be happy here?

If the candidate joined, would they be happy? If people aren't happy, it won't be a pleasant experience, and likely, they won't be very successful. Whether they are happy is the most important criteria because an employee who can't do the job but is happy can be trained or can use their skills for other purposes. But an employee who is unhappy will just drag the morale of the whole team down.

To figure out whether a candidate would be happy, I explain the job (including any office hours/environment/location) and discuss it in contrast to their previous experience. The best person to judge if they would be happy are the candidate themselves - so I ask that question. The tricky part is that it's an interview setting, so they have prepared saying "Yes, that sounds good" to every question. I try and alleviate that by building a rapport with the candidate first, being honest about my experiences, and trying to discuss what they like in the abstract first. If I'm not convinced they are being truthful or properly thinking it through, I ask deeper questions, for example how they like to split their day etc.

A great sign is when a candidate, during the interview, concludes for themselves that this job just isn't what they were looking for. I've had that happen 5 times during the actual interview, and 2 times as an email afterwards. It isn't awkward, and has saved some candidates an inappropriate job (at least 2 would have likely been offered a job otherwise).

While I'm trying to find out if the candidate will be happy, at the same time, I'm also attempting to persuade the candidate that they want to join. It's a hard balance and being open and honest is the only way I have managed it. Assuming I am happy where I work, I can use my enthusiasm to convince the candidate it's a great place, but also give them a sense of what I do.

Can they do the job?

There are two ways I used to figure out if someone can do the job. Firstly, I discuss their background, coding preferences etc. Do the things they've done in the past match the kind of things required in the job. Have they got experience with the non-technical portions of the job, or domain expertise. Most of these aspects are on their CV, so it involves talking about their CV, past projects, what worked well etc.

Secondly, I give them a short technical problem. My standard problem can be solved in under a minute in a single line of code by the best candidates. The problem is not complex, and has no trick-question or clever-approach element. The result can then be used as a springboard to talk about algorithmic efficiency, runtime implementation, parallelism, testing, verification etc. However, my experience is that candidates who struggle at the initial problem go on to struggle with any of the extensions, and candidates that do well at the initial question continue to do well on the extensions. The correlation has been so complete that over time I have started to use the extensions more for candidates who did adequately but not great on the initial problem.

My approach of an incredibly simple problem does not seem to be standard or adopted elsewhere. One reason might be that if it was used at scale, the ability to cheat would be immense (I actually have 2 backup questions for people I've interviewed previously).

Given such a simple question, there have been times when 5 candidates in a row ace the question, and I wonder if the question is just too simple. But usually then the next 5 candidates all struggle terribly and I decide it still has value.

Will I be happy with them doing the job?

The final thing I wonder is would I be happy with them being a part of the team/company. The usual answer is yes. However, if the candidate displays nasty characteristics (belittling, angry, racist, sexist, lying) then it's a no. This question definitely isn't code for "culture fit" or "would I go for a beer with them", but specific negative traits. Generally I answer this question based on whether I see these characteristics reflected in the interactions I have with the candidate, not specific questions. I've never actually had a candidate who was successful at the above questions, and yet failed at this question. I think approximately 5-10 candidates have failed on this question.