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20 May, 2017

The technical interview.

I like interviewing people. I like going to interview. This is how I do it.

1 - Have your objectives clear.

There is no consensus on what an interview is for. Companies work in different ways, teams have different cultures, we have to accept this, there is no "one true way" of making software, of managing creative teams and of building them.

Some companies hire very slowly, very senior staff. Others hire more junior positions more often. Some companies are highly technical, others care more about creativity and the ability for everyone to contribute to the product design. Some prefer hyper-specialized experts, others want to have only generalists and so on.

You could craft an interview process that is aimed at absolutely having zero false positives, probing your candidates until you know them more than they know themselves, and accept to have a high rate of false negatives (and thus piss some candidates) because the company is anyhow very slow in creating new openings.
Or you could argue that a strength of the company is to give a chance to candidates that would otherwise be overlooked, and thus "scout" gems.

Regardless, this means that you have to understand what your objectives are, what the company is looking for and what specifically your role in the overall interview process. 
And that has to be tailored with the specific opening, the goals in an interview for a junior role are obviously not the same as the goals you have when interviewing a senior one.

Personally, I tend to focus on the technical part of the interview process, that's both my strongest suit and also I often interview on the behalf of other teams to provide an extra opinion. So, I don't focus on cultural fit and other aspects of the team dynamics.

Having your objectives clear won't just inform you on what to ask and how, but also how to evaluate the resulting conversations. Ideally one should always have an interview rubric, a chart that clearly defines what is that we are looking for and what traits correspond to what levels of demonstrated competence.

It's hard to make good standardized interviews as the easiest way to standardize a test is to dumb it down to something that has very rigid, predefined answers, but that doesn't mean that we should not strive to engineer and structure the interview in a way that makes it as standard, repeatable and unbiased as possible.

2 - What (usually) I care about.

Almost all my technical interviews have three objectives:
  • Making sure that the candidate fulfills the basic requirements of the position.
  • Understanding the qualities of the candidate: strengths and weaknesses. What kind of engineer I'm dealing with.
  • Not being boring.
This is true for basically any opening, what changes are the specific things I look for.

For a junior role, my objective is usually to hire someone that, once we account for the time spent by the senior staff for mentoring, still manages to create positive value, that doesn't end up being a net loss for the team.

In this case, I tend not to care much about previous experience. If you have something on your resume that looks interesting it might be a good starting point for discussion, but for the most part, the "requirement" part is aimed at making sure the candidate knows enough of the basics to be able to learn the specifics of the job effectively.

What university where you in (if any), what year, what scores, what projects you did, none of these things matter (much) because I want to directly assess the candidate's abilities.

When it comes to the qualities a junior should demonstrate, I look for two characteristics. First: its' ability to unlearn, to be flexible. 
Some junior engineers tend (and I certainly did back in my times) to misjudge their expertise, coming from high education you get the impression that you actually know things, not that you were merely given the starting points to really learn. That disconnect is dangerous because inordinate amounts of time can be spent trying to change someone's mindset.

The second characteristic is, of course, to be eager and passionate about the field, trying to finding someone that really wants to be there, wants to learn, experiment, grow.

For a senior candidate, I still will (almost) always have a "due diligence" part, unless it would truly be ridiculous because I do truly -know- already that the candidate fulfills it. 
I find this to be important because in practice I've seen how just looking at the resume and discussing past experience does not paint the whole picture. I think I can say that with good certainty. In particular, I've found that:
  • Some people are better than others at communicating their achievements and selling their work. Some people might be brilliant but "shy" while others might be almost useless but managed to stay among the right people and absorb enough to present a compelling case.
  • With senior candidates, it's hard to understand how "hands on" one still is. Some people can talk in great detail about stuff they didn't directly build.
  • Some people are more afraid of delving into details and breaking confidentiality than others.
  • Companies have wildly different expectations in given roles. In some, even seniors do not daily concern themselves with "lower level" programming, in others, even juniors are able to e.g. write GPU assembly.
When it comes to the qualities of a senior engineer, things are much more varied than juniors. My aim is not to fit necessarily a given stereotype, but to gain a clear understanding, to then see if the candidate can fit somewhere.
What are the main interests? Research? Management? Workflows? Low-level coding? The tools and infrastructure, or the end-product itself?

Lastly, there is the third point: "not be boring", and for me, it's one of the most interesting aspects. To me, that means two things:
  • Respect the candidate's time.
  • Be aware that an interview is always a bi-directional communication. The interviewer is under exam too!
This is entirely an outcome that is guided by the interviewer's behavior. Nonetheless, it's a fundamental objective of the interview process: you have to demonstrate that you, and your company, are smart and thoughtful.

You have to assume that any good candidate will have many opportunities for employment. Why would they chose you, if you wasted their time, made the process annoying, and didn't demonstrate that you put effort into its design? What would that telegraph about the quality of the other engineers that were hired through it?

You have to be constantly aware of what you're doing, and what you're projecting.

3 - The technical questions.

I won't delve into details of what I do ask, both because I don't want to, and because I don't think it would be of too much value, being questions catered to the specific roles I interview for. But I do want to talk about the process of finding good technical questions, and what I consider to be the main qualities of good questions.

First of all, always, ALWAYS use your own questions and make them relevant to the job and position.  This is the only principle I dare to say is universal, regardless of your company and objectives, should be adhered to.

There is no worse thing than being asked the same lazy, idiotic question taken from the list of the "ten most common questions in programming interviews" off the internet.

It's an absolute sin. 
It both signals that you care so little about the process that just recycled some textbook bullshit, and it's ineffective because some people will just know the answer and be able to recite it with zero thought.
That will be a total waste of time, it's not even good for "due diligence" because people that just prepared and learned the answer to that specific question are not necessarily really knowledgeable in the field you're trying to assess.

The "trick" I use to find such questions is to just be aware during my job. You will face time to time interesting, general, small problems that are good candidates to become interview questions. Just be aware not to bias things too much to reflect your specific area of expertise.

Second, always ask questions that are simple to answer but hard to master. The best question is a conversation starter, something that allows many follow-ups in many different directions.

This serves two purposes. First, it allows people to be more "at ease", relaxed as they don't immediately face an impossibly convoluted problem. Second, it allows you not to waste time, as you established a problem setting, asking a couple follow-ups is faster than asking two separate questions that need to be set-up from scratch.

Absolutely terrible are questions that are either too easy or too hard and left there. On one extreme you'll get no information and communicate that your interview is not aimed at selecting good candidates, on the other you will frustrate the candidate which might start closing up and missing the answers even when you scale back the difficulty.

Also, absolutely avoid trick questions: problems that seem should have some very smart solution but really do not. Some candidates will be stuck thinking there ought to be more to the question and be unable to just dare try the "obvious" answer.

Third, avoid tailoring too much to your knowledge and expectations. There are undoubtedly things that are to be known for a given role, that are necessary to be productive and that are to be expected from someone that actually did certain things in the past. But there are also lots of areas where we have holes and others are experts.

Unfortunately, we can't really ask questions about things we don't know, so we have to choose among our areas of expertise. But we should avoid specializing too much, thinking that what we know is universal.

A good trick to sidestep this problem (other than knowing everything) is to have enough options that is possible to discuss and find an area where the both you and your candidate have experience. Instead of having a single, fixed checklist, have a good pool of options so you can steer things around.
This also allows you to make the interview more interactive, like a discussion where questions seem a natural product of a chat about the candidate's experience, instead of being a rigid quiz or checklist.

Lastly, make sure that each question has a purpose. Optimize for signal to noise. There are lots of questions that could be asked and little time to ask them. A good question leaves you reasonably informed that a given area has been covered. You might want to ask one or two more, but you should not need tens of questions just to probe a single area.

This can also be achieved, again, by making sure that your questions are steerable, so if you're satisfied e.g. with the insight you gained about the candidate's mastery of low-level programming and optimization, you could steer the next question towards more algorithmic concerns instead.


I do not think there is a single good way of performing interviews. Some people will say that technical questions are altogether bad and to be avoided, some will have strong feelings about the presence of whiteboards in the room or computers, of leaving candidates alone solving problems or being always there chatting.

I have no particular feeling about any of these subjects. I do ask technical questions because of the issues discussed above, and for the record, I will draw things on a whiteboard and occasionally have people collaborating on it, if it's the easier way to explain a concept or a solution.

I do not use computers because I do not ask anything that requires real coding. The only exception is for internships, due to the large number of candidates I do pre-screen with an at-home test. That's always real coding, candidates are tasked to make some changes to a small, and hopefully fun and interesting codebase, which either I make ad-hoc from scratch or extract from suitable real-world programs).

I do not think any of this matters that much. What it really matters, is to take time, study, and design a process deliberately. In the real world the worst interviews I've seen were never because they chose a tool or another, but because no real thinking was behind them.

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