In a previous post, I discussed preparing for coding interviews as a candidate. In this post, I want to flip to the other side of the table and talk about conducting better interviews from the interviewer perspective.
Goals of an interview
Before digging into the details, it’s worth reviewing what the purpose of an interview is- figuring out whether a random person who you probably don’t know much about (i.e., “the candidate”) is a good match for a specific position at your company (i.e., “the job”).
The most reliable way to figure out whether the candidate is a match for the job is to observe them doing the job or, at least, doing activities that are as close as possible to the work involved in the job. If you were interviewing a chef for a restaurant, you would ask them to cook a meal for you. Likewise, in the case of a software engineer, your interview should focus on things that software engineers actually do day-to-day- writing, testing, and running code on a computer, debugging issues in systems, and explaining technical ideas to colleagues, among other things.
Questions that are not directly related to day-to-day work on the job may have some correlation with eventual job performance, but it’s not going to be as strong. Given the amount of time required for the interview on both sides as well as the risk of bad hiring decisions, it’s best for all questions to be as practical and job-relevant as possible.
Now that we’ve covered the purpose of interviewing, I’d like to share some “rules” for coding interviews to ensure that they’re productive and provide good signal.
Rule 1: Coding problems should be done on a computer, not a whiteboard
Writing code on a whiteboard made sense 25 years ago, before laptops were ubiquitous and when compiling and testing small programs could be painful due to primitive tooling. Now, there’s really no good reason to do it. Unless you happen to be at a company where software is written without computers, having a candidate use a computer is much closer to the work in the actual job and, therefore, is a more accurate way to predict job performance.
Beyond the high-level, philosophical reasons for doing coding problems on computers, there are a number of practical benefits for both the candidate and the interviewing company:
- Writing code on a computer, ideally their own computer, allows the candidate to use the tooling that they feel comfortable with and that makes them productive. Since engineers at most companies are allowed to pick their own tooling (within reasonable limits), this better matches the conditions of the actual job.
- On a computer, you can’t hand-wave away parts of your solution (e.g., “suppose we had a helper function that implemented a binary search”). The candidate has to either implement it themselves or find some existing code or library to use.
- Coding problems on a computer requires the candidate to write tests and then iteratively debug problems that are found. These testing and debugging skills are an important part of software engineering work, but they’re much harder to evaluate on a whiteboard.
Rule 2: Please, no algorithm trivia
Understanding algorithms and data structures is a prerequisite for writing good software. Having them all committed to memory, however, is not. This is particularly true for the long tail things that are rarely encountered in day-to-day work. Quizzing a candidate on lexicographic sorting, heap implementations, or reversing linked lists is usually just a waste of time- if you need to understand the details of these things on the job, you can just look them up.
The same goes for trivia about programming language syntax, operating systems, hardware, or other technical topics. If it’s something that people can either pick up on the job or look up as needed, you’re not actually measuring something that’s critical for job success; instead, you’re picking up correlated skills (e.g., studying and memorization) that may not be relevant.
Rule 3: Avoid one-shot, all or nothing problems
Most problems that engineers work on are done in stages. You usually don’t deliver a new system in a single commit. Instead, you start with the high-level data models and interfaces, sketch out the key classes, fill in the blanks with basic implementations, then come back and optimize as needed. At each stage, you get feedback from your peers (or, in some cases, your end users), and make course corrections as needed before continuing to the next stage.
Coding problems done in interviews should ideally be structured in a similar way. Instead of just throwing a problem out and asking the candidate to solve it end-to-end in one burst, it’s better to divide it into chunks and work through it in stages- first, have the candidate write a skeleton class, then ask them to add in a method to do X, then a method to do Y that uses X, etc. Ideally, there should be a long list of these extensions so that even the best candidates never make it through all of them.
In addition to being more like real projects, this approach has a number of other, interview-specific benefits:
- Candidates are eased into the problem gradually. If they’re prone to interview anxiety (as I often am), this makes them feel more comfortable.
- Having seen the problem before is less of an advantage. The candidate may be able to get an answer to the core parts very quickly, but then you can just ask harder and harder extensions which they’re less likely to have prepared solutions for.
- Performance is less likely to be binary. Even weak candidates should make some progress, whereas stronger candidates will get to different parts of the problem depending on their skills and background. This allows for more granular feedback than simply “they got the problem so they passed” or “they didn’t get it so they failed”.
Other approaches to more practical interviews
Over the course of my career, I’ve seen companies take other approaches to make their interviews better match real work. These are less common and a bit more controversial than the rules above, so I’m putting them in their own section.
Instead of asking a bunch of smaller, 45-60 minute technical questions, some companies have their candidates do a single “mini-project”. The work is either done at home, before the interview, or for a couple of hours on site. Ideally, the candidate doesn’t just submit their code but also gets a chance to present their work and discuss why they took the approaches that they did with their interviewers.
I think these projects can be a good way to make interviews more realistic and give candidates a chance to show off their skills. On the other hand, when the project is done at home (as is usually the case), there’s pressure to spend a lot of time on it, way more than the suggested amount, to make a good impression. This can be unfair for candidates who have full-time jobs and/or significant personal obligations outside of work.
The idea here is to have the candidate pair with a member of the team instead of solving interview problems by themselves. The pair can work together on either made-up interview questions or, in some cases, actual project work for the company. Ideally, the company has hardware set up that’s designed for pairing, i.e. two monitors, mice, and keyboards connected to a single computer.
The advantages over standard, non-paired interviews are that:
- The interview better tests collaboration, a big part of practical software work, by forcing the candidate to work directly with someone else.
- The interviewer can step in and help the candidate make progress when they get stuck.
- If the pairing is on actual project work, the interview can be a more realistic test of how well the candidate will perform on the job.
I think there are good intentions here. In practice, though, there are a number of issues I’ve seen with pairing interviews that make me have mixed feeling about them:
- If the company provides its own hardware, then the candidate might be forced to use a setup that’s not ideal for them.
- Often, there really isn’t much pairing going on- the interviewer just watches the candidate solve the question like in a normal, non-paired interview.
- If the pairing is on actual project work, then the candidate is basically being forced to do unpaid work. Aside from the potential legal issues involved here, this can feel exploitative.
Interviews should measure how well candidates do on realistic problems under job-like constraints. Despite the common sense behind this, I still see companies routinely asking abstract, one-shot, whiteboard coding questions. Just don’t do this- it’s not the way to hire the best people.