These are the questions banks are asking people they really want to hire

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We are in the age of the mathematician. At Goldman Sachs, there were 500 stock traders 20 years ago, and there are now are three - plus an array of engineers. Strats are all the rage, both at GS and elsewhere. Daniel Pinto, head of the investment bank at J.P. Morgan says, "artificial intelligence, robotics, machine learning, distributed ledgers and big data," will shape the bank's future. - All the hail the quant.

Clearly, you can't become a quant overnight. You're probably going to need a PhD in mathematics, plus knowledge of programming languages like R or Python or C++. Then you're going to survive the onerous quant interview process.

In the past four years, Dirk Bester, a VP-level quant at Barclays with a PhD in Bayesian algorithmic design, has interviewed everywhere from Barclays to Blackrock, to BNP Paribas, Cumulus, G-Research, Goldman Sachs, GSA Capital, J.P. Morgan, Man Group, Oxford Asset Management, RBC, RBS, Squarepoint Capital, Two Sigma, UBS, and...Winton Capital. Bester has written a detailed guide to his quant interview experiences and posted it to Github.  Whether you want to be a quant/strat, or you simply want to be in with a chance of remaining employed in finance in 2025, it's set to become mandatory reading.

What follows are the three easiest questions and answers from Bester's guide. There are many far harder ones listed in the full iteration on Github. If you can't answer the questions below, your chances of being useful to banks in the brave new quantitative world are therefore minimal. - Unless, of course, you can compensate for your mathematical illiteracy with some strong relationships. 

1. The murderer question: 

Murderer in a field

You are guarding 100 murderers in a field, and you have a gun with a single bullet. If any one of the murderers has a non-zero probability of surviving, he will attempt to escape. If a murderer is certain of death, he will not attempt an escape. How do you stop them from escaping?

The murderer answer:

Bester says that solving this question requires you to make some possibly incorrect assumptions about the rationality of the criminals involved. It also requires you to clarify whether you will be able to talk to the murderers (through a megaphone) before they make a run for it. Assuming the murderers are rational and you do have a megaphone, this is the solution...

"Start with n = 1," says Bester. "Were there a single murderer, you would shoot him if he tried to escape. Because he would know the probability of survival is zero, he wouldn’t try. Were there only two murderers in the field, each would have a probability of surviving of 0.5, and thus would attempt to escape. But, if you told one of the two that you would definitely shoot him if the two of them attempted to escape, he wouldn’t make an attempt, bringing you back to the situation with a single murderer."

From there, Bester says you need to generalize this outcome to three or more murderers. For any given group, you need to identify one murderer as the so-called “sacrificial escapee”, and make to him aware of it. One way to do this is to every murderer in the murderer sub-group a number from 1 to n and to tell them that if any group attempts to escape, the group member with the highest number will be shot. That way, no group will attempt an escape.

2. The airplane seating question 

Airplane seating

Bester says he was asked this question repeatedly in successive finance interviews. It's sometimes called the "drunken passenger" and goes like this:

One hundred people are in line to board a plane which has exactly 100 seats. Each passenger has a ticket assigning them to a specific seat, and the passengers board one at a time. The first person to board is drunk, picks a random seat, and sits in it. The remaining passengers board; if they find their assigned seat empty, they sit in it. If they find their seat taken, they pick a random seat to sit in. Everyone boards, and is seated. What is the probability that the final person who boards gets to sit in their assigned seat? 

The airplane seating answer:

Bester says this question is so well known that it's detailed in classic books on quant interviews by Mark Joshi and Paul Wilmott. 

He cites Wilmott's solution. - Start by considering just two people: the drunkard and yourself. In this case, the drunkard will sit in his correct seat 50% of the time and you will get your allocated seat. In another 50% of cases, the drunkard will sit in your seat and you will be displaced.  Then, expand this to three people: the drunkard either sits in his seat, your seat, or in the other person (Peter's) seat. The chances of him sitting your seat and his seat are the same and therefore balance out. If the drunkard sits in Peter's seat, the outcome will depend on whether Peter sits in the drunkard's seat or yours. So, there's a 50% chance that you'll get to sit in your allocated seat (and this holds however many people there are).

3. The sock question

socks 6

A bag contains N socks, some of which are black, and some of which are red. If two random socks are picked, the probability that they are both red is 1/2. What is the smallest possible value of N for which this is possible?

The sock answer:


Bester says you solve this as follows:

Let Ri indicate the event where the ith sock is red.

Let r be the number of red socks in a bag of N socks.

socks 8

If N = 2, both socks will be red with certainty, so this cannot work. When N = 3 you have...

socks 69

...and there aren’t any integer values of r that would make this hold.

But, when N=4, you get the following solution:

socks 99

Therefore, says Bester, the smallest possible value of N is four. Simple! Or not.

Quant questions of your own:

If you want to impress at a quant interview, Bester also suggests a number of questions you might want to ask yourself, partly to establish whether the team you're joining is working with the latest technologies. These include the following:

What operating systems do the team use?

What are the proportions of Windows, Apple, GNU/Linux?

What does the team use for technical documentation? LaTeX, Wiki, Word, or nothing?

What software does the team use for version control? Svn, Git, Mercurial, Team Foundation Server, or appending V1, V2, V3 to foldernames?

What programming languages does the team use? R, Python, C++, Julia, Matlab? For data analysis? For scripting? For production?

Is the team Bayesians, frequentists, or whatever gets the job done?

Are modellers expected to write the code to implement their models, or is this handled by developers? •

Is there a formal process for requesting the installation of new software libraries?

Good luck.

Photo credit: NadyaPhoto/Getty, Marc Dufresne/Getty/slovegrove/Getty

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