In my book I provide many examples of interview questions: informational, technical and case questions that you are likely to encounter at quant interviews.

The following is a really good hybrid technical-cum-case question that I myself was asked when I interviewed for my present full-time financial engineer job. It’s a technical question because it tests whether you know what a median is, how to find the median, and how you would do it via a computer program. It’s also a case question because it involves some serious problem-solving skills — it’s not a trivial technical question but requires some degree of out-of-box thinking. Chapter 4 of my book reveals the exact steps you can take to approaching a question like this.

I won’t disclose the answer, of which there are two approaches, today, because I want you to work it out yourself.

Question: You are given a sample of 10 trillion unsorted numbers. You have an ordinary PC with limited CPU power and memory, but a big enough hard disk to hold the data (plus to cache any necessary calculations). Now, how can you find the median of these 10 trillions numbers quickly? (By "quickly" I mean within a reasonable amount of time.)

Analysis: Usually, finding the

*exact*median of a series of numbers requires sorting the series and then picking out the middle guy. But, alas, it’s pretty much impossible to sort 10*trillion*numbers, no matter which sorting algorithm you use. So, how can you solve this problem?Hint: Is it really necessary to find the

*exact*median? If not, what would you do? But if yes, what would you do then?Note: This question can

*definitely*be solved, i.e., it has a real, non-trivial answer.If you think you have the answer, feel free to e-mail it to me and I’ll let you know if you got it right.

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