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Data Scientist Interview Questions in United States


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You're about to get on a plane to Seattle. You want to know if you should bring an umbrella. You call 3 random friends of yours who live there and ask each independently if it's raining. Each of your friends has a 2/3 chance of telling you the truth and a 1/3 chance of messing with you by lying. All 3 friends tell you that "Yes" it is raining. What is the probability that it's actually raining in Seattle?

30 Answers

Bayesian stats: you should estimate the prior probability that it's raining on any given day in Seattle. If you mention this or ask the interviewer will tell you to use 25%. Then it's straight-forward: P(raining | Yes,Yes,Yes) = Prior(raining) * P(Yes,Yes,Yes | raining) / P(Yes, Yes, Yes) P(Yes,Yes,Yes) = P(raining) * P(Yes,Yes,Yes | raining) + P(not-raining) * P(Yes,Yes,Yes | not-raining) = 0.25*(2/3)^3 + 0.75*(1/3)^3 = 0.25*(8/27) + 0.75*(1/27) P(raining | Yes,Yes,Yes) = 0.25*(8/27) / ( 0.25*8/27 + 0.75*1/27 ) **Bonus points if you notice that you don't need a calculator since all the 27's cancel out and you can multiply top and bottom by 4. P(training | Yes,Yes,Yes) = 8 / ( 8 + 3 ) = 8/11 But honestly, you're going to Seattle, so the answer should always be: "YES, I'm bringing an umbrella!" (yeah yeah, unless your friends mess with you ALL the time ;)

I thought about this a little differently from a non-bayes perspective. It's raining if any ONE of the friends is telling the truth, because if they are telling the truth then it is raining. If all of them are lieing, then it isn't raining because they told you that it was raining. So what you want is the probability that any one person is telling the truth. Which is simply 1-Pr(all lie) = 26/27 Anyone let me know if I'm wrong here!

Here's another perspective on how to answer a question like this: Bring an umbrella. It's Seattle - if it's not raining right now, it probably will be by the time you get there.

Find the second largest element in a Binary Search Tree

16 Answers

Write a function that takes in two sorted lists and outputs a sorted list that is their union.

9 Answers

generating a sorted vector from two sorted vectors.

3 Answers

Why is data important? (or something along those lines; I'd call this an unexpected question)

1 Answer

academic-like questions where you could just look up the answer. just memorize a thing or two about the basic algos like neural nets, random forests, svm, linear regression. that satisfies their data scientists.

2 Answers

I signed an NDA, so I can't give much details, but one interviewer asked me a very open ended question that involved how I would create/design/implement a certain algorithm from start to end.

1 Answer

Bayes' Formular: Marbles: There are 30 red marbles and 10 black marbles in Urn #1. You have 20 red and 20 Black marbles in Urn 2. Randomly you pull a marble from the random urn and find that it is red. What is the probability that it was pulled from Urn #1

1 Answer

Alice and Bob take turns in rolling a fair dice. Whoever gets "6" first wins the game. Alice starts the game. What are the chances that Alice wins.

2 Answers

Two random cards numbered from 1,2...100 are pulled from the deck. What is the probability that one number doubles the other from the deck.

1 Answer
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