# Data Scientice and Machine Learning Interview Questions

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Here are some Data Science and Machine Learning related Interview Questions asked by big companies such as Facebook, Amazon, Microsoft, Yelp, Pinterest, Square, Google, Glassdoor and Groupon.  I also post an article that briefly describes the popular machine learning interview questions.

1. Given a coin you don’t know it’s fair or unfair. Throw it 6 times and get 1 tail and 5 head. Determine whether it’s fair or not. What’s your confidence value?

2. Given Amazon data, how to predict which users are going to be top shoppers in this holiday season.

3. Which regression methods are you familiar? How to evaluate regression result?

4. Write down the formula for logistic regression. How to determine the coefficients given the data?

5. How do you evaluate regression? For example, in this particular case: item click-through-rate  predicted rate
1       0.04        0.06
2       0.68        0.78
3       0.27        0.19
4       0.52        0.57

6. What’s the formula for SVM? What is decision boundary?

7. A field with unknown number of rabbits. Catch 100 rabbits and put a label
on each of them. A few days later, catch 300 rabbits and found 60 with
labels. Estimate how many rabbits are there?

8. Given 10 coins with 1 unfair coin and 9 fair coins. The unfair coin has &
#8532; prob. to be head. Now random select 1 coin and throw it 3 times. You
observe head, head, tail. What’s the probability that the selected coin is
the unfair one?

9. What’s the formula for Naive Bayesian classifier? What’s the assumption
in the formula? What kind of data is Naive Bayesian good at? What is not?

10. What is the real distribution of click-through rate of items? If you
want to build a predictor/classifier for this data, how do you do it? How do
you divide the data?

11. You have a stream of data coming in, in the format as the following:
item_id, views, clicks, time
1            100     10         2013-11-28
1            1000   350       2013-11-29
1            200     14         2013-11-30
2            127     13         2013-12-1

The same id are consecutive.

Click through rate = clicks / views.
On every day, I want to output the item id when its click through rate is
larger than a given threshold.
For example, at day 1, item 1’s rate is 10/100=10%, day2, its (10+350)/(100
+1000)=0.32. day3 it is (10+350+14)/(100+1000+200)=0.28.
If my threshold is 0.3, then at day 1, I don’t output. On day2 I output. On
day3, I don’t output.

11. Given a dictionary and a string. Write a function, if every word is in the dictionary return true, otherwise return false.

12. Generate all the permutation of a string. For example, abc, acb, cba, …

13. We want to add a new feature to our product. How to determine if people like it?
A/B testing. How to do A/B testing? How many ways? pros and cons?

14. 44.3% vs 47.2% is it significant?

15. Design a function to calculate people’s interest to a place against the distance to the place.

16. How to encourage people to write more reviews on Yelp? How to determine  who are likely to write reviews? How to increase the registration rate of Yelp? What features to add for a better Yelp app? We are expanding to other countries. Which country we should enter first?

17. What’s the difference between classification and regression?

18. Can you explain how decision tree works? How to build a decision tree from data?

19. What is regularization in regression? Why do regularization? How to do regularization?

21. We have a database of <product_id, name, description, price>. When user inputs a product name, how to return results fast?

22. If user gives a budget value, how to find the most expensive product under budget? Assume the data fits in memory. What data structure, or algorithm you use to find the product quickly? Write the program for it.

23. Given yelp data, how to find top 10 restaurants in America?

24. Given a large file that we don’t know how many lines are there. It doesn’t fit into memory. We want to sample K lines from the file uniformly. Write a program for it.

25. How to determine if one advertisement is performing better than the other?

26. How to evaluate classification result? What if the results are in probability mode?
If I want to build a classifier, but the data is very unbalanced. I have a
few positive samples but a lot of negative samples. What should I do?

27. Given a lot of data, I want to random sample 1% of them. How to do it efficiently?

28. When a new user signs up Pinterest, we want to know its interests. We decide to show the user a few pins, 2 pins at a time. Let the user choose
which pin s/he likes. After the user clicks on one of the 2, we select another 2 pins.
Question: how to design the system and select the pins so that we can
achieve our goal?

29. Write a function to compute sqrt(X). Write a function to compute pow(x, n) [square root and power)

30. Given a matrix
a b c  d
e f  g  h
i  j  k   l
Print it in this order:
a  f  k
b g l
c h
d
e j
i

31. Given a matrix and an array of words, find if the words are in the matrix. You can search the matrix in all directions:  from left to right, right to left, up to down, down to up, or diagonally.
For example
w o r x b
h  e l  o v
i   n d e m

then the word “world” is in the matrix.

32. Given a coordinates, and two points A and B. How many ways to go from A to B? You can only move up or right.
For example, from (1, 1) to (5, 7), one possible way is 1,1 -> 2, 1… 5, 1 –
> 5,2 -> ..5, 7

33. In a city where there are only vertical and horizontal streets. There are people on the cross point. These people want to meet. Please find a
cross point to minimize the cost for all the people to move.

34. Design a job search ranking algorithm on glassdoor

35. How to identify review spam?

36. Glassdoor has this kind of data about a job : (position, company, location, salary). For example (Software Engineer, Microsoft, Seattle, \$125K
). For some records, all four entires are available. But for others, the salary is missing. Design a way to estimate salary for those records.

37. When to send emails to users in a day can get maximum click through rate?

38. Youtube has video play log like this:
Video ID, time
vid1        t1
vid2        t2
…           …
The log is super large.
Find out the top 10 played videos on youtube in a given week.

39. Write a program to copy a graph

40. A bank has this access log:
ip1      t1
ip2      t2
…        …

If one ip accessed K times within m seconds, it may be an attack. Given the log, identify all IPs that may cause attack.

### Machine learning related questions:

–  Discuss how to predict the price of a hotel given data from previous years
–  SVM formulation
–  Logistic regression
–  Regularization
–  Cost function of neural network
–  What is the difference between a generative and discriminative algorithm
–  Relationship between kernel trick and dimension augmentation
–  What is PCA projection and why it can be solved by SVD
–  Bag of Words (BoW) feature
–  Nonlinear dimension reduction (Isomap, LLE)
–  Supervised methods for dimension reduction
–  What is naive Bayes
–  How to predict the age of a person given everyone’s phone call history
–  Variance and Bias (a very popular question, watch Andrew’s class)
–  Practices: When to collect more data / use more features / etc. (watch Andrew’s class)
–  How to extract features of shoes
–  During linear regression, when using each attribute (dimension) independently to predict the target value, you get a positive weight for each attribute. However, when you combine all attributes to predict, you get some large negative weights, why? How to solve it?
–  Cross Validation
–  Reservoir sampling
–  Explain the difference among decision tree, bagging and random forest
–  What is collaborative filtering
–  How to compute the average of a data stream (very easy, different from
moving average)
–  Given a coin, how to pick 1 person from 3 persons with equal probability.

### Coding related questions:

–  Leetcode: Number of Islands
–  Given the start time and end time of each meeting, compute the smallest
number of rooms to host these meetings. In other words, try to stuff as many
meetings in the same room as possible
–  Given an array of integers, compute the first two maximum products(乘积)
of any 3 elements (O(nlogn))
–  LeetCode: Reverse words in a sentence (follow up: do it in-place)
–  LeetCode: Word Pattern
–  Evaluate a formula represented as a string, e.g., “3 + (2 * (4 – 1) )”
–  Flip a binary tree
–  What is the underlying data structure for JAVA hashmap? Answer: BST, so
that the keys are sorted.
–  Find the lowest common parent in a binary tree
–  Given a huge file, each line of which is a person’s name. Sort the names
using a single computer with small memory but large disk space
–  Design a data structure to quickly compute the row sum and column sum of
a sparse matrix
–  Design a wrapper class for a pointer to make sure this pointer will
always be deleted even if an exception occurs in the middle

Reference: http://www.mitbbs.com/article/JobHunting/32808273_0.html