40 Must know Questions to test a data scientist on Dimensionality Reduction techniques
Introduction
Have you come across a dataset with hundreds of columns and wondered how to build a predictive model on it? Or have come across a situation where a lot of variables might be correlated? It is difficult to escape these situations while working on real life problems.
Thankfully, dimensionality reduction techniques come to our rescue here. Dimensionality Reduction is an important technique in data science. It is a must have skill set for any data scientist. To test your knowledge in dimensionality reduction techniques, we are conducted this skill test. These questions include topics like Principal Component Analysis (PCA), tSNE and LDA.
Check out more challenging competitions coming up here
A total of 582 people participated in this skill test. The questions varied from theoretical to practical.
If you missed taking the test, here is your opportunity for you to find out how many questions you could have answered correctly.
Read on!
Overall Scores
Below is the distribution of scores, this will help you evaluate your performance:
You can access your performance here. More than 180 people participated in the skill test and the highest score was 34. Here are a few statistics about the distribution.
Overall distribution
Mean Score: 19.52
Median Score: 20
Mode Score: 19
Useful Resources
 Beginners Guide To Learn Dimension Reduction Techniques
 Practical Guide to Principal Component Analysis (PCA) in R & Python
 Comprehensive Guide on tSNE algorithm with implementation in R & Python
Are you just getting started with Dimensionality Reduction Techniques? Do you want to learn how to use these techniques to work on reallife projects and improve the model performance? Presenting two comprehensive courses which cover all the important concepts like feature selection and dimentionality reduction,
Questions & Answers
1) Imagine, you have 1000 input features and 1 target feature in a machine learning problem. You have to select 100 most important features based on the relationship between input features and the target features.
Do you think, this is an example of dimensionality reduction?
A. Yes
B. No
Solution: (A)
2) [ True or False ] It is not necessary to have a target variable for applying dimensionality reduction algorithms.
A. TRUE
B. FALSE
Solution: (A)
LDA is an example of supervised dimensionality reduction algorithm.
3) I have 4 variables in the dataset such as – A, B, C & D. I have performed the following actions:
Step 1: Using the above variables, I have created two more variables, namely E = A + 3 * B and F = B + 5 * C + D.
Step 2: Then using only the variables E and F I have built a Random Forest model.
Could the steps performed above represent a dimensionality reduction method?
A. True
B. False
Solution: (A)
Yes, Because Step 1 could be used to represent the data into 2 lower dimensions.
4) Which of the following techniques would perform better for reducing dimensions of a data set?
B. Removing columns which have high variance in data
C. Removing columns with dissimilar data trends
D. None of these
Solution: (A)
If columns have too many missing values, (say 99%) then we can remove such columns.
5) [ True or False ] Dimensionality reduction algorithms are one of the possible ways to reduce the computation time required to build a model.
A. TRUE
B. FALSE
Solution: (A)
Reducing the dimension of data will take less time to train a model.
6) Which of the following algorithms cannot be used for reducing the dimensionality of data?
A. tSNE
B. PCA
C. LDA False
D. None of these
Solution: (D)
All of the algorithms are the example of dimensionality reduction algorithm.
7) [ True or False ] PCA can be used for projecting and visualizing data in lower dimensions.
B. FALSE
Solution: (A)
Sometimes it is very useful to plot the data in lower dimensions. We can take the first 2 principal components and then visualize the data using scatter plot.
8) The most popularly used dimensionality reduction algorithm is Principal Component Analysis (PCA). Which of the following is/are true about PCA?
 PCA is an unsupervised method
 It searches for the directions that data have the largest variance
 Maximum number of principal components <= number of features
 All principal components are orthogonal to each other
A. 1 and 2
B. 1 and 3
C. 2 and 3
D. 1, 2 and 3
E. 1,2 and 4
F. All of the above
Solution: (F)
All options are self explanatory.
9) Suppose we are using dimensionality reduction as preprocessing technique, i.e, instead of using all the features, we reduce the data to k dimensions with PCA. And then use these PCA projections as our features. Which of the following statement is correct?
B. Higher ‘k’ means less regularization
C. Can’t Say
Solution: (B)
Higher k would lead to less smoothening as we would be able to preserve more characteristics in data, hence less regularization.
10) In which of the following scenarios is tSNE better to use than PCA for dimensionality reduction while working on a local machine with minimal computational power?
A. Dataset with 1 Million entries and 300 features
B. Dataset with 100000 entries and 310 features
C. Dataset with 10,000 entries and 8 features
D. Dataset with 10,000 entries and 200 features
Solution: (C)
tSNE has quadratic time and space complexity. Thus it is a very heavy algorithm in terms of system resource utilization.
11) Which of the following statement is true for a tSNE cost function?
A. It is asymmetric in nature.
B. It is symmetric in nature.
C. It is same as the cost function for SNE.
Solution: (B)
Cost function of SNE is asymmetric in nature. Which makes it difficult to converge using gradient decent. A symmetric cost function is one of the major differences between SNE and tSNE.
Question 12
Imagine you are dealing with text data. To represent the words you are using word embedding (Word2vec). In word embedding, you will end up with 1000 dimensions. Now, you want to reduce the dimensionality of this high dimensional data such that, similar words should have a similar meaning in nearest neighbor space.In such case, which of the following algorithm are you most likely choose?
B. PCA
C. LDA
D. None of these
Solution: (A)
tSNE stands for tDistributed Stochastic Neighbor Embedding which consider the nearest neighbours for reducing the data.
13) [True or False] tSNE learns nonparametric mapping.
A. TRUE
B. FALSE
Solution: (A)
tSNE learns a nonparametric mapping, which means that it does not learn an explicit function that maps data from the input space to the map. For more information read from this link.
14) Which of the following statement is correct for tSNE and PCA?
A. tSNE is linear whereas PCA is nonlinear
B. tSNE and PCA both are linear
C. tSNE and PCA both are nonlinear
D. tSNE is nonlinear whereas PCA is linear
Solution: (D)
Option D is correct. Read the explanation from this link
15) In tSNE algorithm, which of the following hyper parameters can be tuned?
A. Number of dimensions
B. Smooth measure of effective number of neighbours
C. Maximum number of iterations
D. All of the above
Solution: (D)
All of the hyperparameters in the option can tuned.
16) What is of the following statement is true about tSNE in comparison to PCA?
A. When the data is huge (in size), tSNE may fail to produce better results.
B. TNSE always produces better result regardless of the size of the data
C. PCA always performs better than tSNE for smaller size data.
D. None of these
Solution: (A)
Option A is correct
17) Xi and Xj are two distinct points in the higher dimension representation, where as Yi & Yj are the representations of Xi and Xj in a lower dimension.
1. The similarity of datapoint Xi to datapoint Xj is the conditional probability p (ji) .
2. The similarity of datapoint Yi to datapoint Yj is the conditional probability q (ji) .
Which of the following must be true for perfect representation of xi and xj in lower dimensional space?
B. p (ji) < q (ji)
C. p (ji) = q (ji)
D. p (ji) > q (ji)
Solution: (C)
The conditional probabilities for similarity of two points must be equal because similarity between the points must remain unchanged in both higher and lower dimension for them to be perfect representations.
18) Which of the following is true about LDA?
A. LDA aims to maximize the distance between class and minimize the within class distance
B. LDA aims to minimize both distance between class and distance within class
C. LDA aims to minimize the distance between class and maximize the distance within class
D. LDA aims to maximize both distance between class and distance within class
Solution: (A)
Option A is correct.
19) In which of the following case LDA will fail?
A. If the discriminatory information is not in the mean but in the variance of the data
B. If the discriminatory information is in the mean but not in the variance of the data
C. If the discriminatory information is in the mean and variance of the data
D. None of these
Solution: (A)
Option A is correct
20) Which of the following comparison(s) are true about PCA and LDA?
 Both LDA and PCA are linear transformation techniques
 LDA is supervised whereas PCA is unsupervised
 PCA maximize the variance of the data, whereas LDA maximize the separation between different classes,
B. 2 and 3
C. 1 and 3
D. Only 3
E. 1, 2 and 3
Solution: (E)
All of the options are correct
21) What will happen when eigenvalues are roughly equal?
A. PCA will perform outstandingly
B. PCA will perform badly
C. Can’t Say
D.None of above
Solution: (B)
When all eigen vectors are same in such case you won’t be able to select the principal components because in that case all principal components are equal.
22) PCA works better if there is?
 A linear structure in the data
 If the data lies on a curved surface and not on a flat surface
 If variables are scaled in the same unit
B. 2 and 3
C. 1 and 3
D. 1 ,2 and 3
Solution: (C)
Option C is correct
23) What happens when you get features in lower dimensions using PCA?
 The features will still have interpretability
 The features will lose interpretability
 The features must carry all information present in data
 The features may not carry all information present in data
B. 1 and 4
C. 2 and 3
D. 2 and 4
Solution: (D)
When you get the features in lower dimensions then you will lose some information of data most of the times and you won’t be able to interpret the lower dimension data.
24) Imagine, you are given the following scatterplot between height and weight.
Select the angle which will capture maximum variability along a single axis?
A. ~ 0 degree
B. ~ 45 degree
C. ~ 60 degree
D. ~ 90 degree
Solution: (B)
Option B has largest possible variance in data.
25) Which of the following option(s) is / are true?
 You need to initialize parameters in PCA
 You don’t need to initialize parameters in PCA
 PCA can be trapped into local minima problem
 PCA can’t be trapped into local minima problem
B. 1 and 4
C. 2 and 3
D. 2 and 4
Solution: (D)
PCA is a deterministic algorithm which doesn’t have parameters to initialize and it doesn’t have local minima problem like most of the machine learning algorithms has.
Question Context 26
The below snapshot shows the scatter plot of two features (X1 and X2) with the class information (Red, Blue). You can also see the direction of PCA and LDA.
26) Which of the following method would result into better class prediction?
A. Building a classification algorithm with PCA (A principal component in direction of PCA)
B. Building a classification algorithm with LDA
C. Can’t say
D. None of these
Solution: (B)
If our goal is to classify these points, PCA projection does only more harm than good—the majority of blue and red points would land overlapped on the first principal component.hence PCA would confuse the classifier.
27) Which of the following options are correct, when you are applying PCA on a image dataset?

 It can be used to effectively detect deformable objects.
 It is invariant to affine transforms.
 It can be used for lossy image compression.
 It is not invariant to shadows.
B. 2 and 3
C. 3 and 4
D. 1 and 4
Solution: (C)
Option C is correct
28) Under which condition SVD and PCA produce the same projection result?
A. When data has zero median
B. When data has zero mean
C. Both are always same
D. None of these
Solution: (B)
When the data has a zero mean vector, otherwise you have to center the data first before taking SVD.
Question Context 29
Consider 3 data points in the 2d space: (1, 1), (0,0), (1,1).
29) What will be the first principal component for this data?
 [ √ 2 /2 , √ 2/ 2 ]
 (1/ √ 3, 1/ √ 3)
 ([ √ 2/ 2 , √ 2/ 2 ])
 ( 1/ √ 3, – 1/ √ 3)
B. 3 and 4
C. 1 and 3
D. 2 and 4
Solution: (C)
The first principal component is v = [ √ 2 /2 , √ 2/ 2 ] T (you shouldn’t really need to solve any SVD or eigenproblem to see this). Note that the principal component should be normalized to have unit length. (The negation v = [− √ 2/ 2 , − √ 2/ 2 ] T is also correct.)
30) If we project the original data points into the 1d subspace by the principal component [ √ 2 /2, √ 2 /2 ] T. What are their coordinates in the 1d subspace?
B. (√ 2 ), (0), (√ 2)
C. ( √ 2 ), (0), (√ 2)
D. (√ 2 ), (0), (√ 2)
Solution: (A)
The coordinates of three points after projection should be z1 = x T 1 v = [−1, −1][ √ 2/ 2 , √ 2 /2 ] T = − √ 2, z2 = x T 2 v = 0, z3 = x T 3 v = √ 2.
31) For the projected data you just obtained projections ( (− √ 2 ), (0), (√ 2) ). Now if we represent them in the original 2d space and consider them as the reconstruction of the original data points, what is the reconstruction error? Context: 2931:
B. 10%
C. 30%
D. 40%
Solution: (A)
The reconstruction error is 0, since all three points are perfectly located on the direction of the first principal component. Or, you can actually calculate the reconstruction: z1 ·v.
xˆ1 = − √ 2·[ √ 2/ 2 , √ 2/2 ] T = [−1, −1]T
xˆ2 = 0*[0, 0]T = [0,0]
xˆ3 = √ 2* [1, 1]T = [1,1]
which are exactly x1, x2, x3.
32) In LDA, the idea is to find the line that best separates the two classes. In the given image which of the following is a good projection?
B. LD2
C. Both
D. None of these
Solution: (A)
LD1 Is a good projection because it best separates the class.
Question Context 33
PCA is a good technique to try, because it is simple to understand and is commonly used to reduce the dimensionality of the data. Obtain the eigenvalues λ1 ≥ λ2 ≥ • • • ≥ λN and plot.
To see how f(M) increases with M and takes maximum value 1 at M = D. We have two graph given below:
33) Which of the above graph shows better performance of PCA? Where M is first M principal components and D is total number of features?
A. Left
B. Right
C. Any of A and B
D. None of these
Solution: (A)
PCA is good if f(M) asymptotes rapidly to 1. This happens if the first eigenvalues are big and the remainder are small. PCA is bad if all the eigenvalues are roughly equal. See examples of both cases in figure.
34) Which of the following option is true?
A. LDA explicitly attempts to model the difference between the classes of data. PCA on the other hand does not take into account any difference in class.
B. Both attempt to model the difference between the classes of data.
C. PCA explicitly attempts to model the difference between the classes of data. LDA on the other hand does not take into account any difference in class.
D. Both don’t attempt to model the difference between the classes of data.
Solution: (A)
Options are self explanatory.
35) Which of the following can be the first 2 principal components after applying PCA?
 (0.5, 0.5, 0.5, 0.5) and (0.71, 0.71, 0, 0)
 (0.5, 0.5, 0.5, 0.5) and (0, 0, 0.71, 0.71)
 (0.5, 0.5, 0.5, 0.5) and (0.5, 0.5, 0.5, 0.5)
 (0.5, 0.5, 0.5, 0.5) and (0.5, 0.5, 0.5, 0.5)
B. 1 and 3
C. 2 and 4
D. 3 and 4
Solution: (D)
For the first two choices, the two loading vectors are not orthogonal.
36) Which of the following gives the difference(s) between the logistic regression and LDA?
 If the classes are well separated, the parameter estimates for logistic regression can be unstable.
 If the sample size is small and distribution of features are normal for each class. In such case, linear discriminant analysis is more stable than logistic regression.
37) Which of the following offset, do we consider in PCA?
B. Perpendicular offset
C. Both
D. None of these
Solution: (B)
We always consider residual as vertical offsets. Perpendicular offset are useful in case of PCA
38) Imagine you are dealing with 10 class classification problem and you want to know that at most how many discriminant vectors can be produced by LDA. What is the correct answer?
A. 20
B. 9
C. 21
D. 11
E. 10
Solution: (B)
LDA produces at most c − 1 discriminant vectors. You may refer this link for more information.
Question Context 39
The given dataset consists of images of “Hoover Tower” and some other towers. Now, you want to use PCA (Eigenface) and the nearest neighbour method to build a classifier that predicts whether new image depicts “Hoover tower” or not. The figure gives the sample of your input training images.
39) In order to get reasonable performance from the “Eigenface” algorithm, what preprocessing steps will be required on these images?
 Align the towers in the same position in the image.
 Scale or crop all images to the same size.
B. 2
C. 1 and 2
D. None of these
Solution: (C)
Both the statements are correct.
40) What are the optimum number of principle components in the below figure ?
B. 30
C. 40
D. Can’t Say
Solution: (B)
We can see in the above figure that the number of components = 30 is giving highest variance with lowest number of components. Hence option ‘B’ is the right answer.
End Notes
I hope you enjoyed taking the test and found the solutions helpful. The test focused on conceptual as well as practical knowledge of dimensionality reduction.
If you have any doubts in the questions above, let us know through comments below. Also, If you have any suggestions or improvements you think we should make in the next skill test, you can let us know by dropping your feedback in the comments section. Also, checkout DATAFEST 2017.
6 thoughts on "40 Must know Questions to test a data scientist on Dimensionality Reduction techniques"
Pratima Joshi says: March 21, 2017 at 7:23 am
Hi, I think the answers and explanations of questions 10 and 11 are not in sync. Please revisit and correct.Ankit Gupta says: March 21, 2017 at 8:33 am
Hi Pratima, Thanks for noticing! I change the explanation of question number 10 which was addressing some other issue. Answers for questions 10 and 11 are remain same Best Regards, Ankit GuptaMarvin says: March 21, 2017 at 4:57 pm
Hi , could it be that in question 33 solution and explanation are contradicting or did I get it wrong?Ankit Gupta says: March 22, 2017 at 3:28 am
Hi Marvin, Explanation is correct but solution was incorrectly marked. Thanks for noticing Best! Ankit Gupta考察数据科学家数据降维知识的40道题，快来测测吧(附答案)  大数据多客 says: June 07, 2017 at 9:28 pm
[…] 查看原文>>> […]Keyuri says: July 17, 2017 at 8:18 pm
Answer for the Q.20 should be C because both LDA and PCA are unsupervised methods.