This article was published as a part of the Data Science Blogathon

## Introduction

K nearest neighbour (KNN) is one of the most widely used and simplest algorithms for classification problems under supervised Machine Learning.

Therefore it becomes necessary for every aspiring Data Scientist and Machine Learning Engineer to have a good knowledge of this algorithm.

In this article, we will discuss the most important questions on the K Nearest Neighbor (KNN) Algorithm which is helpful to get you a clear understanding of the algorithm, and also for Data Science Interviews, which covers its very fundamental level to complex concepts.

## 1. What is the KNN Algorithm?

KNN(K-nearest neighbours) is a supervised learning and non-parametric algorithm that can be used to solve both classification and regression problem statements.

It uses data in which there is a target column present i.e, labelled data to model a function to produce an output for the unseen data. It uses the euclidean distance formula to compute the distance between the data points for classification or prediction.

The main objective of this algorithm is that similar data points must be close to each other so it uses the distance to calculate the similar points that are close to each other. ## 2. Why is KNN a non-parametric Algorithm?

The term “non-parametric” refers to not making any assumptions on the underlying data distribution. These methods do not have any fixed numbers of parameters in the model.

Similarly in KNN, the model parameters grow with the training data by considering each training case as a parameter of the model. So, KNN is a non-parametric algorithm.

## 3. What is “K” in the KNN Algorithm?

K represents the number of nearest neighbours you want to select to predict the class of a given item, which is coming as an unseen dataset for the model.

## 4. Why is the odd value of “K” preferred over even values in the KNN Algorithm?

The odd value of K should be preferred over even values in order to ensure that there are no ties in the voting. If the square root of a number of data points is even, then add or subtract 1 to it to make it odd.

## 5. How does the KNN algorithm make the predictions on the unseen dataset?

The following operations have happened during each iteration of the algorithm. For each of the unseen or test data point, the kNN classifier must:

Step-1: Calculate the distances of test point to all points in the training set and store them

Step-2: Sort the calculated distances in increasing order

Step-3: Store the K nearest points from our training dataset

Step-4: Calculate the proportions of each class

Step-5: Assign the class with the highest proportion ## 6. Is Feature Scaling required for the KNN Algorithm? Explain with proper justification.

Yes, feature scaling is required to get the better performance of the KNN algorithm.

For Example, Imagine a dataset having n number of instances and N number of features. There is one feature having values ranging between 0 and 1. Meanwhile, there is also a feature that varies from -999 to 999. When these values are substituted in the formula of Euclidean Distance, this will affect the performance by giving higher weightage to variables having a higher magnitude.

## 7. What is space and time complexity of the KNN Algorithm?

Time complexity:

The distance calculation step requires quadratic time complexity, and the sorting of the calculated distances requires an O(N log N) time. Together, we can say that the process is an O(N3 log N) process, which is a monstrously long process.

Space complexity:

Since it stores all the pairwise distances and is sorted in memory on a machine, memory is also the problem. Usually, local machines will crash, if we have very large datasets.

## 8. Can the KNN algorithm be used for regression problem statements?

Yes, KNN can be used for regression problem statements.

In other words, the KNN algorithm can be applied  when the dependent variable is continuous. For regression problem statements, the predicted value is given by the average of the values of its k nearest neighbours.

## 9. Why is the KNN Algorithm known as Lazy Learner?

When the KNN algorithm gets the training data, it does not learn and make a model, it just stores the data. Instead of finding any discriminative function with the help of the training data, it follows instance-based learning and also uses the training data when it actually needs to do some prediction on the unseen datasets.

As a result, KNN does not immediately learn a model rather delays the learning thereby being referred to as Lazy Learner.

## 10. Why is it recommended not to use the KNN Algorithm for large datasets?

The Problem in processing the data:

KNN works well with smaller datasets because it is a lazy learner. It needs to store all the data and then make a decision only at run time. It includes the computation of distances for a given point with all other points. So if the dataset is large, there will be a lot of processing which may adversely impact the performance of the algorithm.

Sensitive to noise:

Another thing in the context of large datasets is that there is more likely a chance of noise in the dataset which adversely affects the performance of the KNN algorithm since the KNN algorithm is sensitive to the noise present in the dataset.

## 11. How to handle categorical variables in the KNN Algorithm?

To handle the categorical variables we have to create dummy variables out of a categorical variable and include them instead of the original categorical variable. Unlike regression, create k dummies instead of (k-1).

For example, a categorical variable named “Degree” has 5 unique levels or categories. So we will create 5 dummy variables. Each dummy variable has 1 against its degree and else 0.

## 12. How to choose the optimal value of K in the KNN Algorithm?

There is no straightforward method to find the optimal value of K in the KNN algorithm.

You have to play around with different values to choose which value of K should be optimal for my problem statement. Choosing the right value of K is done through a process known as Hyperparameter Tuning.

The optimum value of K for KNN is highly dependent on the data itself. In different scenarios, the optimum K may vary. It is more or less a hit and trial method.

There is no one proper method of finding the K value in the KNN algorithm. No method is the rule of thumb but you should try the following suggestions:

1. Square Root Method: Take the square root of the number of samples in the training dataset and assign it to the K value.

2. Cross-Validation Method: We should also take the help of cross-validation to find out the optimal value of K in KNN. Start with the minimum value of k i.e, K=1, and run cross-validation, measure the accuracy, and keep repeating till the results become consistent.

As the value of K increases, the error usually goes down after each one-step increase in K, then stabilizes, and then raises again. Finally, pick the optimum K at the beginning of the stable zone. This technique is also known as the Elbow Method. 3. Domain Knowledge: Sometimes with the help of domain knowledge for a particular use case we are able to find the optimum value of K (K should be an odd number).

I would therefore suggest trying a mix of all the above points to reach any conclusion.

## 13. How can you relate KNN Algorithm to the Bias-Variance tradeoff?

Problem with having too small K:

The major concern associated with small values of K lies behind the fact that the smaller value causes noise to have a higher influence on the result which will also lead to a large variance in the predictions.

Problem with having too large K:

The larger the value of K, the higher is the accuracy. If K is too large, then our model is under-fitted. As a result, the error will go up again. So, to prevent your model from under-fitting it should retain the generalization capabilities otherwise there are fair chances that your model may perform well in the training data but drastically fail in the real data. The computational expense of the algorithm also increases if we choose the k very large.

So, choosing k to a large value may lead to a model with a large bias(error).

The effects of k values on the bias and variance is explained below :

• As the value of k increases, the bias will be increases
• As the value of k decreases, the variance will increases
• With the increasing value of K, the boundary becomes smoother

So, there is a tradeoff between overfitting and underfitting and you have to maintain a balance while choosing the value of K in KNN. Therefore, K should not be too small or too large. ## 14. Which algorithm can be used for value imputation in both categorical and continuous categories of data?

KNN is the only algorithm that can be used for the imputation of both categorical and continuous variables. It can be used as one of many techniques when it comes to handling missing values.

To impute a new sample, we determine the samples in the training set “nearest” to the new sample and averages the nearby points to impute. A Scikit learn library of Python provides a quick and convenient way to use this technique.

Note: NaNs are omitted while distances are calculated. Hence we replace the missing values with the average value of the neighbours. The missing values will then be replaced by the average value of their “neighbours”.

## 15. Explain the statement- “The KNN algorithm does more computation on test time rather than train time”.

The above-given statement is absolutely true.

The basic idea behind the kNN algorithm is to determine a k-long list of samples that are close to a sample that we want to classify. Therefore, the training phase is basically storing a training set, whereas during the prediction stage the algorithm looks for k-neighbours using that stored data. Moreover, KNN does not learn anything from the training dataset as well.

## 16. What are the things which should be kept in our mind while choosing the value of k in the KNN Algorithm?

If K is small, then results might not be reliable because the noise will have a higher influence on the result. If K is large, then there will be a lot of processing to be done which may adversely impact the performance of the algorithm.

So, the following things must be considered while choosing the value of K:

• K should be the square root of n (number of data points in the training dataset).
• K should be chosen as the odd so that there are no ties. If the square root is even, then add or subtract 1 to it.

## 17. What are the advantages of the KNN Algorithm?

Some of the advantages of the KNN algorithm are as follows:

1. No Training Period: It does not learn anything during the training period since it does not find any discriminative function with the help of the training data. In simple words, actually, there is no training period for the KNN algorithm. It stores the training dataset and learns from it only when we use the algorithm for making the real-time predictions on the test dataset.

As a result, the KNN algorithm is much faster than other algorithms which require training. For Example, SupportVector Machines(SVMs), Linear Regression, etc.

Moreover, since the KNN algorithm does not require any training before making predictions as a result new data can be added seamlessly without impacting the accuracy of the algorithm.

2. Easy to implement and understand: To implement the KNN algorithm, we need only two parameters i.e. the value of K and the distance metric(e.g. Euclidean or Manhattan, etc.). Since both the parameters are easily interpretable therefore they are easy to understand.

## 18. What are the disadvantages of the KNN Algorithm?

Some of the disadvantages of the KNN algorithm are as follows:

1. Does not work well with large datasets: In large datasets, the cost of calculating the distance between the new point and each existing point is huge which decreases the performance of the algorithm.

2. Does not work well with high dimensions: KNN algorithms generally do not work well with high dimensional data since, with the increasing number of dimensions, it becomes difficult to calculate the distance for each dimension.

3. Need feature scaling: We need to do feature scaling (standardization and normalization) on the dataset before feeding it to the KNN algorithm otherwise it may generate wrong predictions.

4. Sensitive to Noise and Outliers: KNN is highly sensitive to the noise present in the dataset and requires manual imputation of the missing values along with outliers removal.

## 19. Is it possible to use the KNN algorithm for Image processing?

Yes, KNN can be used for image processing by converting a 3-dimensional image into a single-dimensional vector and then using it as the input to the KNN algorithm. ## 20. What are the real-life applications of KNN Algorithms?

The various real-life applications of the KNN Algorithm includes:

1. KNN allows the calculation of the credit rating. By collecting the financial characteristics vs. comparing people having similar financial features to a database we can calculate the same. Moreover, the very nature of a credit rating where people who have similar financial details would be given similar credit ratings also plays an important role. Hence the existing database can then be used to predict a new customer’s credit rating, without having to perform all the calculations.

2. In political science: KNN can also be used to predict whether a potential voter “will vote” or “will not vote”, or to “vote Democrat” or “vote Republican” in an election.

Apart from the above-mentioned use cases, KNN algorithms are also used for handwriting detection (like OCR), Image recognition, and video recognition.

## End Notes

I hope you enjoyed the questions and were able to test your knowledge about K Nearest Neighbor (KNN) Algorithm.

If you liked this and want to know more, go visit my other articles on Data Science and Machine Learning by clicking on the Link

Something not mentioned or want to share your thoughts? Feel free to comment below And I’ll get back to you.

## Chirag Goyal

Currently, I pursuing my Bachelor of Technology (B.Tech) in Computer Science and Engineering from the Indian Institute of Technology Jodhpur(IITJ). I am very enthusiastic about Machine learning, Deep Learning, and Artificial Intelligence.

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