21 Must-Know Machine Learning Interview Questions and Answers


The following machine learning interview questions and answers are broken in 9 major topics, as outlined in our guide to machine learning.

21 Machine Learning Interview Questions and Answers

Table of Contents

  1. The Big Picture
  2. Optimization
  3. Data Preprocessing
  4. Sampling & Splitting
  5. Supervised Learning
  6. Unsupervised Learning
  7. Model Evaluation
  8. Ensemble Learning
  9. Business Applications

For an additional 100 ML interview questions and answers, check out our book: 121 Essential Machine Learning Q's & A's.

1. The Big Picture

Essential ML theory, such as the Bias-Variance tradeoff.

Explain machine learning to a layperson.

Imagine a curious kid who sticks his palm over a candle flame and pulls back in a brief moment of sharp pain.

The next day, he comes across a hot stove top, seeing the red color and feeling the heat waves pulsing from it like the candle from the day before.

The kid has never touched a stove top, but fortunately, he has learned from previous data to avoid red things that pulse heat.


What does it mean to "fit" a model? How do hyperparameters relate?

Fitting a model is the process of learning the parameters of a model using training data.

Parameters help define the mathematical formulas behind machine learning models.

However, there are also "higher-level" parameters that cannot be learned from the data, called hyperparameters.

Hyperparameters define properties of the models, such as model complexity or learning rate.


Explain the Bias-Variance tradeoff.

Predictive models have a tradeoff between bias (how well the model fits the data) and variance (how much the model changes based on changes in the inputs).

Simpler models are stable (low variance) but they don't get close to the truth (high bias).

More complex models are more prone to being overfit (high variance) but they are expressive enough to get close to the truth (low bias).

The best model for a given problem usually lies somewhere in the middle.

2. Optimization

Algorithms for finding the best parameters for a model.

What is the difference between stochastic gradient descent (SGD) and gradient descent (GD)?

Both algorithms are methods for finding a set of parameters that minimize a loss function by evaluating parameters against data and then making adjustments.

In standard gradient descent, you'll evaluate all training samples for each set of parameters. This is akin to taking big, slow steps toward the solution.

In stochastic gradient descent, you'll evaluate only 1 training sample for the set of parameters before updating them. This is akin to taking small, quick steps toward the solution.


When would you use GD over SDG, and vice-versa?

GD theoretically minimizes the error function better than SGD. However, SGD converges much faster once the dataset becomes large.

That means GD is preferable for small datasets while SGD is preferable for larger ones.

In practice, however, SGD is used for most applications because it minimizes the error function well enough while being much faster and more memory efficient for large datasets.

3. Data Preprocessing

Dealing with missing data, skewed distributions, outliers, etc.

What is the Box-Cox transformation used for?

The Box-Cox transformation is a generalized "power transformation" that transforms data to make the distribution more normal.

For example, when its lambda parameter is 0, it's equivalent to the log-transformation.

It's used to stabilize the variance (eliminate heteroskedasticity) and normalize the distribution.


What are 3 data preprocessing techniques to handle outliers?

1. Winsorize (cap at threshold).

2. Transform to reduce skew (using Box-Cox or similar).

3. Remove outliers if you're certain they are anomalies or measurement errors.


What are 3 ways of reducing dimensionality?

1. Removing collinear features.

2. Performing PCA, ICA, or other forms of algorithmic dimensionality reduction.

3. Combining features with feature engineering.

4. Sampling & Splitting

How to split your datasets to tune parameters and avoid overfitting.

How much data should you allocate for your training, validation, and test sets?

You have to find a balance, and there's no right answer for every problem.

If your test set is too small, you'll have an unreliable estimation of model performance (performance statistic will have high variance).

If your training set is too small, your actual model parameters will have high variance.

A good rule of thumb is to use an 80/20 train/test split.

Then your train set can be further split into train/validation or into partitions for cross-validation.


If you split your data into train/test splits, is it still possible to overfit your model?

Yes, it's definitely possible.

One common beginner mistake is re-tuning a model or training new models with different parameters after seeing its performance on the test set.

In this case, its the model selection process that causes the overfitting.

The test set should not be tainted until you're ready to make your final selection.

5. Supervised Learning

Learning from labeled data using classification and regression models.

What are the advantages and disadvantages of k-nearest neighbors?

Advantages: K-Nearest Neighbors have a nice intuitive explanation, and then tend to work very well for problems where comparables are inherently indicative. For example, you could build a kNN housing price model by modeling on other houses in the area with similar number of bedrooms, floor space, etc.

Disadvantages: They are memory-intensive.They also do not have built-in feature selection or regularization, so they do not handle high dimensionality well.


What are the advantages and disadvantages of neural networks?

Advantages: Neural networks (specifically deep NNs) have led to performance breakthroughs for unstructured datasets such as images, audio, and video. Their incredible flexibility allows them to learn patterns that no other ML algorithm can learn.

Disadvantages: However, they require a large amount of training data to converge. It's also difficult to pick the right architecture, and the internal "hidden" layers are incomprehensible.


How can you choose a classifier based on training set size?

If training set is small, high bias / low variance models (e.g. Naive Bayes) tend to perform better because they are less likely to be overfit.

If training set is large, low bias / high variance models (e.g. Logistic Regression) tend to perform better because they can reflect more complex relationships.

6. Unsupervised Learning

Learning from unlabeled data using factor and cluster analysis models.

Explain Latent Dirichlet Allocation (LDA).

Latent Dirichlet Allocation (LDA) is a common method of topic modeling, or classifying documents by subject matter.

LDA is a generative model that represents documents as a mixture of topics that each have their own probability distribution of possible words.

The "Dirichlet" distribution is simply a distribution of distributions. In LDA, documents are distributions of topics that are distributions of words.


What are hierarchical cluster models? Give an example.

Hierarchical (or connectivity) cluster models are distance-based models that represent clusters using dendrograms.

They do not provide a single partition of the dataset, but instead produce a hierarchy of clusters that merge at certain distances.

An example is single-linkage clustering.

7. Model Evaluation

Making decisions based on various performance metrics.

What is the ROC Curve and what is AUC (a.k.a. AUROC)?

The ROC (receiver operating characteristic) the performance plot for binary classifiers of True Positive Rate (y-axis) vs. False Positive Rate (x-

AUC is area under the ROC curve, and it's a common performance metric for evaluating binary classification models.

It's equivalent to the expected probability that a uniformly drawn random positive is ranked before a uniformly drawn random negative.


Why is Area Under ROC Curve (AUROC) better than raw accuracy as an out-of- sample evaluation metric?

AUROC is robust to class imbalance, unlike raw accuracy.

For example, if you want to detect a type of cancer that's prevalent in only 1% of the population, you can build a model that achieves 99% accuracy by simply classifying everyone has cancer-free.

8. Ensemble Learning

Combining multiple models for better performance.

Why are ensemble methods superior to individual models?

They average out biases, reduce variance, and are less likely to overfit.

There's a common line in machine learning which is: "ensemble and get 2%."

This implies that you can build your models as usual and typically expect a small performance boost from ensembling.


Explain bagging.

Bagging, or Bootstrap Aggregating, is an ensemble method in which the dataset is first divided into multiple subsets through resampling.

Then, each subset is used to train a model, and the final predictions are made through voting or averaging the component models.

Bagging is performed in parallel.

9. Business Applications

How machine learning can help different types of businesses.

What are some key business metrics for (S-a-a-S startup | Retail bank | e- Commerce site)?

Thinking about key business metrics, often shortened as KPI's (Key Performance Indicators), is an essential part of a data scientist's job. Here are a few examples, but you should practice brainstorming your own.

Tip: When in doubt, start with the easier question of "how does this business make money?"

  • S-a-a-S startup: Customer lifetime value, new accounts, account lifetime, churn rate, usage rate, social share rate
  • Retail bank: Offline leads, online leads, new accounts (segmented by account type), risk factors, product affinities
  • e-Commerce: Product sales, average cart value, cart abandonment rate, email leads, conversion rate


How can you help our marketing team be more efficient?

The answer will depend on the type of company. Here are some examples.

Clustering algorithms to build custom customer segments for each type of marketing campaign.

Natural language processing for headlines to predict performance before running ad spend.

Predict conversion probability based on a user's website behavior in order to create better retargeting campaigns.


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