10 Deep Learning Interview Questions and Answers for ML Engineers

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If you're preparing for ml engineer interviews, see also our comprehensive interview questions and answers for the following ml engineer specializations:

1. Can you explain the difference between supervised and unsupervised learning?

Supervised and unsupervised learning are two main categories of machine learning algorithms based on different types of input data and approaches to achieve specific goals.

  • Supervised Learning: In supervised learning, the algorithm is trained on labeled data, where the input data is already tagged with the desired output. The model learns to map the input data to the expected output during the training phase. This type of learning is useful when we have a target variable that we want to predict. For example, in image recognition, we might want to predict the type of an image. A model that has been trained with images and their associated labels can then be used to correctly classify new images.
  • Unsupervised Learning: In unsupervised learning, the algorithm is trained on untagged data, where there is no predetermined correct answer. The model looks for patterns in the data and creates its own logical structures to represent that data. Clustering is a common use case for unsupervised learning. For example, we can cluster similar customers based on their behavior on a website. Without any prior knowledge of the customers, the algorithm will identify different groups based on their shared attributes.

Overall, while both types of learning have their unique use cases, supervised learning is often used when we have a labeled dataset and want to predict labels for new data. On the other hand, unsupervised learning is often used when we have an unlabeled dataset and want to discover patterns or structures within the data.

2. What is a neural network, and what are some common architecture types?

A neural network is an advanced machine learning model that is based on the structure and function of the human brain. It is a collection of interconnected nodes that contain mathematical operations and transfer functions to process data. Neural networks can learn and generalize from examples, making them ideal for a wide range of applications, including image recognition, speech recognition, natural language processing, and predictive analytics.

Here are some common architecture types:

  1. Feedforward Neural Networks: These are the simplest form of neural networks and consist of input, hidden, and output layers. In this architecture, the data flows in one direction, from the input layer through the hidden layers to the output layer.
  2. Recurrent Neural Networks: These networks have an internal memory that stores information about previous inputs. This architecture is used for applications such as speech recognition, natural language processing, and time series prediction.
  3. Convolutional Neural Networks: These are mainly used for image recognition and object detection. They consist of convolutional layers, pooling layers, and fully connected layers. Convolutional layers extract features from input images, pooling layers downsample the feature maps, and fully connected layers classify the features.
  4. Generative Adversarial Networks: These consist of a generator model that creates fake data and a discriminator model that tries to distinguish between the fake and real data. These networks can be trained to generate realistic images, videos, and sounds.

For example, a convolutional neural network (CNN) was used to achieve state-of-the-art results in the image recognition task of the ImageNet challenge. The CNN achieved a top-5 error rate of 4.9%, which is better than the 5.1% error rate of human participants.

3. What is backpropagation, and how does it work?

Backpropagation is a supervised learning algorithm used to train deep neural networks. It is a method for calculating the gradient of the loss function with respect to the weights of the neural network. In other words, it is a way to adjust the weights of the network in order to minimize the error between the predicted and actual output.

  1. The forward pass: The input data is fed into the input layer of the neural network and then processed through each layer of hidden neurons until the output layer is reached. Each neuron in each layer applies an activation function to the weighted sum of its inputs and output the result to the next layer.
  2. The backward pass: Using the results from the forward pass, the error between the predicted output and the actual output is calculated using a loss function such as mean squared error. The goal of backpropagation is to minimize this loss function by adjusting the weights of the neurons.
  3. The update step: The gradient of the loss function with respect to the weights is calculated using the chain rule of calculus. Then, the weights are adjusted according to this gradient using an optimization algorithm such as stochastic gradient descent. The process is repeated for each batch of training data until the neural network reaches convergence.

To illustrate how backpropagation works, consider a neural network that is trained to recognize handwritten digits. Given an image of a digit, the network would output a probability distribution over the ten possible digits. For example, if the digit is a "5", the probability distribution might look like this:

  • 0: 0.01
  • 1: 0.02
  • 2: 0.01
  • 3: 0.03
  • 4: 0.05
  • 5: 0.70
  • 6: 0.01
  • 7: 0.02
  • 8: 0.03
  • 9: 0.12

The goal of backpropagation is to adjust the weights of the network so that when the network is given an image of a "5", the probability of the network outputting a "5" is increased, and the probabilities of the other digits are decreased. Backpropagation achieves this by calculating the gradient of the loss function with respect to the weights of the network and adjusting the weights accordingly.

4. How do you handle overfitting in a deep learning model?

Overfitting occurs when a model is too complex, leading it to perform well on the training data but poorly on new, unseen data. There are several techniques to overcome overfitting in deep learning models, such as:

  1. Early stopping: We can monitor the performance of the model during training and stop when the performance on the validation set stops improving. This prevents the model from overfitting to the training data.
  2. Regularization: Regularization methods impose constraints on the model parameters, forcing it to favor simpler models that are less prone to overfitting. Techniques such as L1 and L2 regularization or dropout can be effective in reducing overfitting.
  3. Data augmentation: Adding more data can help generalize the model by increasing the variety of examples it has seen. For example, horizontally flipping an image of a cat does not change the fact that it is a cat, so we can create new training data by doing so.
  4. Ensemble methods: Ensembles combine multiple models to improve performance. Bagging, boosting and stacking are examples of ensemble methods that can be used to reduce overfitting.
  5. Cross-validation: Cross-validation can help evaluate the performance of a model and reduce overfitting by randomly dividing the training data into multiple sets and training the model on each set while evaluating it on the others.

In a recent project I worked on, I noticed that the model was overfitting to the training data. I applied early stopping and regularization techniques such as L1 and L2 regularization, which helped reduce the overfitting. Specifically, applying L2 regularization to the fully connected layers and using early stopping led to a 10% reduction in overfitting and a 3% improvement in accuracy on the validation set.

5. What is regularization, and how does it prevent overfitting?

Regularization is a technique used to prevent overfitting in machine learning models. Overfitting occurs when a model is too complex and learns information from the training data that is not useful for making predictions on new data.

One common type of regularization is L2 regularization, also known as ridge regression. This technique adds a penalty term to the loss function during training, which encourages the model to have small weights. The penalty term is proportional to the square of the magnitudes of the weights, so it has a greater effect on larger weights. This helps prevent the model from placing too much emphasis on any one feature, which can lead to overfitting.

To demonstrate the effect of regularization, let's consider a binary classification problem with 100 examples and 20 features. We will train logistic regression models with and without L2 regularization on this data, and evaluate their performance on a held-out test set.

  1. Without regularization:
    • Train accuracy: 99%
    • Test accuracy: 80%
  2. With regularization:
    • Train accuracy: 95%
    • Test accuracy: 85%

As we can see from these results, the model without regularization has almost perfect accuracy on the training data, but performs poorly on the test data. This is a clear case of overfitting. On the other hand, the model with regularization has slightly lower accuracy on the training data, but generalizes better to the test data. This is because the regularization term helps prevent the model from becoming too complex and overfitting to the training data.

6. What is a convolutional neural network, and how is it used in computer vision?

A Convolutional Neural Network (CNN) is a deep learning algorithm that is commonly used in computer vision tasks such as image classification, object detection, and segmentation. It is inspired by the human visual cortex and consists of several layers of interconnected nodes. Each layer performs a specific operation on the input data and produces an output that is passed on to the next layer.

The first layer of a CNN is a convolutional layer, which applies a set of filters to the input image to extract relevant features. The filters are typically small matrices that are convolved with the input image to produce a feature map. Each filter detects a specific pattern or feature such as edges, corners, or blobs.

The second layer of a CNN is a pooling layer, which reduces the spatial size of the feature maps by downsampling them. This helps to reduce the number of parameters in the network and makes it less likely to overfit the training data.

After several convolutional and pooling layers, the output is flattened and passed through one or more fully connected layers, which perform classification or regression tasks depending on the application.

One example of the successful application of CNNs in computer vision is the ImageNet Large Scale Visual Recognition Challenge, where teams compete to develop the best image classification algorithm. In 2012, a CNN called AlexNet achieved a top-5 error rate of 15.3%, which was significantly better than the human error rate of 26.2%. Since then, CNNs have become the go-to algorithm for many computer vision tasks.

7. What is a recurrent neural network, and how is it used in natural language processing?

What is a recurrent neural network, and how is it used in natural language processing? A recurrent neural network (RNN) is a type of neural network that is designed for processing sequential data. This means that the network is capable of processing data that comes in a time series, such as speech or text. Unlike traditional feedforward neural networks, RNNs have feedback loops that allow them to use their own output as input. This allows them to process sequences of arbitrary length. In natural language processing (NLP), RNNs are commonly used for tasks such as language modelling, machine translation, and sentiment analysis. One particular type of RNN that is used in NLP is the Long Short-Term Memory (LSTM) network, which is designed to overcome the vanishing gradient problem that can occur in standard RNNs. For example, suppose we want to use an RNN to generate text based on a given input. We might train the network on a dataset of text sequences, such as a collection of news articles. During training, the RNN would learn to predict the next word in the sequence based on the previous words. Once the network is trained, we can use it to generate new text by feeding it a starting sequence and letting it predict the next word in the sequence. This can be repeated to generate an entire paragraph or article. In a study conducted by Google, a variant of the LSTM network was used for machine translation. The researchers trained the network on a dataset of English and French sentence pairs and tested its ability to translate new sentences. The LSTM network outperformed traditional machine translation techniques and achieved state-of-the-art results. This demonstrates the power of RNNs, especially when combined with other deep learning techniques, for natural language processing tasks.

8. Can you explain the concept of transfer learning, and when is it useful?

Transfer learning is the practice of utilizing pre-trained models in order to achieve faster and more accurate results when building a new model. It involves taking a pre-existing model that has already been trained on a large dataset and using it as a starting point for building a related model.

One example of the usefulness of transfer learning is in the field of image classification. If a pre-existing model has already been trained to recognize objects within images, this model can be utilized as a starting point for a new model that aims to classify different types of flowers. The model will have already learned how to recognize basic features such as edges and shapes, so the new model can utilize these pre-existing features to classify specific types of flowers more accurately and with less training data.

Another benefit of transfer learning is that it allows developers to train models with smaller datasets. Take for instance, a new model needs to be trained to recognize different types of birds. Instead of starting from scratch, a pre-trained model on a large collection of labeled bird images can be used as a starting point. This not only improves the accuracy of the new model but also reduces training time and computational resources needed.

One study conducted at Google showed that transfer learning can lead up to a 25% improvement in accuracy for image classification tasks, while reducing the time and effort required to develop successful models. This demonstrates the significance of transfer learning in the field of machine learning and why it is an important concept for ML Engineers to understand.

9. What is a generative adversarial network, and how does it work?

A Generative Adversarial Network (GAN) is a subset of deep learning architecture that learns to generate synthetic data by training one neural network to generate samples, and another neural network to differentiate between real and generated samples. The generator creates synthetic data from a random noise signal, and the discriminator determines whether the data is real or generated.

  • The generator network creates synthetic data from a random noise vector, which is fed into the network as input. The generator network then maps the input noise vector to a latent vector, which is a compressed representation of the input.
  • The latent vector is then decoded or transformed into a desired output, such as an image, text or sound. The output of the generator network is compared to the real data by the discriminator, which is trained to distinguish between real and generated samples.
  • During training, the generator tries to create synthetic samples that fool the discriminator into classifying them as real. The discriminator, on the other hand, tries to correctly identify which samples are real and which are generated
  • Eventually, the generator learns to create synthetic samples that are so convincing that the discriminator cannot distinguish them from real samples, thus achieving the objective of the GAN.

One impressive application of GANs is the ability to generate realistic images of non-existent objects, people or scenery. For example, Nvidia researchers have used a GAN to generate images of photorealistic rooms, faces, and landscapes that are indistinguishable from real photographs.

10. How do you evaluate the performance of a deep learning model?

How do you evaluate the performance of a deep learning model?

Evaluating the performance of a deep learning model is crucial in order to understand how well it is performing on a given task.

  1. Accuracy: A common metric for evaluating the performance of a model is accuracy, which is the percentage of correct predictions on a dataset. For example, if we have a classification problem with two classes and our model correctly predicts 80 out of 100 samples, then the accuracy would be 80%. However, accuracy alone is not always sufficient, especially if the classes are imbalanced.
  2. Precision and Recall: We can use precision and recall to evaluate the performance of a class in a classification problem. Precision is the ratio of true positives to the total number of positive predictions, while recall is the ratio of true positives to the total number of actual positive samples. Using these metrics can be especially useful in imbalanced problems.
  3. Confusion Matrix: Another popular way to evaluate the performance of a classification model is by using a confusion matrix. This matrix displays the number of samples true positives, true negatives, false positives and false negatives.
  4. ROC Curve: ROC (Receiver Operating Characteristic) curves can be used to evaluate the performance of a binary classifier. ROC curves show the trade-off between true positives and false positives for different classification thresholds. A better classifier will have an ROC curve that is close to the upper left corner, indicating high true positive rates and low false positive rates.
  5. F1 score: The F1 score is a metric that combines precision and recall into a single value. It is the harmonic mean of precision and recall and is often used when there is an uneven distribution of classes. A perfect F1 score is 1.0, while a score of 0 indicates that the model failed to make any correct predictions.

When evaluating a deep learning model, it's important to consider all of these metrics and understand their strengths and limitations. It can be useful to combine multiple metrics to get a better understanding of the model's performance. For example, we might use accuracy to get a broad sense of how well the model is doing, and then examine precision and recall to get more detail on how well it is performing on specific classes.

As an example, we trained a deep learning model to classify images of cats and dogs. We used a test set of 1000 images and measured the following performance metrics:

  • Accuracy: 85%
  • Precision for 'cats': 90%
  • Recall for 'cats': 80%
  • Precision for 'dogs': 80%
  • Recall for 'dogs': 90%
  • F1 score: 85%

Examining these metrics, we can see that the model is doing quite well overall, with an accuracy of 85%. However, if we look more closely, we see that it is better at identifying cats than dogs, as the precision for 'cats' is higher than the precision for 'dogs'. We can also see that it has a slight bias towards identifying dogs, as the recall for 'dogs' is higher than the recall for 'cats'. Finally, looking at the F1 score, we see that the model is doing a good job overall, but there is room for improvement in specific areas.


As a Machine Learning Engineer, preparing for an interview can be challenging. By knowing the types of questions that interviewers may ask, you can be better equipped to provide impressive answers. With the answers to these 10 deep learning interview questions, you’ve got a great starting point for your preparation. But remember, interview preparation doesn't end here. You should also write a great cover letter, which you can learn how to do here and prepare an impressive ML Engineering CV, which can be found at this link.

If you’re looking for a new job, we encourage you to search through our remote ML Engineering job board to see if there are any opportunities that fit what you're looking for.

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