The Bias-Variance Trade-Off

Imagine you’re tuning a musical instrument, like a guitar. Bias is like having the strings too tight or too loose—your instrument produces the wrong note consistently, no matter what you play. This is similar to bias in machine learning, where your model consistently predicts something wrong because it’s stuck in a certain perspective.

Now, variance is like having the strings of your guitar at different tensions. Each string produces a slightly different sound, and the overall result is chaotic and inconsistent. This relates to variance in machine learning, where your model is too sensitive to small changes in the data and gives unpredictable results.

Balancing bias and variance is like tuning your guitar perfectly 🎸👇
You adjust the strings just right so that each note is accurate and consistent across different parts of the instrument. Similarly, in machine learning, you aim to find the sweet spot where your model neither underfits (bias) nor overfits (variance), resulting in reliable and accurate predictions. Just as a well-tuned guitar produces beautiful music, a well-balanced model produces meaningful insights.

You can think about very complex models as the ones that will experience a lot of variance in their outputs/solutions.

An idiot admires complexity, a genius admires simplicity." -Terry A. Davis

But very simple models, tend to be biased, that means to provide very simplistic ‘replies’ that might not capture all the insights of the domain that its trying to make predictions about.

Supervised Learning

A computer is educated to carry out a task by being provided samples of desired input and output in the supervised learning method of machine learning. The examples are then used by the computer to deduce how to carry out the work by itself.

For this, you should get familiar with these concepts:

  • Features - Your independent variable, the X, your model input.
  • Labels - Your model’s prediction, the dependent variable (on the model), the Y.

Predictive models that can make precise forecasts based on new data (not seen before by the algorithm) are created using supervised learning techniques. This is particularly helpful in programs like:

  • Regression:
  • Classification:
    • Image recognition, where the computer can recognize items in pictures
    • Sentiment Analysis

Supervised Models require training, for that we need a dataset that includes historical features, characteristics of the entity for which you want to make a prediction, and known label values.

The train/test split - Normally we train the model using a subset of the data, while holding back some data with which to test the trained model. This enables you to compare the labels that the model predicts with the actual known labels in the original dataset.

Steps required for Supervised ML models: Prepare data (Pre-process, or clean and transform), train model (train/test split), evaluate performance, deploy the model.


Regression in machine learning is a technique used to investigate the relationship between independent variables (or features) and a dependent variable (or outcome).

It can be used to predict a continuous outcome (such as salary or weight) based on the input data.


Classification is a supervised machine learning method where the model attempts to predict the correct label of a given input data. It involves categorizing a given set of input data into classes based on one or more variables. Classification algorithms use a set of labeled data points to learn how to assign labels to unseen data points. Common classification algorithms include: Support Vector Machines (SVM), Logistic Regression, Decision Trees, Random Forests, and Naive Bayes.

Evaluating Classification Models

The confusion matrix is a tool used to assess the quality of a classification model’s predictions. It compares predicted labels against actual labels.

The Matrix with: Binary vs MultiClass Models 👇
  • In a binary classification model where you’re predicting one of two possible values, the confusion matrix is a 2x2 grid showing the predicted and actual value counts for classes 1 and 0. It categorizes the model’s results into four types of outcomes.

  • For a multi-class classification model (where there are more than two possible classes), the same approach is used to tabulate each possible combination of actual and predicted value counts - so a model with three possible classes would result in a 3x3 matrix with a diagonal line of cells where the predicted and actual labels match.

There are several metrics for measuring the performance of a classification model

Check Them Out 👇
  • Accuracy: The number of correct predictions (true positives + true negatives) divided by the total number of predictions.

  • Precision: The number of the cases classified as positive that are actually positive: the number of true positives divided by (the number of true positives plus false positives).

  • Recall: The fraction of positive cases correctly identified: the number of true positives divided by (the number of true positives plus false negatives). {Another term for recall is True positive rate}

  • F1 Score: An overall metric that essentially combines precision and recall. The harmonic mean of precision and recall.

They can be obtained from the confussion matrix.

One important concept for Classification algorithms is the threshold in the case of a binary classification model, the predicted probability is a value between 0 and 1.

By default, a predicted probability including or above 0.5 results in a class prediction of 1, while a prediction below this threshold means that there’s a greater probability of a negative prediction (remember that the probabilities for all classes add up to 1), so the predicted class would be 0.

  • Other ways for evaluating classification models:
    • Log loss - the negative logarithmic of the likelihood ratio
    • Confussion Matrix - visualize the model predictions vs the ground truth labels
    • Receiver Operating Characteristic curve - You will use it to visualize how different thresholds affect the predictions performance of your model. The x axis is the FPR (false possitive rate) and the y axis is the TPR (true possitive rate = precission)
    • AUC - Area under the ROC curve - It will be between 1 (perfect model) and 0 (always wrong model). AUC of 0.5 is as good as guessing could be.

Unsupervised Learning

Unsupervised learning is a type of machine learning algorithm that looks for patterns in a dataset without pre-existing labels.

The goal is to identify relationships between data points and group them into clusters or assign them to categories.

Unlike supervised learning, which uses labeled data to train a model, unsupervised learning does not require any labels or outcomes to be known beforehand. Rather, the algorithm itself must discover the patterns in the data, without any guidance.

It can be used for a variety of tasks, such as:

  • Clustering data points - to group similar entities based on their features
  • Anomaly detection
  • Dimensionality reduction


Clustering is a Machine Learning technique that involves the grouping of data points. Given a set of data points, we can use a clustering algorithm to identify and group similar points into clusters.

Clustering is an unsupervised learning technique, meaning that the algorithm is not explicitly provided with labels for how the data points should be grouped. Instead, it is left to the algorithm to identify similarities between the data points and group them accordingly.

Popular clustering algorithms include k-means clustering, hierarchical clustering, and density-based clustering. Clustering can be used in a variety of applications, such as customer segmentation, image segmentation, and anomaly detection.


The K-Means Clustering algorithm groups items into the number of clusters, or centroids, you specify - a value referred to as K.

This is how KMeans works 👇
  • Initializing K coordinates as randomly selected points called centroids in n-dimensional space (where n is the number of dimensions in the feature vectors).
  • Plotting the feature vectors as points in the same space, and assigning each point to its closest centroid.
  • Moving the centroids to the middle of the points allocated to it (based on the mean distance).
  • Reassigning the points to their closest centroid after the move. Repeating steps 3 and 4 until the cluster allocations stabilize or the specified number of iterations has completed.


Key Concepts

  • Neural Networks (NN): Remember that not all ML models use NN.

I could not finish this topic without mentioning it.

  • These are very famous NN architectures 👇
    • FNN - Feedforward Neuronal Networks
      • Used for supervised learning tasks, such as classification and regression
    • CNN - Convolutional Neuronal Networks
      • Used for image and video recognition tasks
    • RNN - Recurrent Neuronal Networks
      • Used for sequence-based tasks, such as language modeling and speech recognition. They use a technique called recurrent connections to remember data from previous inputs, allowing them to produce an output based on the context of the previous data
    • GANN - Generative Adversarial NN
      • Used for unsupervised learning tasks
  • Deep Neural Network (DNN) - a NN with 3+ hidden layers

  • Backpropagation - adjust weights - this is how NN learn

  • Loss Function - compares the ground truth to the prediction to get the error rate (rates how well/bad the NN performed)

  • Activation Functions - algorithm applied to a hidden layer node that affects connected outputs, e.g ReLu

  • Dense Layer - next layer increase amount of nodes

  • Sparse Layer - next layer decrease amount of nodes

Software for DSc

  • Knime
  • Alteryx
  • AMZ (AWS) Sagemaker - Propietary

Other Resources