Machine learning

Machine learning algorithms can be classified according to

• the type of problems they solve
• the model they use
• the way they learn

Types of problems

• classification
  • output of the model is a discrete set of categories
    • examples:
      • spam detection
      • pion/proton discrimination
      • positive/negative COVID test
• regression
  • output of the model is a continous variable
  • examples:
    • value of stock
    • country GDP

The boundary between the two types can be blured:

• when the categories have an ordering we can use regression and bin the result into categories
  • A*, A, B, C, ... grades
  • number of stars for a review
  • energy rating of a building
• for classification we can fit a function for the probability of belonging to one class

Types of learning

• supervised learning
  • we have a training set with labelled examples
• unsupervised learning
  • no labelled examples
  • model has to find features
  • can be used as a first step before supervised learning
  • dimensionality reduction
    • many features
    • need to find the most relevant ones as input to supervised learning model

Learning modes

• batch learning
• online learning
• mini-batch

Batch learning

The entire training set is used for each iteration of the model optimisation

Online learning

The model is updated for each new training example.

Mini-batch

The model is optimised for subsets of the training set

Instance based vs Model based

• instance based
  • uses examples to learn
  • need "similarity" measure to compare new data to training data
• model based
  • we use a model to quantify the relationship between the data
  • the data fixes the parameters of the model

Example: predicting final grade $g_4$ of a student given their 1st, 2nd and 3rd year result $g_1$, $g_2$ and $g_3$.

• instance-based:
  • look at historical results and find the student who has the closest marks to the student we want to predict the result from
  • use the final grade of the past student as the prediction for the new student
  • could look at a set of historical students and average

• model-based:

  • we can hypothesize a linear dependency:

  $$ g_4 = c_1 g_1 + c_2 g_2 +c_3 g_3$$

  • fit the coefficients $c_1$, $c_2$, $c_3$ to historical data and use them to predict the new student's final grade.

Examples of model-based algorithms

• linear models
  • perceptron
  • logistic regression
  • SVM (support vector machine)
  • ...
• non-linear models
  • polynomial features
  • neural networks
  • ...

Examples of instance-based algorithms

• $k$-neighbour
• SVM with RBF kernel
• ...