Task \(T\)¶
Process of learning itself is not the task; learning is the means of attaining ability to perform the task
Usually described in terms of how the machine learning system should process an instance (collection of features), which is usually represented as a vector.
Tasks¶
| Function Mapping | Example | ||
|---|---|---|---|
| Regression | Predicting a continuous numerical output | \({\mathbb R}^n \to {\mathbb R}\) | Stock value prediction |
| Classification | Categorizing input into a discrete output or outputing a probability dist over classes Derived from regression If binary and very imbalanced dataset, use anomaly detection instead | \({\mathbb R}^n \to [1, C]\) | Categorizing images Fraud detection |
| Anomaly Detection | Identify abnormal events | \(\mathbb R \to [0, 1]\) | Fraud detection |
| Classification w/ missing inputs | Learn distribution over all variables, solve by marginalizing over missing variables | \({\mathbb R}^n \to [1, C]\) | |
| Clustering | Assigning class label to set of unclassified items, to group observations into clusters | \(\hat y = \hat f(x) = \text{Cluster}(x)\) \({\mathbb R}^n \to [1, C ]\) | Grouping similar images |
| Density estimation | Estimating probability distribution from data | \({\mathbb R}^n \to P(x)\) | |
| Transcription | Convert unstructured data intro discrete textual form | OCR Speech Recognition | |
| Machine Translation | Convert it into a sequence of symbols into another language | Natural Language Translation | |
| Structured Output | Output data structure has relationships between elements | Parsing Image segmentation Image captioning | |
| Synthesis & Sampling | Generate new samples similar to those in training data | Texture generation Speech synthesis Supersampling images | |
| Data Imputation | Predict values of missing entries | ||
| Denoising | Predict clean output from corrupt input | Image/Video denoising | |
| Density Estimation | Identify underlying probability distribution of set of inputs |
Types of Predictions¶
| \(x_\text{new}\) | Uncertainty | |
|---|---|---|
| Intrapolation? | \(\in X_\text{train}\) | Low |
| Interpolation | \(\in [X_{\text{train}_\text{min}}, X_{\text{train}_\text{max}}]\) | Moderate |
| Extrapolation | \(\not \in [X_{\text{train}_\text{min}}, X_{\text{train}_\text{max}}]\) | High |
| Smoothing |
Regression¶
Probabilistic Regression¶
- Probability of prediction is required
- Understand impact of input
- Regression target is the sum of individual binary outcomes
\[ y'_i = p_i = \dfrac{y_i}{n_i} \]
Binary Aggregate Outcomes¶
\[ \begin{aligned} y_i &\sim \text{Binomial}(n_i, p_i) \\ p_i &= \sigma(\beta x_i) \\ \implies y_i &\sim \text{Bernoulli}\Big( \sigma(x_i' \beta) \Big) \end{aligned} \]
temperature fields cultivated percentCultivated
1 13.18475 63 49 0.7777778
2 12.35680 165 147 0.8909091
3 17.57882 38 30 0.7894737
4 20.86867 152 95 0.6250000
5 13.88084 88 69 0.7840909
6 17.18088 191 141 0.7382199
Multiple Aggregate Outcomes¶
\[ \begin{aligned} y_i &\sim \text{Multinomial}(n, p) \\ p_j &= \text{Softmax}(\beta_j x) \\ &= \dfrac{\exp(\beta_j x)}{\sum_k^K \exp(\beta_k x) } \end{aligned} \]
When \(n_i=1,\) this becomes multi-class classification
Example
temperature rainfall fields noncrop corn wheat rice
1 13.18475 75.26666 63 8 31 17 7
2 12.35680 102.37572 165 7 100 30 28
3 17.57882 101.61363 38 1 26 3 8
4 20.86867 64.35788 152 45 78 12 17
5 13.88084 107.54101 88 4 54 15 15
Classification¶
Decision Boundary/Surface¶
The boundary/surface that separates different classes
Generated using decision function
If we have \(d\) dimensional data, our decision boundary will have \((d-1)\) dimensions
Linear Separability¶
Means the ability to separate points of different classes using a line, with/without a non-linear activation function
\[ f(u) = \begin{cases} 1, & u \ge 0 \\ 0, & \text{otherwise} \end{cases} \]
| Logic Gate | Linearly-Separable? | Comment |
|---|---|---|
| AND | ✅ | |
| OR | ✅ | |
| XOR | ❌ | Linearly separable if we add \((x \cdot y)\) as a feature |
| XNOR | ❌ | Linearly separable if we add \((x \cdot y)\) as a feature |
Linearly Non-Separable¶
| Slightly | Seriously |
|---|---|
 |  |
Discriminant Function¶
Functions which takes an input vector \(x\) and assigns it to one of the \(k\) classes
Multi-Class Classification¶
| One-vs-Rest | One-vs-One | |
|---|---|---|
| No of classifiers | \(k\) | \(\frac{k(k-1)}{2}\) |
| Retains valid probabilistic interpretation | ✅ | ❌ |
| Limitation | Some point may have multiple classes/no classes at all | Multiple classes assigned to some points |
Clustering¶
Analyzing clusters¶
Understand the characteristics of each cluster
- Select features \(x_j\)
- Do not use all features to perform clustering
- where \(x_j=\) sensible features such as
- Spending habits
- Perform clustering
- Perform statistical analysis on \(x_{\centernot{j}}\)
For example: 4 clusters
| \(x_1=0\) | \(x_1=1\) | |
|---|---|---|
| \(x_2=0\) | A | B |
| \(x_2=1\) | C | D |