ML | Active Learning

What is Active Learning?
Active Learning is a special case of Supervised Machine Learning. This approach is used to construct a high performance classifier while keeping the size of the training dataset to a minimum by actively selecting the valuable data points.

Where should we apply active learning?

  1. We have a very small amount or a huge amount of dataset.
  2. Annotation of the unlabeled dataset cost human effort, time and money.
  3. We have access to limited processing power.

Example

On a certain planet, there are various fruits of different size(1-5), some of them are poisonous and others don’t. The only criteria to decide a fruit is poisonous or not is it’s size. our task is to train a classifier which predicts the given fruit is poisonous or not. The only information we have, is fruit with size 1 is not poisonous, the fruit of size 5 is poisonous and after a particular size, all fruits are poisonous.



The first approach is to check each and every size of the fruit, which consume time and resources.
The second approach is to apply the binary search and find the transition point (decision boundary). This approach uses fewer data and gives the same results as of linear search.

 General Algorithm : 

1. train classifier with the initial training dataset
2. calculate the accuracy
3. while(accuracy < desired accuracy):
4.    select the most valuable data points (in general points close to decision boundary)
5.    query that data point/s (ask for a label) from human oracle
6.    add that data point/s to our initial training dataset
7.    re-train the model
8.    re-calculate the accuracy

Approaches Active Learning Algorithm
1. Query Synthesis

  • Generally this approach is used when we have a very small dataset.
  • This approach we choose any uncertain point from given n-dimensional space. we don’t care about the existence of that point.
  • In this querry synthesis can pick any point(valuable) from 3*3 2-D plane.

  • Sometime it would be difficult for human oracle to annotate the queried data point.
  • These are some queries generated by Query Synthesis approach for a model trained for handwritten recognition. It is very difficult to annotate these queries.

2. Sampling