ML | Locally weighted Linear Regression

Difficulty Level : Easy
Last Updated : 08 Jan, 2019

Linear regression is a supervised learning algorithm used for computing linear relationships between input (X) and output (Y).

The steps involved in ordinary linear regression are:

Training phase: Compute $\theta$ to minimize the cost.
$J(\theta) = $\sum_{i=1}^{m} (\theta^Tx^{(i)} - y^{(i)})^2$
Predict output: for given query point $x$ ,
$return: \theta^Tx$

As evident from the image below, this algorithm cannot be used for making predictions when there exists a non-linear relationship between X and Y. In such cases, locally weighted linear regression is used.

Locally Weighted Linear Regression:

Locally weighted linear regression is a non-parametric algorithm, that is, the model does not learn a fixed set of parameters as is done in ordinary linear regression. Rather parameters $\theta$ are computed individually for each query point $x$ . While computing $\theta$ , a higher “preference” is given to the points in the training set lying in the vicinity of $x$ than the points lying far away from $x$ .

The modified cost function is: $J(\theta) = $\sum_{i=1}^{m} w^{(i)}(\theta^Tx^{(i)} - y^{(i)})^2$

where, $w^{(i)}$ is a non-negative “weight” associated with training point $x^{(i)}$ .
For $x^{(i)}$ s lying closer to the query point $x$ , the value of $w^{(i)}$ is large, while for $x^{(i)}$ s lying far away from $x$ the value of $w^{(i)}$ is small.

A typical choice of $w^{(i)}$ is: $w^{(i)} = exp(\frac{-(x^{(i)} - x)^2}{2\tau^2})$
where, $\tau$ is called the bandwidth parameter and controls the rate at which $w^{(i)}$ falls with distance from $x$

Clearly, if $|x^{(i)} - x|$ is small $w^{(i)}$ is close to 1 and if $|x^{(i)} - x|$ is large $w^{(i)}$ is close to 0.

Thus, the training-set-points lying closer to the query point $x$ contribute more to the cost $J(\theta)$ than the points lying far away from $x$ .

For example –

Consider a query point $x$ = 5.0 and let $x^{(1)}$ and $x^{(2)$ be two points in the training set such that $x^{(1)}$ = 4.9 and $x^{(2)}$ = 3.0.
Using the formula $w^{(i)} = exp(\frac{-(x^{(i)} - x)^2}{2\tau^2})$ with $\tau$ = 0.5:
$w^{(1)} = exp(\frac{-(4.9 - 5.0)^2}{2(0.5)^2}) = 0.9802$
$w^{(2)} = exp(\frac{-(3.0 - 5.0)^2}{2(0.5)^2}) = 0.000335$
$So, \ J(\theta) = 0.9802*(\theta^Tx^{(1)} - y^{(1)}) + 0.000335*(\theta^Tx^{(2)} - y^{(2)})$
Thus, the weights fall exponentially as the distance between $x$ and $x^{(i)}$ increases and so does the contribution of error in prediction for $x^{(i)}$ to the cost.

Consequently, while computing $\theta$ , we focus more on reducing $(\theta^Tx^{(i)} - y^{(i)})^2$ for the points lying closer to the query point (having larger value of $w^{(i)}$ ).

Steps involved in locally weighted linear regression are:

Compute $\theta$ to minimize the cost. $J(\theta) = $\sum_{i=1}^{m} w^{(i)}(\theta^Tx^{(i)} - y^{(i)})^2$
Predict Output: for given query point $x$ ,
$return: \theta^Tx$

Points to remember:

Locally weighted linear regression is a supervised learning algorithm.
It a non-parametric algorithm.
There exists No training phase. All the work is done during the testing phase/while making predictions.

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