# Class WeightedTau

java.lang.Object
it.unimi.dsi.law.stat.CorrelationIndex
it.unimi.dsi.law.stat.WeightedTau

```public class WeightedTau
extends CorrelationIndex```
Computes the weighted τ between two score vectors. More precisely, this class computes the formula given by Sebastiano Vigna in “A weighted correlation index for rankings with ties”, Proc. of the 24th International World–Wide Web Conference, pages 1166−1176, 2015, ACM Press, using the algorithm therein described (see details below).

Given two scores vectors for a list of items, this class provides a method to compute efficiently the weighted τ using an `ExchangeWeigher`.

Instances of this class are immutable. At creation time you can specify a weigher that turns indices into weights, and whether to combine weights additively or multiplicatively. Ready-made weighers include `HYPERBOLIC_WEIGHER`, which is the weigher of choice. Alternatives include `LOGARITHMIC_WEIGHER` and `QUADRATIC_WEIGHER`. Additional methods inherited from `CorrelationIndex` make it possible to compute directly the weighted τ bewteen two files, to bound the number of significant digits, or to reverse the standard association between scores and ranks (by default, a larger score corresponds to a higher rank, i.e., to a smaller rank index; the largest score gets rank 0).

The weighted τ is defined as follows: consider a rank function ρ (returning natural numbers or ∞) that provides a ground truth—it tells us which elements are more or less important. Consider also a weight function w(−, −) associating with each pair of ranks a nonnegative real number. We define the rank-weighted τ by

r, sρ,w = ∑ij sgn(rirj) sgn(sisj) w(ρ(i), ρ(j))
rρ,w = 〈r, rρ,w1/2
τρ,w(r, s) = 〈r, sρ,w / (‖rρ,wsρ,w).

The weight function can be specified by giving a weigher f (e.g., `HYPERBOLIC_WEIGHER`) and a combination strategy, which can be additive or multiplicative. The weight of the exchange between i and j is then f(i) ● f(j), where ● is the chosen combinator.

Now, consider the rank function ρr, s induced by the lexicographical order by r and s. We define

τw = (τρr, s, w + τρs, r, w) / 2.

In particular, the (additive) hyperbolic τ is defined by the weight function h(i) = 1 / (i + 1) combined additively:

τh = (τρr, s, h + τρs, r, h) / 2.

The methods inherited from `CorrelationIndex` compute the formula above using the provided weigher and combination method. A ready-made instance `HYPERBOLIC` can be used to compute the additive hyperbolic τ. An ad hoc method can instead compute τρ,w.

A main method is provided for command-line usage.

• ## Nested Class Summary

Nested Classes
Modifier and Type Class Description
`static class ` `WeightedTau.AbstractWeigher`
• ## Field Summary

Fields
Modifier and Type Field Description
`static WeightedTau` `HYPERBOLIC`
A singleton instance of the symmetric hyperbolic additive τ.
`static Int2DoubleFunction` `HYPERBOLIC_WEIGHER`
A hyperbolic weigher (the default one).
`static Int2DoubleFunction` `LOGARITHMIC_WEIGHER`
A logarithmic weigher.
`static Int2DoubleFunction` `QUADRATIC_WEIGHER`
`static Int2DoubleFunction` `ZERO_WEIGHER`
A constant zero weigher.
• ## Constructor Summary

Constructors
Constructor Description
`WeightedTau()`
`WeightedTau​(Int2DoubleFunction weigher)`
Create an additive weighted τ using the specified weigher.
```WeightedTau​(Int2DoubleFunction weigher, boolean multiplicative)```
Create an additive or multiplicative weighted τ using the specified weigher and combination strategy.
• ## Method Summary

Modifier and Type Method Description
`double` ```compute​(double[] v0, double[] v1)```
Computes the symmetrized weighted τ between two score vectors.
`double` ```compute​(double[] v0, double[] v1, int[] rank)```
Computes the weighted τ between two score vectors, given a reference rank.
`static void` `main​(String[] arg)`

### Methods inherited from class it.unimi.dsi.law.stat.CorrelationIndex

`compute, compute, compute, compute, computeDoubles, computeDoubles, computeDoubles, computeDoubles, computeFloats, computeFloats, computeFloats, computeFloats, computeInts, computeInts, computeLongs, computeLongs, loadAsDoubles, parseInputTypes`

### Methods inherited from class java.lang.Object

`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`
• ## Field Details

• ### HYPERBOLIC_WEIGHER

public static final Int2DoubleFunction HYPERBOLIC_WEIGHER
A hyperbolic weigher (the default one). Rank x has weight 1 / (x + 1).

A quadratic weigher. Rank x has weight 1 / (x + 1)2.
• ### LOGARITHMIC_WEIGHER

public static final Int2DoubleFunction LOGARITHMIC_WEIGHER
A logarithmic weigher. Rank x has weight 1 / ln(x + e).
• ### ZERO_WEIGHER

public static final Int2DoubleFunction ZERO_WEIGHER
A constant zero weigher.
• ### HYPERBOLIC

public static final WeightedTau HYPERBOLIC
A singleton instance of the symmetric hyperbolic additive τ.
• ## Constructor Details

• ### WeightedTau

public WeightedTau()
• ### WeightedTau

public WeightedTau​(Int2DoubleFunction weigher)
Create an additive weighted τ using the specified weigher.
Parameters:
`weigher` - a weigher.
• ### WeightedTau

public WeightedTau​(Int2DoubleFunction weigher, boolean multiplicative)
Create an additive or multiplicative weighted τ using the specified weigher and combination strategy.
Parameters:
`weigher` - a weigher.
`multiplicative` - if true, weights are combined multiplicatively, rather than additively.
• ## Method Details

• ### compute

public double compute​(double[] v0, double[] v1)
Computes the symmetrized weighted τ between two score vectors.
Specified by:
`compute` in class `CorrelationIndex`
Parameters:
`v0` - the first score vector.
`v1` - the second score vector.
Returns:
the symmetric weighted τ.
• ### compute

public double compute​(double[] v0, double[] v1, int[] rank)
Computes the weighted τ between two score vectors, given a reference rank.

Note that this method must be called with some care. More precisely, the two arguments should be built on-the-fly in the method call, and not stored in variables, as the first argument array will be `null`'d during the execution of this method to free some memory: if the array is referenced elsewhere the garbage collector will not be able to collect it.

Parameters:
`v0` - the first score vector.
`v1` - the second score vector.
`rank` - the “ground truth” ranking used to weight exchanges, or `null` to use the ranking induced lexicographically by `v1` and `v0` as ground truth.
Returns:
the weighted τ.
• ### main

public static void main​(String[] arg) throws
Throws:
`NumberFormatException`
`IOException`
`JSAPException`