## Rank correlation in statistics.

In statistics Kendall's rank correlation produces a distribution-free test of independence and a measure of the strength of ordinal association between two variables.
It is named after Maurice Kendall, who developed it in 1938.
Kendall's tau, like Spearman's rank correlation, is carried
out on the ranks of the data.

The Kendall correlation between two variables will be high when observations have a similar rank between the two variables, and low when observations have a different rank between the two variables.

Spearman's rank correlation is a more widely used measure of rank correlation because it is much easier to compute than Kendall's tau. The main advantages of using Kendall's tau are that the distribution of this statistic has slightly better statistical properties and there is a direct interpretation of Kendall's tau in terms of probabilities of observing concordant and discordant pairs. In almost all situations the values of Spearman's rank correlation and Kendall's tau are very close and would invariably lead to the same conclusions.

## Kendall's rank for correlated data - movie ratings and education level:

```
import matplotlib.pyplot as plt
import numpy as np
import scipy.stats as stats
N = 40
# movie ratings
docuRatings = np.random.randint(low=1,high=6,size=N)
# education level (1-4, correlated with docuRatings)
eduLevel = np.ceil( (docuRatings + np.random.randint(low=1,high=5,size=N) )/9 * 4 )
# compute the correlations
cr = [0,0,0]
cr[0] = stats.kendalltau(eduLevel,docuRatings)[0]
cr[1] = stats.pearsonr(eduLevel,docuRatings)[0]
cr[2] = stats.spearmanr(eduLevel,docuRatings)[0]
# round for convenience
cr = np.round(cr,4)
# plot the data
plt.plot(eduLevel+np.random.randn(N)/30,docuRatings+np.random.randn(N)/30,'ks',markersize=10,markerfacecolor=[0,0,0,.25])
plt.xticks(np.arange(4)+1)
plt.yticks(np.arange(5)+1)
plt.xlabel('Education level')
plt.ylabel('Documentary ratings')
plt.grid()
plt.title('$r_k$ = %g, $r_p$=%g, $r_s$=%g'%(cr[0],cr[1],cr[2]))
plt.show()
```

## Correlation estimation errors under H0:

```
numExprs = 1000
nValues = 50
nCategories = 6
c = np.zeros((numExprs,3))
for i in range(numExprs):
# create data
x = np.random.randint(low=0,high=nCategories,size=nValues)
y = np.random.randint(low=0,high=nCategories,size=nValues)
# store correlations
c[i,:] = [ stats.kendalltau(x,y)[0],
stats.pearsonr(x,y)[0],
stats.spearmanr(x,y)[0] ]
print(c)
```

## Correlation comparison - the graphs:

```
plt.bar(range(3),np.mean(c**2,axis=0))
plt.errorbar(range(3),np.mean(c**2,axis=0),yerr=np.std(c**2,ddof=1,axis=0))
plt.xticks(range(3),('Kendall','Pearson','Spearman'))
plt.ylabel('Squared correlation error')
plt.title('Noise correlation ($r^2$) distributions')
plt.show()
plt.plot(c[:100,:],'s-')
plt.xlabel('Experiment number')
plt.ylabel('Correlation value')
plt.legend(('K','P','S'))
plt.show()
plt.imshow(np.corrcoef(c.T),vmin=.9,vmax=1)
plt.xticks(range(3),['K','P','S'])
plt.yticks(range(3),('K','P','S'))
plt.colorbar()
plt.title('Correlation matrix')
plt.show()
```