6.8 Spearmanís Rank Correlation Coefficient
This method is based on the ranks of the items rather than on their actual values. The advantage of this method over the others in that it can be used even when the actual values of items are unknown. For example if you want to know the correlation between honesty and wisdom of the boys of your class, you can use this method by giving ranks to the boys. It can also be used to find the degree of agreements between the judgements of two examiners or two judges. The formula is :
where R = Rank correlation coefficient
D = Difference between the ranks of two
N = The number of observations.
Note: -1 £
R £ 1.
i) When R = +1
Þ Perfect positive correlation or complete
in the same direction
ii) When R = -1
Þ Perfect negative correlation or complete agreement
in the opposite direction.
iii) When R = 0
Þ No Correlation.
Give ranks to the values of items. Generally the item with the highest value is ranked 1 and then the others are given ranks 2, 3, 4, .... according to their values in the decreasing order.