Correlation Co-efficient Definition:
     A measure of the strength of linear association between two variables. Correlation will always between -1.0 and +1.0. If the correlation is positive, we have a positive relationship. If it is negative, the relationship is negative.

  Formula:

Correlation Co-efficient :
Correlation(r) = NΣXY - (ΣX)(ΣY) / Sqrt([NΣX² - (ΣX)²][NΣY² - (ΣY)²])
where
              N = Number of values or elements
              X = First Score
              Y = Second Score
              ΣXY = Sum of the product of first and Second Scores
              ΣX = Sum of First Scores
              ΣY = Sum of Second Scores
              ΣX² = Sum of square First Scores
              ΣY² = Sum of square Second Scores



Correlation Co-efficient Example: To find the Correlation of

X Values	Y Values
60	3.1
61	3.6
62	3.8
63	4
65	4.1

  Step 1: Count the number of values.
            N = 5

  Step 2: Find XY, X², Y²
            See the below table

X Value	Y Value	X*Y	X*X	Y*Y
60	3.1	60 * 3.1 = 186	60 * 60 = 3600	3.1 * 3.1 = 9.61
61	3.6	61 * 3.6 = 219.6	61 * 61 = 3721	3.6 * 3.6 = 12.96
62	3.8	62 * 3.8 = 235.6	62 * 62 = 3844	3.8 * 3.8 = 14.44
63	4	63 * 4 = 252	63 * 63 = 3969	4 * 4 = 16
65	4.1	65 * 4.1 = 266.5	65 * 65 = 4225	4.1 * 4.1 = 16.81

  Step 3: Find ΣX, ΣY, ΣXY, ΣX², ΣY².
            ΣX = 311
            ΣY = 18.6
            ΣXY = 1159.7 1159.7
            ΣX² = 19359
            ΣY² = 69.82

  Step 4: Now, Substitute in the above formula given.
            Correlation(r) = NΣXY - (ΣX)(ΣY) / Sqrt([NΣX² - (ΣX)²][NΣY² - (ΣY)²])
            = ((5)*(1159.7)-(311)*(18.6))/sqrt([(5)*(19359)-(311)²]*[(5)*(69.82)-(18.6)²])
            = (5798.5 - 5784.6)/sqrt([96795 - 96721]*[349.1 - 345.96])
            = 13.9/sqrt(74*3.14)
            = 13.9/sqrt(232.36)
            = 13.9/15.24336
            = 0.9119

This example will guide you to find the relationship between two variables by calculating the Correlation Co-efficient from the above steps.