Example Find the number of pair of observations
from the following data.

r = 0.25, S
(x_{i} - x ) ( y_{i} - y ) = 60, s_{y}
= 4, S ( x_{i} - x )2 = 90.

Solution: Given - r = 0.25

If the values of x and y are very big, the calculation becomes
very tedious and if we change the variable x to u =
and y to
where x_{0} and y_{0} are the assumed means
for variable x and y respectively, then r_{xy} = ruv