Calculate the correlation coefficient
Mathematica 34 bytes
Here are a few ways to obtain the Pearson product moment correlation.  They all produce the same result.
From Dr. belisarius: 34 bytes
Dot@@Normalize/@(#-Mean@#&amp;)/@{x,y}


Built-in Correlation function I: 15 chars
This assumes that x and y are lists corresponding to each variable.
x~Correlation~y


0.76909


Built-in Correlation function II: 31 chars
This assumes d is a list of ordered pairs.
d[[;;,1]]~Correlation~d[[;;,2]]


0.76909

The use of ;; for All thanks to A Simmons.

Relying on the Standard Deviation function: 118 115 chars
The correlation can be determined by:
s=StandardDeviation;
m=Mean;
n=Length@d;
x=d[[;;,1]];
y=d[[;;,2]];
Sum[((x[[i]]-m@x)/s@x)((y[[i]]-m@y)/s@y),{i,n}]/(n-1)


0.76909


Hand-rolled Correlation: 119 chars
Assuming x and y are lists...
s=Sum;n=Length@d;m@p_:=Tr@p/n;
(s[(x[[i]]-m@x)(y[[i]]-m@y),{i,n}]/Sqrt@(s[(x[[i]]-m@x)^2,{i,n}] s[(y[[i]] - m@y)^2,{i,n}]))


0.76909