I-MAG STS    Corporation
In this case, the dimension of time flows from left
to right, suggesting that if there is any correlation
between the blue events and the red events, blue
must cause red. A straight rho  (product-moment)
correlation gives an  r of -.356 but when
covariance in effect shifts the blue bars to the
right there is such a high correlation that even a
jaded statistician would concede causality.

Readers will find Bradley E. Huitema's (professor
of psychology at Western Michigan University)
book Analysis of Covariance and Alternatives
(Wiley, New York, 1980) instructive.   ISBN:
0471420441 : 9780471420446 -
no copies at the publisher or on Amazon, but
Professor Huitema tells us a new edition is
coming in January 2009