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 -
Professor Huitema told us a new edition was
coming in January 2009. Made it in October 2011.
$125 in hardcover; $80 on Nook.