STATISTICAL METHODS
The association between individual dietary
factors and CRC rates was examined first by
calculating simple (Pearson) correlation coeYcients.
CRC rates were used, rather than data
for cancers of the colon and rectum separately,
because fuller data were available for the
former. CRC rates were taken as the combined
rates for men and women as these were highly correlated with one another (r= 0.95) and also
because food supply data were available per
capita rather than separately for men and
women. Because the various dietary factors
were correlated with each other, the partial
correlation coeYcients (PCC) were also calculated.
These coeYcients measure the correlation
for each dietary factor after controlling for
the others. This procedure is similar to that
used by Armstrong and Doll3 in their study of
environmental factors, mostly dietary, and
cancer.
We then examined the association between
the dietary factors and CRC using stepwise
regression. This procedure starts by selecting
the model in which each individual factor fits
the data best. A second factor is selected for
inclusion in the model, in combination with the
first, as the one in which the model of the two
terms again produces the best fit. This
procedure is continued until the addition of no
other factor produces a significant (p