The concepts that case-referent studies provide for the estimation of "relative risk"
only if the illness is "rare", and that the rates and risks themselves are inestimable,
are overly superficial and restrictve. The ratio of incidence densities (forces of
morbidity)-and thereby the instantaneous risk-ratio-is estimable without any rarity-assumption.
Long-term risk-ratio can be computed through the coupling of case-referent data on
exposure rates for various age-categories with estimates, possibly from the study
itself, or the corresponding age-specific incidence-densities for the exposed and
nonexposed combined-but again, no rarity-assumption is involved. Such data also provide
for the assessment of exposure-specific absolute incidence-rates and risks. Point
estimation of the various parameters can be based on simple relationships among them,
and in interval estimation it is sufficient simply to couple the point estimate with
the value of the chi square statistic used in significance testing.