Although an observed dose-response effect supports the hypothesis of a causal relationship, studies of a genuinely causal relationship may fail to show a dose-response effect for several reasons. Which choice below is the generally the least plausible reason?
a. all doses are too high to reveal the effect
b. all doses are too low to reveal the effect
c. a confounder obscures the effect
d. random variation obscures the effect
e. there is no real dose-response effect