> Second, there is a known effect -- the placebo effect --
> that can affect results. In other words your null
> hypothesis is that there may be some effect. These
> things mean that, without a control group, the evidence
> in favor of a drug's effectiveness is not very strong.
Placebo effect is but one confound variable you'll encounter in pharmacological and all other "non-quantitative" prediction experiments. There are plenty of others; for instance, order of measurement, influence of experimenter, subtle pre-existing differences, and so on.
In order to keep confounds in check, scientists attempt to keep everything either equivalent (by matching samples as precisely as scientifically feasible) or randomly distributed (by using, for instance, Latin squares and other randomization techniques).
In order to keep confounds in check, scientists attempt to keep everything either equivalent (by matching samples as precisely as scientifically feasible) or randomly distributed (by using, for instance, Latin squares and other randomization techniques).