An observational study differs from an experiment in that the random assign ment of treatments (i.e. agents, programs, procedures) to units is absent. As has been pointed out by many writers since Fisher (1925), this randomization is a powerful tool in that many systematic sources of bias are made random. If randomization is absent, it is virtually impossible in many practical circumstances to be convinced that the estimates of the effects of treatments are in fact unbiased. This follows because other variables that affect the dependent variable besides the treatment may be differently distributed across treatment groups, and thus any estimate of the treatment is confounded by these extraneous variables.
read the following:
An observational study differs from an experiment in that the random assign ment of treatments (i.e. agents, programs, procedures) to units is absent. As has been pointed out by many writers since Fisher (1925), this randomization is a powerful tool
in that many systematic sources of bias are made random. If randomization is absent,
it is virtually impossible in many practical circumstances to be convinced that the estimates of the effects of treatments are in fact unbiased. This follows because
other variables that affect the dependent variable besides the treatment may be differently distributed across treatment groups, and thus any estimate of the treatment
is confounded by these extraneous variables.
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