  Pearl has provided the back door criterion, the front door criterion and the
conditional instrumental variable (IV) method as identifiability criteria for
total effects. In some situations, these three criteria can be applied to
identifying total effects simultaneously. For the purpose of increasing
estimating accuracy, this paper compares the three ways of identifying total
effects in terms of the asymptotic variance, and concludes that in some
situations the superior of them can be recognized directly from the graph
structure.
