In LHC analyses, as in the rest of life, we are inevitably faced with the problem of how to deal with less than perfect knowledge. Our experimental searches force us to deal with uncertainty from e.g. the PDFs in the initial state, imperfect experimental reconstruction, and from undetected particles. A variety of techniques have been developed for dealing with uncertain and incomplete information. In this seminar we will examine some more (and less) motivated methods that have been developed for dealing with imperfect knowledge, with illustrative highlights from recent ATLAS SUSY and Higgs analyses.