  In many risk analyses the results are only given as mean values and often the
input data are also mean values. However the required accuracy of the result is
often an interval of values e. g. for the derivation of a Safety Integrity
Level (SIL). In this paper we reason what should be the accuracy of the input
data of risk analyses if a particular certainty of the result is demanded. Also
the backside of the coin, the SIL composition is discussed. The results show
that common methods for risk analysis are faulty and that SIL allocation by a
kind of SIL calculus seems infeasible without additional requirements on the
composed components. A justification of a common practice for parameter scaling
in well-constructed semi-quantitative risk analysis is also provided.
