  When a measurement of a physical quantity is reported, the total uncertainty
is usually decomposed into statistical and systematic uncertainties. This
decomposition is not only useful to understand the contributions to the total
uncertainty, but also required to propagate these contributions in subsequent
analyses, such as combinations or interpretation fits including results from
other measurements or experiments. In profile-likelihood fits, widely applied
in high-energy physics analyses, contributions of systematic uncertainties are
routinely quantified using "impacts", which are not adequate for such
applications. We discuss the difference between impacts and actual uncertainty
components, and establish methods to determine the latter in a wide range of
statistical models.
