As a general scientific principle, the most reliable data comes from actual observation and the most reliable theories depend on it. This also holds true in military analysis, where analysts look towards live field-exercises and trials as providing data likely to be realistic (depending on the realism of the exercise) and verifiable (it has been gathered by actual observation). One can readily discover, for example, how long it takes to construct a pontoon bridge under given conditions with given manpower, and this data can then generate norms for expected performance under similar conditions in the future, or serve to refine the bridge-building process.