When measuring and analysing the data it is desirable that the data has statistical properties that are invariant over time, i.e. stationary data.
In practice, this is not the case and the data will often be non-stationary seen over the whole period, e.g. the ship will vary its RPM and speed as it will change course depending on the area of operation.
It is possible though to find cases where the ship is in a steady state condition, i.e. conditions where mean and variance are more or less constant.
The conditions can be processed in the model and thus a piecewise analysis of the performance of the ship can be made.