FEPEST - Head Difference

FEPEST allows to use head, topo minus head, etc as an observation to compare with the simulated value. It doesn't allow to include head difference (increment) as an observation. What is the work flow to use head difference as observation? Should it be implemented as IFM observation or there are alternatives?
Thanks!

FePEST allows you to use [b]difference between values [/b] as a reference method but I am not sure whether or not the feature provides the method you are looking for. You indicated a head difference (increment). Do you refer to the head difference between two time-steps?

Thanks Bjorn. Yes I am referring head difference between two time steps. Let's say observation 1 has heads at different times with values of h1, h2, h3, h4 etc... I am looking for h1-h2, h1-h3, h1-h4 etc On the other hand hand observation 2 has heads starting from a different time say 12 - like h12, h13, h14, 15 etc... in this case it will be h12-h13, h12-h14, h12-h15 etc. Different observations will start at different times. I am looking for the difference between their first time and the subsequent values.

The [b] difference between values [/b] allows you to use the observation at t - t-1. At the moment FePEST does not have a feature which allows you to calculate the difference to a constant reference time. Does IFM / Scripting (C/C++, Python) represents an option for you?

'Difference between values' doesn't seem an option. I also couldn't see 'difference between values' in the 'Observation' section of FePest 7.1 (build 4).

IFM would be the option for the problem. Is there a sample code for interacting with FePest through IFM?

Thanks again.