What experimental units should be used?
It is desirable for experimental units to be as similar as possible, so every attempt should be made to make the experimental units homogeneous.
Often however, the experimenter has little influence on the choice of experimental units and must contend with whatever variability exists. Grouping them into block of similar units (and using a randomised block design) will make the results more accurate.
What response variable should be recorded?
In an experiment, there is sometimes a single obvious response measurement from an experimental unit, but often there are several variables which can be considered as response measurements.
A subject expert would need to decide on which was most relevant to the aims of the experiment.
Controlled variables
Thought also needs to be given to which variables need to be controlled (the input variables) and what settings should be used for these variables in different experimental runs.