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. We should therefore characterise the process in terms of 'nuisance' variables and endeavour to find ways of minimising their variability for the experiment.
Variability is the enemy of the experimenter!
Often however, the experimenter has little influence on the choice of experimental units and must contend with whatever variability exists. If possible, the experimental units should be grouped into blocks which can be used later in the design process to obtain more precise answers to the questions of interest.
What response variable should be recorded?
In an experiment, there is sometimes a single obvious response measurement from an experimental unit (e.g. number of computers sold, concentration of impurities, exam mark), but often there are several variables which can be considered as response measurements.
For example, in a study of the effect of an epidural pain killer during birth, what do you measure?
There are many possibilities here and a physician would need to decide on which was most important from a biological perspective.
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. In an agricultural experiment, do we only want to assess the difference in yields for three crop varieties, or do we simultaneously want to determine the effects of different levels of application of fertiliser? (And if so, what levels should be used?)