For pilot projects or research, it may sometimes be possible to define two or more groups, one target group on which prevention actions are being set up, the other groups being control
groups. This approach removes exogenous effects (principally the market effects) and allows a
better evaluation of the only prevention effects. (See European Foundation for the
Improvement of Living and Working Conditions, Stress prevention in the workplace: assessing
the costs and benefits to organisations, 1996.)
For practical cost assessments, it is therefore necessary to construct specific lists of variables.