Elasticity is required , because the line capacity should be rapidly adjusted to the variation in the customer orders received. Sometime the operators should move from one workstation to an other one, the time spent is not included in the Useful Time calculation. Useful time is calculated for a defined process. If the process is in the middle of being re-engineered, the useful time is calculated for the new process organisation. Do not define a Useful Time based on potential improvements but only on real commitments (action plan, investments, deadlines for its implementation).