Abstract: The thickness of successive sheets which are delivered by the feeders of a gathering machine to a collating conveyor are monitored and a computer which is associated with each feeder calculates the extent of deviation of the monitored thickness from an acceptable value. Signals which are generated by the computers in the event of excessive deviation are used to arrest the motor of the gathering machine and/or to expel the respective groups of sheets from the collating conveyor. Each computer is caused to calculate a reference value on the basis of monitored thicknesses of n successively delivered sheets whose thicknesses are within the acceptable range.