The machine comprises one or more weighers for producing electrical output signals dependent on the weights of samples weighed thereby, and a plurality of sample holders, greater in number than the weighers, for holding respective samples. The sample holders may include a respective weigher holder associated with each weigher, and two or more storage holders associated with each weigher for holding samples discharged from the associated weigher holder. The machine further comprises memories for storing signals indicative of the weights of the samples in the storage holders as determined by the weighers, and selection circuitry for making a selection from amongst the samples in all the storage holders in dependence on the weights of the samples as indicated by the memories and for discharging the selected samples from the storage holders to form a batch of an appropriate weight.