A semiconductor wafer fabrication metrology method in which process steps are characterised by a change in wafer mass, whereby during fabrication mass is used as a measurable parameter to implement statistical process control on the one or more of process steps. In one aspect, the shape of a measured mass distribution is compared with the shape of a predetermined characteristic mass distribution to monitor the process. An determined empirical relationship between a control variable of the process and the characteristic mass change may enable differences between the measured mass distribution and characteristic mass distribution to provide information about the control variable. In another aspect, the relative position of an individual measured wafer mass change in a current distribution provides information about individual wafer problems independently from general process problems.