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Variability in software process models justifies tailoring them to meet the specific goals and characteristics of organisations and projects. Existing process modelling notations typically do not have constructs which are appropriate for expressing process variability. To fill this gap, the authors have extended software process engineering metamodel (SPEM) to vSPEM, by adding new variability constructs (such as variants and variation points). This article presents an empirical validation to check whether the variability constructs supported through vSPEM are more appropriate for modelling variant-rich processes than SPEM, in terms of understandability of the notation, as well as of their variability mechanisms. The results indicate that the vSPEM variability mechanisms understandability is a 126.99% higher than for SPEM. On the other hand, process diagram understandability is a 34.87% lower with vSPEM than with SPEM. If we compare the relative results obtained with regard to understandability of diagrams and understandability of variation mechanisms of both vSPEM and SPEM, the enhancement just mentioned is 3.64 times that fall. The results indicate accepting that a slight decrease in understandability of the diagrams might lead to a large increase in understandability when using variability mechanisms of vSPEM.