Some particle size instruments, known as single particle counters, measure individual particle size, while others measure surface area as a function of particle size. Further instruments measure mass or volume versus size, while others still measure multiple functions of scattered light intensity as a function of size. Each of these instruments can generate particle size distributions and, in theory, one can transform between types in order to correlate the results. In practice, if a measurement with a single particle counter generated a differential number-weighted size distribution, how would one undertake a comparison of the results with a measurement deriving from a different type of instrument, which generated a differential volume-weighted size distribution?