For various technical and operational reasons, many power quality surveys are carried out using non-standard data aggregation intervals. The data aggregation interval is the time interval that rapidly sampled data is reduced to by the monitoring instrument for subsequent analysis and reporting. Some of the rationales for using non-standard data aggregation intervals include instrumentation limitations, memory restrictions, a belief that more insights may be obtained from data captured at faster aggregation intervals and dual use of instrumentation (such is the case for many smart revenue meters). There is much conjecture over the effect which the data aggregation interval will have on the final outcomes of a power quality survey. IEC61000-4-30 which is the international standard describing power quality monitoring methodology suggests 10 minute data aggregation intervals are appropriate for routine power quality monitoring of most power quality disturbances including magnitude of supply voltage. This paper investigates the variation observed for magnitude of supply voltage monitoring when data is captured at a range of data aggregation intervals.