A filter test method is needed to compare cabin air filters from different manufacturers. The performance comparison between different filters requires a test method with repeatable results and low variability. The accurate determination of filter performance can only be achieved by understanding the influence different test parameters and instrumentation can have on the results. A good test standard will minimize these sources of variability, be simple to operate and not deter potential users.The SAE J1669 proposal allows for different test options that can be a source of variability. This paper highlights some of the sources of variability and common errors possible. The effect of pressure tap and sampling probe location is investigated and discussed. Also examined is the impact background particle measurement and calculation can have on the final results of filter efficiency.