Second, OEI methods rely on emission factors when measurements cannot be made. These factors typically originate from prior eld studies conducted years earlier on facilities operated by multiple O&G operators. While well-performed studies include the full range of emissions from each source category, emission distributions are inherently averages across facility types, operating methods, failure modes, etc. Further, OEI methods use only the mean emission factor, and therefore do not account for extremes in emission rates which may exist on any one facility or for any one source.