Combined uncertainty factor for sampling and analysis

Measurement uncertainty that arises from primary sampling can be expressed as an uncertainty factor, which recognises its sometimes approximately log-normal probability distribution. By contrast, uncertainty arising from chemical analysis is usually expressed as relative uncertainty, based upon the assumptions of its approximately normal distribution. A new method is proposed that enables uncertainty from these two sources, expressed in these different ways, to be combined to produce an estimate of the total combined uncertainty of the measurement values that result when the measurement process is considered as a whole.