以钢铁中锰的测定为例，介绍“精密度法则”，即“保持影响测量各因素对同一测量系列中各个样品影响的一致性”，在光度法中的两大作用：1) 它使各种影响因素对于同一测定系列各个样品的干扰趋于一致，显出了光度法的真实规律，确立了“两标准光度法”，奠定了光度法测定各种含量的理论基础，改变了传统观念；2) 它是正态分布理论在光度法中的体现，使样本测得值近似正态分布。不仅易于发现系统误差和统计离群值，保证了测定结果的可靠性，而且简化了不确定度评定，满足了JJF1059.1-2012《测量不确定度评定与表示》的各项要求。
Taking the determination of manganese in iron and steel as an example, this paper introduces the “precision rule”, i.e. “to maintain infection consistency of each factor for the each sample in the same measurement series”, which has two functions in photometry: 1) it makes the interference of various influencing factors on each sample in the same determination series tend to be the same, shows the true law of photometry, establishes the “two standard photometry”, establishes the theoretical basis for photometry to determine various contents, and changes the traditional concept; 2) It is the embodiment of normal distribution theory in photometry, which makes the measured values of samples approximate to normal distribution. Therefore, it is easy to find system errors and statistical outliers, to ensure the reliability of the measurement results, and to simplify the evaluation of uncertainty, to meet the requirements of JJF1059.1-2012 “Evaluation and Expression of Uncertainty in Measurement”.
