Evaluation of Measurement Uncertainty in an Environmental Test Laboratory by Quality Assurance, Control Charting and Robust Statistics
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摘要: 基于实验室长期积累的质控数据评估测量不确定度的方法具有广泛应用前景,但常见的质控图法只能处理单一浓度,而处理多浓度水平的线性校准法建立模型时需要成套、完整的质控数据,不利于基层实验室的应用。稳健统计是指不用识别、剔除离群值,直接应用全部测量数据,将离群值对统计分析结果影响降低到最小的统计分析方法。本文尝试用回收率将不同浓度数据归一化,然后用质控图方法处理。如果存在离群数据时,可用稳健统计法计算期间精密度sR′。利用本实验室积累的5套和其他实验室提供的19套环境检测领域常规项目质控数据验证了新方法的可行性。验证结果表明,对单一浓度数据,不经任何处理,稳健统计-迭代法可得到与质控图法基本相符的结果,sR′(相对值)平均偏差为0.15%。对于多浓度水平数据,经归一化后,质控图法、稳健统计-迭代法与线性校准法的结果平均偏差分别为0.43%和0.20%,质控图法与稳健统计-迭代法的结果平均偏差为0.26%,三种方法计算结果基本相符;稳健统计-迭代法更接近于线性校准法计算结果,且方法原理简单,计算步骤明显简化,适用于线性校准法比例模型数据的处理。Abstract: There are broad application prospects for evaluation of measurement uncertainty in the environmental test laboratory based on quality control data accumulated in long-term routine analysis. The quality control charting method is used only for the same concentration data. Linear calibration using reference materials can be used in different concentration measurement data but the complete quality control data cover different concentrations with the same number of measurements and should be prepared before the mathematical mode is established, which makes its application in most testing laboratories unsuitable. Robust statistics is a type of statistical analysis method where it is unnecessary to identify and delete outliers but it can also reduce the effect of outliers on the final results based on all measurement data. Quality control charting methods and robust statistics (iteration method), when outliers exist, are used to calculate intermediate precision (sR′) after normalizing different concentration data by recovery rate and are described in this paper. Five sets of data collected in our laboratory and 19 sets of data from the other laboratories, which cover routine testing items in environmental protection field, were used to verify the feasibility of the new method. It can be shown that the average difference of relative intermediate precision (ΔsR′-rsd) between robust statistics and quality control charting methods are almost in agreement (i.e. 0.15%) for the single concentration data. For the multi-level concentration data after normalization, the average difference (ΔsR′-rsd) between quality control charting and linear calibration, between robust statistics and linear calibration, are 0.43% and 0.20%, respectively. The average of difference (ΔsR′-rsd) between robust statistics and quality control charting method is 0.26%, which indicates that the results of all three methods are generally in line with each other. The principle of the new methods proposed in this paper is easy to understand and the calculation procedure is significantly simplified, making it suitable for cases of linear calibration using reference materials with direct proportion mode.
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表 1 作者实验室不同检测项目质控样品标称值及测定结果
Table 1. Measured items of standard sample and their analysis results in author′s laboratory
检测项目 质控样品标称值 不同时间测定结果 氨氮(mg/L) 2.55±0.10 2.57,2.54,2.57 0.501±0.027 0.524,0.507,0.508,0.497,0.504 0.491,0.504,0.517,0.490 0.513,0.493,0.503,0.523,0.491 0.778±0.042 0.756,0.776,0.764,0.763,0.772 0.766,0.752,0.774,0.757 0.798,0.780,0.764,0.806,0.775 8.75±0.35 8.61 1.22±0.06 1.22,1.11 0.425±0.025 0.435 总磷(mg/L) 1.46±0.05 1.46,1.46,1.45,1.46,1.48 1.47,1.48,1.44,1.44,1.43 0.539±0.017 0.527,0.537,0.536,0.529 0.531,0.540,0.534,0.542 0.356±0.021 0.352,0.361 COD(mg/L) 148±7 145,146,145,151,150,145,145 112±6 115,114,111,112,111 99.9±5.0 102,104,100 61.0±4.3 61.4,65.0,63.7,61.8 76.1±5.3 80.4,78.3,74.6,76.9 64.3±4.4 66.5,64.6 73.5±4.4 75.4,72.9 土壤铅(mg/kg) 23±3 21.4,22.4,22.3 22.6±1.7 23.9,23.9,22.1,24.0 27±2 25.9,26.0,27.2 30±5 31.5,28.5,28.9,29.9 32.3,32.1,30.3,33.0 58±5 62.6,62.3,60.0 98±6 91.8 314±13 319,324,309 552±29 533 土壤铜(mg/kg) 21±2 20.4 24.3±1.2 23.9,23.6,23.3,22.9 26.3±1.7 24.3,26.6,25.8,26.1,25.0,25.8 22.6±1.3 22.8 32±2 31.3 40±3 39.0,39.7,39.3,38.9 144±6 140,141,139,139,147 注: 5套数据编号分别为12~16。 表 2 其他实验室提供的质控数据
Table 2. Quality control data from other laboratories collected by author
编号 检测项目 数据个数 质控样浓度单位及范围 数据来源 不确定度评估方法 1 COD 30 500 (mg/L) 澳实分析检测(上海)有限公司 质控图法 2 三氯乙烯 25 100 (mg/L) 上海市环境监测中心 质控图法 3 苯并[a]芘 27 5.0 (mg/kg) 澳实分析检测(上海)有限公司 质控图法 4 土壤锌 25 68±8 (mg/kg) 上海市环境监测中心 质控图法 5 挥发酚 45 0.163 (mg/L) 鞍山市环境监测中心站 质控图法 6 氟化物 30 1.20 (mg/L) 光大水务(济南)有限公司 质控图法 7 土壤总氮 50 0.130%±0.010% 中国科学院南京地理与湖泊研究所 质控图法 8 土壤总氮 50 0.072%±0.009% 中国科学院南京地理与湖泊研究所 质控图法 9 亚硝酸盐氮 23 0.0500 (mg/L) 上海市供水调度监测中心水质监测站 质控图法 10 BOD 30 186 (mg/L) 哈尔滨市环境监测中心站 质控图法 11 总油 30 92.0 (mg/L) 北京生态岛科技有限责任公司实验室 质控图法 17 BOD 50 22.8~151 (mg/L) 上海市环境监测中心 线性校准法 18 氯离子 45 40~500 (mg/L) 中石化中原油田环保监测总站 线性校准法 19 COD 34 25~500 (mg/L) 澳实分析检测(上海)有限公司 线性校准法 20 氨氮 34 1~10 (mg/L) 苏州吴中供水有限公司化验中心 线性校准法 21 镉 34 1~10 (μg/L) 苏州市自来水公司水质检测中心 线性校准法 22 TOC 34 12.5~200 (mg/L) 上海海洋大学船舶压载水检测实验室 线性校准法 23 苯并[a]芘 26 96~169 (μg/kg) 上海市环境监测中心 线性校准法 24 总烃 34 42.9~214 (mg/m3) 上海市仪表电子工业环境监测站 线性校准法 表 3 COD的质控图法计算示例
Table 3. Example for evaluation of measurement uncertainty by quality control charting for COD
序号 检测数据 s式计算结果 MR式计算结果 x′i |MR| 升序排列 wi(s) pi 1-pn+1-i Ai wi(MR) pi 1-pn+1-i Ai 1 1.0459 - 0.9666 -1.63 0.0514 0.9858 -7.223 -1.46 0.0720 0.9752 -6.328 2 1.0120 0.0339 0.9717 -1.38 0.0833 0.9820 -19.502 -1.24 0.1077 0.9697 -17.181 3 1.0140 0.0020 0.9730 -1.32 0.0932 0.9567 -27.570 -1.18 0.1183 0.9376 -24.546 4 0.9920 0.0220 0.9780 -1.08 0.1403 0.9356 -32.942 -0.97 0.1670 0.9131 -29.631 5 1.0060 0.0140 0.9800 -0.98 0.1629 0.8885 -36.073 -0.88 0.1894 0.8624 -32.826 6 0.9800 0.0259 0.9800 -0.98 0.1629 0.8716 -42.538 -0.88 0.1894 0.8451 -38.814 7 1.0060 0.0259 0.9807 -0.95 0.1711 0.8672 -49.203 -0.85 0.1975 0.8407 -44.967 8 1.0319 0.0259 0.9820 -0.89 0.1873 0.7431 -45.510 -0.80 0.2132 0.7206 -42.308 9 0.9780 0.0539 0.9820 -0.89 0.1873 0.7111 -49.586 -0.80 0.2132 0.6909 -46.233 10 1.0240 0.0459 0.9840 -0.79 0.2142 0.6472 -49.066 -0.71 0.2391 0.6324 -46.201 11 0.9840 0.0399 0.9840 -0.79 0.2147 0.6396 -53.741 -0.71 0.2396 0.6256 -50.637 12 1.0040 0.0200 0.9846 -0.76 0.2224 0.6396 -58.046 -0.68 0.2469 0.6256 -54.765 13 1.0439 0.0399 0.9920 -0.41 0.3426 0.6057 -50.044 -0.36 0.3583 0.5948 -48.249 14 0.9800 0.0639 0.9923 -0.39 0.3474 0.6057 -53.670 -0.35 0.3627 0.5948 -51.780 15 1.0078 0.0278 0.9949 -0.27 0.3942 0.5682 -51.351 -0.24 0.4050 0.5611 -50.095 16 0.9961 0.0118 0.9961 -0.21 0.4170 0.5411 -51.265 -0.19 0.4255 0.5369 -50.347 17 1.0078 0.0118 0.9961 -0.21 0.4182 0.4918 -51.104 -0.18 0.4267 0.4926 -50.501 18 1.0235 0.0157 0.9974 -0.14 0.4426 0.4426 -48.985 -0.13 0.4485 0.4485 -48.893 19 0.9717 0.0518 1.0000 -0.02 0.4918 0.4182 -46.301 -0.02 0.4926 0.4267 -46.777 20 0.9974 0.0257 1.0026 0.10 0.5411 0.4170 -44.991 0.09 0.5369 0.4255 -45.878 21 0.9820 0.0154 1.0040 0.17 0.5682 0.3942 -43.725 0.15 0.5611 0.4050 -44.977 22 0.9807 0.0013 1.0060 0.27 0.6057 0.3474 -39.915 0.24 0.5948 0.3627 -41.706 23 0.9923 0.0116 1.0060 0.27 0.6057 0.3426 -41.440 0.24 0.5948 0.3583 -43.336 24 0.9846 0.0077 1.0078 0.36 0.6396 0.2224 -32.828 0.32 0.6256 0.2469 -35.376 25 0.9666 0.0180 1.0078 0.36 0.6396 0.2147 -33.740 0.32 0.6256 0.2396 -36.405 26 0.9949 0.0283 1.0083 0.38 0.6472 0.2142 -34.488 0.34 0.6324 0.2391 -37.305 27 0.9730 0.0219 1.0120 0.56 0.7111 0.1873 -29.061 0.50 0.6909 0.2132 -32.305 28 1.0257 0.0527 1.0140 0.65 0.7431 0.1873 -27.739 0.58 0.7206 0.2132 -31.211 29 1.0026 0.0231 1.0235 1.11 0.8672 0.1711 -18.814 1.00 0.8407 0.1975 -22.431 30 0.9820 0.0206 1.0240 1.13 0.8716 0.1629 -18.600 1.02 0.8451 0.1894 -22.322 31 1.0360 0.0540 1.0257 1.22 0.8885 0.1629 -18.059 1.09 0.8624 0.1894 -21.839 32 0.9961 0.0398 1.0319 1.52 0.9356 0.1403 -13.721 1.36 0.9131 0.1670 -17.236 33 0.9840 0.0121 1.0360 1.71 0.9567 0.0932 -9.230 1.54 0.9376 0.1183 -12.369 34 1.0000 0.0160 1.0439 2.10 0.9820 0.0833 -7.044 1.88 0.9697 0.1077 -9.690 35 1.0083 0.0083 1.0459 2.19 0.9858 0.0514 -4.627 -1.46 0.0720 0.0720 -6.889 平均值 1.000 0.0261 - - ∑A(i)=-1241.7 - ∑A(i)=-1236.0 标准偏差 0.0207 0.0232 =sR′ - A(s) =0.478 - A(MR)=0.315 数据量(n ) 35 - - - A*(s) =0.490 - A*(MR) =0.322 表 4 稳健统计-迭代法(方法1)计算示例
Table 4. Example for robust analysis-algorithm A (method 1)
序号 第1轮 第2轮 第3轮 第4轮 第5轮 第6轮 1 0.9666 0.9666 0.9666 0.9669 0.9671 0.9671 2 0.9717 0.9717 0.9717 0.9717 0.9717 0.9717 3 0.9730 0.9730 0.9730 0.9730 0.9730 0.9730 4 0.9780 0.9780 0.9780 0.9780 0.9780 0.9780 5 0.9800 0.9800 0.9800 0.9800 0.9800 0.9800 6 0.9800 0.9800 0.9800 0.9800 0.9800 0.9800 7 0.9807 0.9807 0.9807 0.9807 0.9807 0.9807 8 0.9820 0.9820 0.9820 0.9820 0.9820 0.9820 9 0.9820 0.9820 0.9820 0.9820 0.9820 0.9820 10 0.9840 0.9840 0.9840 0.9840 0.9840 0.9840 11 0.9840 0.9840 0.9840 0.9840 0.9840 0.9840 12 0.9846 0.9846 0.9846 0.9846 0.9846 0.9846 13 0.9920 0.9920 0.9920 0.9920 0.9920 0.9920 14 0.9923 0.9923 0.9923 0.9923 0.9923 0.9923 15 0.9949 0.9949 0.9949 0.9949 0.9949 0.9949 16 0.9961 0.9961 0.9961 0.9961 0.9961 0.9961 17 0.9961 0.9961 0.9961 0.9961 0.9961 0.9961 18 0.9974 0.9974 0.9974 0.9974 0.9974 0.9974 19 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 20 1.0026 1.0026 1.0026 1.0026 1.0026 1.0026 21 1.0040 1.0040 1.0040 1.0040 1.0040 1.0040 22 1.0060 1.0060 1.0060 1.0060 1.0060 1.0060 23 1.0060 1.0060 1.0060 1.0060 1.0060 1.0060 24 1.0078 1.0078 1.0078 1.0078 1.0078 1.0078 25 1.0078 1.0078 1.0078 1.0078 1.0078 1.0078 26 1.0083 1.0083 1.0083 1.0083 1.0083 1.0083 27 1.0120 1.0120 1.0120 1.0120 1.0120 1.0120 28 1.0140 1.0140 1.0140 1.0140 1.0140 1.0140 29 1.0235 1.0235 1.0235 1.0235 1.0235 1.0235 30 1.0240 1.0240 1.0240 1.0240 1.0240 1.0240 31 1.0257 1.0257 1.0257 1.0257 1.0257 1.0257 32 1.0319 1.0319 1.0319 1.0319 1.0319 1.0319 33 1.0360 1.0357 1.0333 1.0325 1.0322 1.0321 34 1.0439 1.0357 1.0333 1.0325 1.0322 1.0321 35 1.0459 1.0357 1.0333 1.0325 1.0322 1.0321 平均值 1.0004 0.9999 0.9997 0.9996 0.9996 0.9996 标准偏差s 0.0207 0.0197 0.0193 0.0191 0.0191 0.0191 sR′=1.134×s 0.0235 0.0223 0.0219 0.0217 0.0216 0.0216 1.5×sR′ 0.0353 0.0334 0.0328 0.0326 0.0325 0.0324 -1.5×sR′ 0.9651 0.9665 0.9669 0.9671 0.9671 0.9672 +1.5×sR′ 1.0357 1.0333 1.0325 1.0322 1.0321 1.0320 表 5 稳健统计-迭代法(方法2)计算示例
Table 5. Example for robust analysis-algorithm A (method 2)
编号 x′i-中位值 第1轮 第2轮 第3轮 第4轮 第5轮 1 0.03082 0.9666 0.9676 0.9674 0.9673 0.9672 2 0.02568 0.9717 0.9717 0.9717 0.9717 0.9717 3 0.02439 0.9730 0.9730 0.9730 0.9730 0.9730 4 0.01936 0.9780 0.9780 0.9780 0.9780 0.9780 5 0.01736 0.9800 0.9800 0.9800 0.9800 0.9800 6 0.01736 0.9800 0.9800 0.9800 0.9800 0.9800 7 0.01668 0.9807 0.9807 0.9807 0.9807 0.9807 8 0.01539 0.9820 0.9820 0.9820 0.9820 0.9820 9 0.01539 0.9820 0.9820 0.9820 0.9820 0.9820 10 0.01340 0.9840 0.9840 0.9840 0.9840 0.9840 11 0.01337 0.9840 0.9840 0.9840 0.9840 0.9840 12 0.01282 0.9846 0.9846 0.9846 0.9846 0.9846 13 0.00538 0.9920 0.9920 0.9920 0.9920 0.9920 14 0.00511 0.9923 0.9923 0.9923 0.9923 0.9923 15 0.00254 0.9949 0.9949 0.9949 0.9949 0.9949 16 0.00132 0.9961 0.9961 0.9961 0.9961 0.9961 17 0.00126 0.9961 0.9961 0.9961 0.9961 0.9961 18 0.00003 0.9974 0.9974 0.9974 0.9974 0.9974 19 0.00260 1.0000 1.0000 1.0000 1.0000 1.0000 20 0.00517 1.0026 1.0026 1.0026 1.0026 1.0026 21 0.00659 1.0040 1.0040 1.0040 1.0040 1.0040 22 0.00859 1.0060 1.0060 1.0060 1.0060 1.0060 23 0.00859 1.0060 1.0060 1.0060 1.0060 1.0060 24 0.01044 1.0078 1.0078 1.0078 1.0078 1.0078 25 0.01044 1.0078 1.0078 1.0078 1.0078 1.0078 26 0.01086 1.0083 1.0083 1.0083 1.0083 1.0083 27 0.01458 1.0120 1.0120 1.0120 1.0120 1.0120 28 0.01657 1.0140 1.0140 1.0140 1.0140 1.0140 29 0.02613 1.0235 1.0235 1.0235 1.0235 1.0235 30 0.02655 1.0240 1.0240 1.0240 1.0240 1.0240 31 0.02831 1.0257 1.0257 1.0257 1.0257 1.0257 32 0.03454 1.0298 1.0311 1.0316 1.0318 1.0319 33 0.03859 1.0298 1.0311 1.0316 1.0318 1.0319 34 0.04651 1.0298 1.0311 1.0316 1.0318 1.0319 35 0.04851 1.0298 1.0311 1.0316 1.0318 1.0319 平均值 - 0.9993 0.9995 0.9996 0.9996 0.9996 标准偏差s - 0.0187 0.0189 0.0190 0.0190 0.0190 sR′=1.134×s - 0.0212 0.0214 0.0215 0.0216 0.0216 1.5×sR′ 0.03242 0.0317 0.0321 0.0323 0.0323 0.0324 -1.5×sR′ 0.96500 0.9676 0.9674 0.9673 0.9672 0.9672 +1.5×sR′ 1.02985 1.0311 1.0316 1.0318 1.0319 1.0320 注: 测量数据中位值为0.9974,标准偏差估计值s0=0.01458,s0×1.483=0.0216。 表 6 单一浓度数据归一化前后质控图法及稳健统计-迭代法结果比较
Table 6. Comparison of results from quality control charting and robust analysis (algorithm A) based on the same concentration data before and after normalization processing
数据编号 检测项目 数据个数 质控图法 稳健统计-迭代法 可疑数据 备注 或 s sR′ A*(s) A*(MR) 或 sR′ 1 COD (mg/L) 30 498.9 5.63 6.64 0.27 0.68 498.9 6.39 0 归一化前结果 30 0.9977 0.0113 0.0133 0.27 0.68 0.9977 0.0128 0 归一化后结果 30 498.9 5.65 6.65 0.27 0.68 498.9 6.40 0 归一化后结果 2 三氯乙烯(mg/L) 25 104 12 13 0.31 0.36 105 12 1 归一化前结果 25 1.04 0.12 0.13 0.31 0.36 1.05 0.12 1 归一化后结果 3 苯并[a]芘(mg/L) 12 4.99 0.141 0.145 0.46 0.43 - - 0 sR′合并前结果 15 4.98 0.125 0.149 0.41 0.40 - - 0 sR′合并前结果 27 4.99 - 0.145 - - - - 0 sR′合并后结果 27 0.998 0.026 0.029 0.72 0.61 0.998 0.029 0 归一化后结果 27 4.99 0.130 0.144 0.72 0.61 4.99 0.146 0 归一化后结果 4 土壤Zn(mg/kg) 25 66.9 4.0 4.0 0.58 0.59 66.9 4.5 0 归一化前结果 25 0.984 0.059 0.058 0.58 0.59 0.983 0.066 0 归一化后结果 25 66.9 4.0 3.9 0.58 0.59 66.8 4.5 0 归一化后结果 注: 表中测量数据取自CNAS组织的用 “top-down” 技术评估不确定度培训班教材和学员提交的报告。第3套数据前3行为数据归一化前结果。 表 7 单一浓度数据质控图法及稳健统计-迭代法结果比较
Table 7. Comparison of results from quality control charting and robust analysis (algorithm A) based on the same concentration data
数据编号 检测项目 数据个数 质控图法 稳健统计-迭代法 可疑数据 s sR′ A*(s) A*(MR) sR′ 5 挥发酚(mg/L) 45 0.163 0.0027 0.0029 0.64 0.58 0.163 0.0031 1 6 氟化物(mg/L) 30 1.20 0.020 0.024 0.44 0.49 1.20 0.022 0 7 土壤总氮(%) 50 0.129 0.0037 0.0034 0.40 0.64 0.129 0.0036 3 8 土壤总氮(%) 50 0.070 0.0023 0.0022 0.86 0.85 0.070 0.0022 3 9 亚硝酸盐氮(mg/L) 23 0.0502 0.0011 0.0011 0.55 0.58 0.0502 0.0012 2 10 BOD(mg/L) 30 188 5.6 6.6 0.46 0.48 188 6.2 0 11 总油(mg/L) 30 92.0 1.2 1.2 0.48 0.58 92.0 1.4 0 注: 表中测量数据取自CNAS组织的用“top-down”技术评估不确定度培训班教材和学员提交的报告。 表 8 不同浓度水平质控数据归一化后质控图与稳健统计-迭代法结果比较
Table 8. Comparison of results from quality control charting and robust analysis (algorithm A) based on the quality control samples analysis data of different concentration after normalization processing
数据编号 检测项目 数据个数 质控图法 稳健统计-迭代法 RMS 可疑数据 ′ s sR′ A*(s) A*(MR) ′ sR′ 12 氨氮(mg/L) 35 1.000 0.021 0.023 0.49 0.32 1.000 0.022 0.023 2 13 总磷(mg/L) 28 1.001 0.015 0.010 0.30 2.32 1.001 0.016 0.015 0 14 COD(mg/L) 27 1.011 0.024 0.023 0.37 0.51 1.011 0.026 0.026 0 15 土壤Pb(mg/kg) 26 1.012 0.051 0.049 0.60 0.71 1.011 0.057 0.051 0 16 土壤Cu(mg/kg) 22 0.977 0.022 0.022 0.44 0.43 0.977 0.022 0.032 1 表 9 不同浓度水平质控数据归一化后三种计算方法结果比较
Table 9. Comparison of results from three calculation methods based on the analysis data of different concentration quality control samples after normalization processing
数据编号 检测项目 数据个数 质控图法 稳健统计-迭代法 线性校准法 可疑数据 ′ s sR′ A*(s) A*(MR) ′ sR′ 17 BOD(mg/L) 50 1.020 0.036 0.038 0.38 0.42 1.019 0.035 0.030 2 18 氯离子(mg/L) 45 1.004 0.012 0.010 0.69 0.81 1.004 0.012 0.012 0 19 COD(mg/L) 34 1.009 0.021 0.027 0.32 0.65 1.008 0.022 0.020 1 20 氨氮(mg/L) 34~36 1.001 0.011 0.010 1.04 1.31 1.001 0.013 0.014 0 21 Cd(mg/L) 34 1.000 0.013 0.013 0.46 0.47 1.000 0.014 0.013 0 22 TOC(mg/L) 34 0.996 0.011 0.0075 1.60 1.96 0.998 0.0080 0.010 2 23 苯并[a]芘(mg/kg) 26 0.855 0.039 0.043 0.20 0.27 0.856 0.040 0.037 1 24 总烃(mg/L) 34 0.997 0.037 0.043 0.41 0.47 0.999 0.040 0.038 1 注: 表中测量数据取自CNAS组织的不确定度培训班学员实习报告。 -
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