Tham khảo tài liệu 'statistical process control 5 episode 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Upper specification limit UAL UWL LWL LAL x fl Lower specification limit Figure Median chart for herbicide batch moisture content Other types of control charts for variables 163 The advantage of using sample medians over sample means is that the former are very easy to find particularly for odd sample sizes where the method of circling the individual item values on a chart is used. No arithmetic is involved. The main disadvantage however is that the median does not take account of the extent of the extreme values - the highest and lowest. Thus the medians of the two samples below are identical even though the spread of results is obviously different. The sample means take account of this difference and provide a better measure of the central tendency. Sample No. Item values Median Mean 1 134 134 135 139 143 135 137 2 120 123 135 136 136 135 130 This failure of the median to give weight to the extreme values can be an advantage in situations where outliers - item measurements with unusually high or low values - are to be treated with suspicion. A technique similar to the median chart is the chart for mid-range. The middle of the range of a sample may be determined by calculating the average of the highest and lowest values. The mid-range M of the sample of five 553 555 561 554 551 is Highest Lowest 561 551 ---------- 556 2 The central-line on the mid-range control chart is the median of the sample mid-ranges MR . The estimate of process spread is again given by the median of sample ranges and the control chart limits are calculated in a similar fashion to those for the median chart. Hence Action Lines at MR AR Warning Lines at MR 2 3 A4R. Certain quality characteristics exhibit variation which derives from more than one source. For example if cylindrical rods are being formed their diameters may vary from piece to piece and along the length of each
