Formulas and Tables
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Median and range chart formulas
X-bar control limits are based on either range or sigma, depending on which chart it is paired with. When the X-bar chart is paired with a range chart, the most common (and recommended) method of computing control limits based on 3 standard deviations is:
Median chart formulas
Upper control limit
Lower control limit:
Range
k is the number of subgroups
Upper control limit
Lower control limit
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Tabular values for median and range charts
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Subgroup Size
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A2
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D3
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D4
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2
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1.880
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—–
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3.268
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3
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1.023
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—–
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2.574
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4
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0.729
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—–
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2.282
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5
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0.577
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—–
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2.114
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6
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0.483
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—–
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2.004
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7
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0.419
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0.076
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1.924
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8
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0.373
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0.136
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1.864
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9
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0.337
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0.184
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1.816
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10
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0.308
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0.223
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1.777
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11
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0.285
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0.256
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1.276
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12
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0.266
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0.283
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1.717
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13
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0.249
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0.307
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1.693
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14
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0.235
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0.328
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1.672
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15
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0.223
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0.347
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1.653
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16
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0.212
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0.363
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1.637
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17
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0.203
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0.378
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1.622
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18
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0.194
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0.391
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1.608
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19
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0.187
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0.403
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1.597
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20
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0.180
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0.415
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1.585
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21
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0.173
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0.425
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1.276
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22
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0.167
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0.434
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1.566
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|
23
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0.162
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0.443
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1.557
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24
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0.157
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0.451
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1.548
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25
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0.153
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0.459
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1.541
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