# Variables Control Charts

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1 MINITAB ASSISTANT WHITE PAPER This paper explains the research conducted by Minitab statisticians to develop the methods and data checks used in the Assistant in Minitab 17 Statistical Software. Variables Control Charts Overview Control charts are used to regularly monitor a process to determine whether it is in control. The Minitab Assistant includes two of the most widely used control charts for continuous data: Xbar-R or Xbar-S charts. These charts are used when data are collected in subgroups. Minitab uses the pooled standard deviation to estimate within-subgroup standard deviation. The R chart provides an effective estimate of variation for subgroups of size up to approximately 10 (AIAG, 1995; Montgomery, 2001). For larger subgroup sizes, an S chart provides a better estimate of individual within-subgroup standard deviation. To follow Minitab past conventions and to be conservative, we recommend an S chart when the subgroup size is greater than 8. For subgroup sizes less than or equal to 8, the R and S chart provide similar results. Individuals and Moving Range (I-MR) chart. This chart is used when there are no subgroups. Minitab uses an average moving range method of length 2 to estimate the standard deviation. The control limits for a control chart are typically established in the control phase of a Six Sigma project. A good control chart should be sensitive enough to quickly signal when a special cause exists. This sensitivity can be assessed by calculating the average number of subgroups needed to signal a special cause. Also, a good control chart rarely signals a false alarm when the process is in control. The false alarm rate can be assessed by calculating the percentage of subgroups that are deemed out-of-control when the process is in control.

2 In general, control charts are optimized when each observation comes from a normal distribution, each observation is independent, and only common cause variability exists within subgroups. Therefore, the Assistant Report Card automatically performs the following data checks to evaluate these conditions: Normality Stability Amount of data Correlated data In this paper, we investigate how a variables control chart behaves when these conditions vary, and we describe how we established a set of guidelines to evaluate requirements for these conditions. VARIABLES CONTROL CHARTS 2

3 Data checks Normality Control charts are not based on the assumption that the process data are normally distributed, but the criteria used in the tests for special causes are based on this assumption. If the data are severely skewed, or if the data fall too heavily at the ends of the distribution ( heavy-tailed ), the test results may not be accurate. For example, the chart may signal false alarms at a higher rate than expected. Objective We investigated the effect of nonnormal data on the Xbar chart and I chart. We wanted to determine how nonnormality affects the false alarm rate. Specifically, we wanted to determine whether nonnormal data significantly increases the rate that a chart indicates that points are out of control when the process is actually in control (false alarms). Method We performed simulations with 10,000 subgroups and different levels of nonnormality and recorded the percentage of false alarms. Simulations allow us to test various conditions to determine the effects of nonnormality. We chose the right-skewed distribution and symmetric distributions with heavy tails because these are common cases of nonnormal distributions in practice. See Appendix A for more details. Results XBAR CHART (SUBGROUP SIZE 2) Our simulation showed that the false alarm rate does not increase significantly when the data are nonnormal if the subgroup size is 2 or more. Based on this result, we do not check normality for the Xbar-R or Xbar-S charts. Even when the data are highly skewed or extremely heavy tailed, the false alarm rate for test 1 and test 2 is less than 2%, which is not markedly higher than false alarm rate of 0.7% for the normal distribution. I CHART (SUBGROUP SIZE = 1) Our simulation showed that the I chart is sensitive to nonnormal data. When the data are nonnormal, the I chart produces a false alarm rate that is 4 to 5 times higher than when the data are normal. To address the sensitivity of the I chart to nonnormal data, the Assistant does the following: Performs an Anderson-Darling test if the data could be highly nonnormal, as indicated by a greater than expected number of points outside the control limits (that is, 2 or more points and 2% or more of the points are outside of the control limits). VARIABLES CONTROL CHARTS 3

4 If the Anderson-Darling test suggests that the data are nonnormal, the Assistant transforms the data using the optimal Box-Cox lambda. An Anderson-Darling test is performed on the transformed data. If the test fails to reject the null (that the data are normal), the Assistant suggests using the transformed data if the process naturally produces nonnormal data. The Box-Cox transformation is effective only for nonnormal data that are right-skewed. If the transformation is not effective for your nonnormal data, you may need to consider other options. In addition, because the Anderson-Darling test and Box-Cox transformation are affected by extreme observations, you should omit points with known special causes before you transform your data. When checking for normality, the Assistant Report Card for the I chart displays the following status indicators: Status Condition If < 2 points or < 2% of the points are outside the control limits on the I chart or The Anderson-Darling normality test for either the data or the Box-Cox transformed data fails to reject the null hypothesis (that the data are normal) at alpha = If the above conditions do not hold. Stability For variables control charts, eight tests can be performed to evaluate the stability of the process. Using these tests simultaneously increases the sensitivity of the control chart. However, it is important to determine the purpose and added value of each test because the false alarm rate increases as more tests are added to the control chart. Objective We wanted to determine which of the eight tests for stability to include with the variables control charts in the Assistant. Our first goal was to identify the tests that significantly increase sensitivity to out-of-control conditions without significantly increasing the false alarm rate. Our second goal was to ensure the simplicity and practicality of the chart. Our research focused on the tests for the Xbar chart and the I chart. For the R, S, and MR charts, we use only Test 1, which signals when a point falls outside of the control limits. Method We performed simulations and reviewed the literature to evaluate how using a combination of tests for stability affects the sensitivity and the false alarm rate of the control charts. In addition, VARIABLES CONTROL CHARTS 4

5 we evaluated the prevalence of special causes associated with the test. For details on the methods used for each test, see the Results section below and Appendix B. Results We found that Tests 1, 2, and 7 were the most useful for evaluating the stability of the Xbar chart and the I chart: TEST 1: IDENTIFIES POINTS OUTSIDE OF THE CONTROL LIMITS Test 1 identifies points > 3 standard deviations from the center line. Test 1 is universally recognized as necessary for detecting out-of-control situations. It has a false alarm rate of only 0.27%. TEST 2: IDENTIFIES SHIFTS IN THE MEANS Test 2 signals when 9 points in a row fall on the same side of the center line. We performed a simulation using 4 different means, set to multiples of the standard deviation, and determined the number of subgroups needed to detect a signal. We set the control limits based on the normal distribution. We found that adding Test 2 significantly increases the sensitivity of the chart to detect small shifts in the mean. When test 1 and test 2 are used together, significantly fewer subgroups are needed to detect a small shift in the mean than are needed when test 1 is used alone. Therefore, adding test 2 helps to detect common out-of-control situations and increases sensitivity enough to warrant a slight increase in the false alarm rate. TEST 7: IDENTIFIES CONTROL LIMITS THAT ARE TOO WIDE Test 7 signals when points in a row fall within 1 standard deviation of the center line. Test 7 is used only for the Xbar chart when the control limits are estimated from the data. When this test fails, the cause is usually a systemic source of variation (stratification) within a subgroup, which is often the result of not forming rational subgroups. Because forming rational subgroups is critical for ensuring that the control chart can accurately detect out-of-control situations, Minitab uses a modified test 7 when estimating control limits from the data. Test 7 signals a failure when the number of points in a row is between 12 and 15, depending on the number of subgroups: k = (Number of Subgroups) x 0.33 Points required k < k 12 and k 15 Integer k k > VARIABLES CONTROL CHARTS 5

6 Tests not included in the Assistant TEST 3: K POINTS IN A ROW, ALL INCREASING OR ALL DECREASING Test 3 is designed to detect drifts in the process mean (Davis and Woodall, 1988). However, when test 3 is used in addition to test 1 and test 2, it does not significantly increase the sensitivity of the chart to detect drifts in the process mean. Because we already decided to use tests 1 and 2 based on our simulation results, including test 3 would not add any significant value to the chart. TEST 4: K POINTS IN A ROW, ALTERNATING UP AND DOWN Although this pattern can occur in practice, we recommend that you look for any unusual trends or patterns rather than test for one specific pattern. TEST 5: K OUT OF K=1 POINTS > 2 STANDARD DEVIATIONS FROM CENTER LINE To ensure the simplicity of the chart, we excluded this test because it did not uniquely identify special cause situations that are common in practice. TEST 6: K OUT OF K+1 POINTS > 1 STANDARD DEVIATION FROM THE CENTER LINE To ensure the simplicity of the chart, we excluded this test because it did not uniquely identify special cause situations that are common in practice. TEST 8: K POINTS IN A ROW > 1 STANDARD DEVIATION FROM CENTER LINE (EITHER SIDE) To ensure the simplicity of the chart, we excluded this test because it did not uniquely identify special cause situations that are common in practice. When checking stability, the Assistant Report Card displays the following status indicators: Status Condition No test failures on the chart for the mean (I chart or Xbar chart) and the chart for variation (MR, R, or S chart). The tests used for each chart are: I chart: Test 1 and Test 2. Xbar chart: Test 1, Test 2 and Test 7. Test 7 is performed only when the control limits are estimated from the data. MR, R and S charts: Test 1. If above condition does not hold. VARIABLES CONTROL CHARTS 6

7 Amount of data If you do not have known values for the control limits, they must be estimated from the data. To obtain precise estimates of the limits, you must have enough data. If the amount of data is insufficient, the control limits may be far from the true limits due to sampling variability. To improve precision of the limits, you can increase the number of observations. Objective We investigated the number of observations that are needed to obtain precise control limits. Our objective was to determine the amount of data required to ensure that false alarms due to test 1 are no more than 1%, with 95% confidence. Method When the data are normally distributed and there is no error due to sampling variability, the percent of points above the upper control limit is 0.135%. To determine whether the number of observations is adequate, we followed the method outlined by Bischak (2007) to ensure that the false alarm rate due to points above the upper control limit is no more than 0.5% with 95% confidence. Due to the symmetry of the control limits, this method results in a total false alarm rate of 1% due to test 1. See Appendix C for more details. Results We determined that, for nearly all subgroup sizes, 100 total observations are adequate to obtain precise control limits. Although subgroup sizes of 1 and 2 required slightly more observations, the false alarm rate was still reasonably low (1.1%) with 100 observations. Therefore, for simplicity, we use the cutoff of 100 total observations for all subgroup sizes. Based on these results, the Assistant Report Card displays the following status indicators when checking the amount of data: Status Condition Number of observations 100. Number of observations < 100. VARIABLES CONTROL CHARTS 7

8 Correlated data Autocorrelation is a measure of the dependence between data points that are collected over time. Most process data exhibit at least a small degree of autocorrelation. If the autocorrelation is moderate or high, it can lead to incorrect test results. Typically, autocorrelated data exhibit positive autocorrelation, which can reduce the within-subgroup variation and lead to a higher false alarm rate. Objective We investigated the relationship between autocorrelation and the false alarm rate. Our objective was to determine the level of autocorrelation that generates an unacceptable false alarm rate. For simplicity, we considered autocorrelation for lag 1 because the autocorrelation for lag 1 is likely to be greater than autocorrelation for lags 2. Method Using a standard model for an autocorrelated process, we performed simulations with = 0.2, 0.4, 0.5, 0.6, and 0.8 ( is the lag 1 autocorrelation) for three subgroup sizes (n = 1, 3, and 5). We used an initial set of 10,000 subgroups to establish the control limits. Then, we recorded the percentage of false alarms for an additional 2,500 subgroups. We performed 10,000 iterations and recorded the average percentage of false alarms. See Appendix D for more details. Results Our simulations showed that even moderate levels of autocorrelation significantly increase the false alarm rate. When the autocorrelation 0.4, the false alarm rate is very high and the control chart becomes meaningless. To address this, the Assistant performs an autocorrelation test if the data could be autocorrelated, as indicated by a greater than expected number of points outside the control limits (when 2 or more points and 2% or more of the points are outside of the control limits). In that case, the Assistant first tests whether the autocorrelation between successive data points (lag = 1) is significantly greater than 0.2. If the autocorrelation is significantly greater than 0.2, the Assistant then tests whether the autocorrelation between successive data points (lag = 1) is significantly greater than 0.4. VARIABLES CONTROL CHARTS 8

9 When checking for correlated data, the Assistant Report Card displays the following status indicators: Status Condition The number of points outside the control limits is not higher than expected; that is, < 2 points or < 2% of the points are outside the control limits. The number of points outside the control limits is higher than expected, but a test of autocorrelation = 0.2 versus autocorrelation > 0.2 fails to reject the null hypothesis at alpha = Therefore, there is not enough evidence to conclude that at least a moderate level of autocorrelation exists. If the above conditions do not hold. Note: If the null hypothesis that autocorrelation = 0.2 is rejected, we perform a follow-up test of autocorrelation = 0.4 versus autocorrelation > 0.4. If the autocorrelation = 0.4 test is rejected, we increase the severity of the caution message. For more details on the hypothesis test for autocorrelation, see Appendix D. VARIABLES CONTROL CHARTS 9

10 References AIAG (1995). Statistical process control (SPC) reference manual. Automotive Industry Action Group. Bischak, D.P., & Trietsch, D. (2007). The rate of false signals in X control charts with estimated limits. Journal of Quality Technology, 39, Bowerman, B.L., & O' Connell, R.T. (1979). Forecasting and time series: An applied approach. Belmont, CA: Duxbury Press. Chan, L. K., Hapuarachchi K. P., & Macpherson, B.D. (1988). Robustness of and R charts. IEEE Transactions on Reliability, 37, Davis, R.B., & Woodall, W.H. (1988). Performance of the control chart trend rule under linear shift. Journal of Quality Technology, 20, Montgomery, D. (2001). Introduction to statistical quality control, 4 th edition. John Wiley & Sons. Schilling, E.G., & Nelson, P.R. (1976). The effect of non-normality on the control limits of charts. Journal of Quality Technology, 8, Trietsch, D. (1999). Statistical quality control: A loss minimization approach. Singapore: World Scientific Publishing Company. Wheeler, D.J. (2004). Advanced topics in statistical process control. The power of Shewhart s charts, 2 nd edition. Knoxville, TN: SPC Press. Yourstone, S.A., & Zimmer, W.J. (1992). Non-normality and the design of control charts for averages. Decision Sciences, 23, VARIABLES CONTROL CHARTS 10

11 Appendix A: Normality Simulation A1: How nonnormality affects the false alarm rate To investigate how nonnormal data affect the performance of the I chart and Xbar chart, we performed a simulation to evaluate the false alarm rate associated with nonnormal data distributions. We focused our attention on right-skewed distributions and symmetric distributions with heavy tails because these are common nonnormal distributions in practice. In particular, we examined 3 skewed distributions (chi-square with df=3, 5 and 10) and 2 heavytailed distributions (t with df=3 and 5). We established the control limits using an initial set of 10,000 subgroups. We recorded the percentage of false alarms for an additional 2,500 subgroups. Then we performed 10,000 iterations and calculated the average percentage of false alarms using test 1 and test 2 for special causes. The results are shown in Table 1. Table 1 % of false alarms for test 1, test 2 for the I chart (n=1), and Xbar chart (n=2, 3, 4 or 5) Subgroup size Distribution n = 1 n = 2 n = 3 n = 4 n = 5 Normal (0,1) 0.27, , , , , 0.39 Chi (3) 2.06, , , , , 0.53 Chi (5) 1.54, , , , , 0.47 Chi (10) 1.05, , , , , 0.43 t (3) 2.18, , , , , 0.39 t (5) 1.43, , , , , 0.39 Each cell in Table 1 indicates the calculated false alarm rate for test 1 and test 2, respectively. For example, the cell associated with the standard normal distribution (Normal (0, 1)) and the I chart (n=1) indicates that the I chart has a false alarm rate 0.27% for test 1 and 0.39% for test 2 when the data are normally distributed. By comparing the false alarm rates for the Xbar chart (n=2, 3, 4, 5) between the normal and nonnormal distributions, you can see that the false alarm rate does not increase significantly when the normality assumption is violated. Even with highly skewed distributions (chi-square, 3) and distributions with heavy tails (t,3), the false alarm rate for test 1 and test 2 combined is less than 2.0% (compared to 0.7% for a normal distribution) for subgroup sizes as small as 2. Thus, VARIABLES CONTROL CHARTS 11

12 we conclude for practical purposes that the Xbar chart is robust to violations of the normality assumption. For the I chart, Table 1 shows a false alarm rate of approximately 3.2% for test 1 and test 2 combined when the distribution is heavily skewed (chi-square, 3); this false alarm rate is nearly 5 times higher than the expected false alarm rate when the data are normally distributed. The false alarm rate for test 1 and test 2 combined is 2.6% for distributions with heavy tails (t, 3), which is almost 4 times the expected rate when the data are normally distributed. Therefore, the data for the I-MR chart should first be tested for normality. If the null hypothesis of the Anderson-Darling goodness-of-fit test for normality is rejected (there is statistically significant evidence that the data do not follow a normal distribution), a Box-Cox transformation can be used to normalize the data. However, the Box-Cox transformation is effective only when the data are skewed to the right. For other cases of nonnormal data, further investigation is needed to determine the best course of action. Also, because the Anderson-Darling test and Box-Cox transformation are affected by extreme observations, points with known special causes should be omitted from the calculations before performing the analysis. VARIABLES CONTROL CHARTS 12

13 Appendix B: Stability Simulation B1: How adding test 2 to test 1 affects sensitivity Test 1 detects out-of-control points by signaling when a point is greater than 3 standard deviations from the center line. Test 2 detects shifts in the mean by signaling when 9 points in a row fall on the same side of the center line. To evaluate whether using test 2 with test 1 improves the sensitivity of the means charts (I chart and Xbar chart), we established control limits for a normal (0, SD) distribution. We shifted the mean of the distribution by a multiple of the standard deviation and then recorded the number of subgroups needed to detect a signal for each of 10,000 iterations. The results are shown in Table 2. Table 2 Average number of subgroups until a test 1 failure (Test 1), test 2 failure (Test 2), or test 1 or test 2 failure (Test 1 or 2). The shift in mean equals a multiple of the standard deviation (SD) and the simulation was performed for subgroup sizes (n) = 1, 3 and 5. n=1 n=3 n=5 Shift Test 1 Test 2 Test 1 or 2 Test 1 Test 2 Test 1 or 2 Test 1 Test 2 Test 1 or SD SD SD SD As seen in the results for the I chart (n=1), when both tests are used (Test 1 or 2 column) an average of 57 subgroups are needed to detect a 0.5 standard deviation shift in the mean, compared to an average of 154 subgroups needed to detect a 0.5 standard deviation shift when test 1 is used alone. Similarly, using both tests increases the sensitivity for the Xbar chart (n=3, n=5). For example, for a subgroup size of 3, an average of 22 subgroups are needed to detect a 0.5 standard deviation shift when both test 1 and test 2 are used, whereas 60 subgroups are needed to detect a 0.5 standard deviation shift when test 1 is used alone. Therefore, using both tests significantly increases sensitivity to detect small shifts in the mean. As the size of the shift increases, adding test 2 does not significantly increase the sensitivity. VARIABLES CONTROL CHARTS 13

14 Simulation B2: How effectively does test 7 detect stratification (multiple sources of variability in subgroups)? Test 7 typically signals a failure when between 12 and 15 points in a row fall within 1 standard deviation of the center line. The Assistant uses a modified rule that adjusts the number of points required based on the number of subgroups in the data. We set k = (number of subgroups * 0.33) and define the points in a row required for a test 7 failure as shown in Table 3. Table 3 Points in a row required for a failure on test 7 k = (Number of Subgroups) x 0.33 Points required k < k 12 and k 15 Integer k k > Using common scenarios for setting control limits, we performed a simulation to determine the likelihood that test 7 will signal a failure using the above criteria. Specifically, we wanted to evaluate the rule for detecting stratification during the phase when control limits are estimated from the data. We randomly chose m subgroups of size n from a normal distribution with a standard deviation (SD). Half of the points in each subgroup had a mean equal to 0 and the other half had a mean equal to the SD shift (0 SD, 1 SD, or 2 SD). We performed 10,000 iterations and recorded the percentage of charts that showed at least one test 7 signal, as shown in Table 4. Table 4 Percentage of charts that have at least one signal from Test 7 Number of subgroups m = 50 m = 75 m = 25 m = 38 m = 25 Subgroup size n = 2 n = 2 n = 4 n = 4 n = 6 Test 15 in a row 15 in a row 12 in a row 13 in a row 12 in a row 0 SD 5% 8% 7% 8% 7% Shift 1 SD 23% 33% 17% 20% 15% 2 SD 83% 94% 56% 66% 50% As seen in the first Shift row of the table (shift = 0 SD), when there is no stratification, a relatively small percentage of charts have at least one test 7 failure. However, when there is stratification (shift = 1 SD or shift = 2 SD), a much higher percentage of charts as many as 94% have at VARIABLES CONTROL CHARTS 14

15 least one test 7 failure. In this way, test 7 can identify stratification in the phase when the control limits are estimated. VARIABLES CONTROL CHARTS 15

16 Appendix C: Amount of data Formula C1: Number of observations required based on a 95% CI for the upper control limit When the data are normally distributed and there is no error due to sampling variability, the percentage of observations above the upper control limit is 0.135% for a process in control. To determine whether there are enough observations to ensure that the false alarm rate stays reasonably low, we follow Bischak (2007) and calculate a 95% lower confidence limit for the upper control limit as follows: where = estimate of center line h = 3 σ 1 1 = unbiasing constant for an Xbar chart n = number of observations k = subgroup size inverse cdf evaluated at p = 0.95 for the Normal distribution with mean = 0 and standard deviation = 1 We set = 2.65 σ, which yields a false alarm rate of 0.5% above the upper control limit and solve for n. For the I chart, we estimate σ using an average moving range of length 2; therefore, we simply use the number of observations from the Xbar chart with subgroup size equal to 2 to determine the amount of data required. Based on these calculations, the number of observations required for various subgroup sizes are shown in Table 5. Table 5 Number of observations required for various subgroup sizes Subgroup size Number of observations VARIABLES CONTROL CHARTS 16

17 Note The number of observations should decrease as the subgroup size increases. However, exceptions to this rule appear in Table 8. These exceptions occur because the number of subgroups was rounded to the next integer before being multiplied by the number of observations in each subgroup, in order to calculate the total number of observations required. The results in Table 5 show that the total number of observations required is less than or equal to 100 for all subgroup sizes, except when the subgroup size is 1 or 2. However, even when the subgroup size is 1 or 2, the false alarm rate is only approximately 1.1% with 100 observations. Therefore, 100 observations is an effective cutoff value for all subgroup sizes. The above analysis assumes that each subgroup will have the same amount of common cause variation. In practice, data collected at different points in time may have different amounts of common cause variability. Therefore, you may want to sample the process at more points in time than required to increase the chances of having a representative estimate of the process variation. VARIABLES CONTROL CHARTS 17

18 Appendix D: Autocorrelation Simulation D1: How autocorrelation affects the false alarm rate We defined an autocorrelated process using the following model: where = Observation at time t = mean of the process = correlation between two consecutive points error term at time t; ~ Normal (0, 1) For simplicity, we considered only lag 1 autocorrelation because it is likely that lag 1 autocorrelation will be greater than the autocorrelation for lags 2. Using the above model, we performed a simulation with = 0.2, 0.4, 0.5, 0.6, and 0.8 and with three subgroup sizes (n=1, 3, and 5). We used an initial set of 10,000 subgroups to establish the control limits and then recorded the percentage of false alarms for an additional 2,500 subgroups. We performed 10,000 iterations and recorded the average percentage of false alarms using test 1 and test 2 for each combination of autocorrelation and subgroup size. The results are shown in Table 6 below. Table 6 Average percentage of false alarms for test 1, test 2 for a process with autocorrelation () and subgroup size (n) Subgroup size n = , , , , , n = , , , , , 7.91 n = , , , , , 3.84 Notice that as the value of increases, the false alarm rate increases. In fact, even moderate levels of autocorrelation ( = 0.4) lead to significant increases in the false alarm rate. When the autocorrelation is 0.4, the control chart shows such a large percentage of false alarms that the results of the chart are almost meaningless. VARIABLES CONTROL CHARTS 18

19 Formula D1: Test for autocorrelation To determine the level of autocorrelation in the data, we follow Bowerman (1979) and perform a test to evaluate the null hypothesis of = r versus the alternative hypothesis of > r using the following test statistic: where 1 = number of observations = sample variance When calculating the autocorrelation of data in Xbar charts, we do not include the dependence between the last point of a subgroup and the first point of the next subgroup because including these comparisons would lead to an underestimate of the autocorrelation. Following Bowerman (1979), we calculate the p-value based on a standard normal distribution. VARIABLES CONTROL CHARTS 19

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