INTERPRETING CONTROL CHARTS Chapter 10

Random Distribution of Points Characteristics of a normally distributed random pattern of points includes:  The sequence of points being unpredictable (random)  Majority (68%) of the points should be within 1 standard deviation of the center line (34% on each side).  28% of the points should be between 1 and 2 standard deviations from the center line (14% on each side).  About 2% of the points should be near each of the control limit lines.  No points should be beyond the control limits.

Analyzing Control Charts Points classified as out of control should be marked with an “X.” The “X” should be placed outside of the broken-line graph away from the center line. If a single point is out of control, just that point is marked. If several points are out of control within a classification, the first point that satisfied the rule and all points after it are marked out of control by the same classification.

Freaks A freak is a single point that is beyond a control limit. It signifies that something changed dramatically in the process for a short time or that a mistake was made.

Freak Patterns Freak patterns exist when:  4 out of 5 consecutive points are in zone B or beyond on the same side of the center line.  2 out of 3 consecutive points fall in zone A or beyond on the same side of the center line.

Shifts Shifts are sets of 7 or more consecutive points that are all on one side of the center line. Something was introduced into the process that changed the whole process. Special causes of shifts are usually due to operators, material, methods, tooling, machines, or environments. Shifts can indicate improvement as well as problems.  Shift up on R chart is trouble, variation has increased.  Shift down on R chart is improvement, variation has decreased.

Runs and Trends A run is a pattern of points that are steadily climbing or steadily falling:  7 consecutive points that steadily increase or decrease  10 out of 11 points that steadily increase or decrease A trend is a sequence of points that show a general pattern of climbing or falling (trends can have increasing or decreasing points mixed in):  Long fluctuating pattern of steadily increase or decrease

Cycles Cycles are patterns that repeat on a regular basis. Signals something that is systematically affecting the process. There is no number rule when it comes to cycles, it comes down to recognizing repeating patterns.

Grouping / Bunching Grouping occurs when points on a chart occur in clusters. One trouble classification can be embedded in another.

Instability (Unstable Mixture) Instability exists when:  More than 1/3 of the points lie outside zone C or more than 4% of the points fall in or beyond zone A.  The chart has a steep, zigzag pattern.

Stability (Stable Mixture) A stable mixture is similar to an unstable mixture:  5 or more consecutive points that are outside zone C.  Steep zigzag lines on the control chart with very few points in the middle of the chart.

Stratification Stratification exists all of the points are close to the averages line on the control chart, they “hug” the middle:  14 or more consecutive points in zone C.