Explanation of the rule of seven (heuristic) on a Control Chart with the help of an example and exercise
The rule of seven states that if seven or more consecutive measurements fall on one side of the mean that there’s an assignable cause that needs investigation.
Key Points
Rule of seven is a rule of thumb or heuristic.
On a control chart, when seven consecutive data points fall on the same side of the mean, either above or below, the process is said to be out of control and in need of adjustment.
All the seven points may be within the control limits.
Such a situation is not considered normal (or common) and warrants an investigation.
It does not necessarily mean that a corrective action is needed, but it does need to be investigated to find an assignable cause.
It also doesn’t imply that the frequency at which a process becomes out of control is acceptable (or not acceptable).
Rule of Seven on a Control Chart
Refer to the figure below for an example of seven data points below the mean on a control chart.
Sample Question 1
A process is designed to produce high precision cylindrical rods. The control limits for the rod diameter are 11.90 mm to 12.10 mm. During a quality inspection, 20 random samples were selected from the production floor, and the following measurements were observed (all values are in mm):
12.09, 11.90, 12.06, 12.08, 12.10, 12.10, 12.09, 12.07, 12.00, 11.99, 12.02, 12.01, 12.07, 12.03, 12.10, 11.95, 12.05, 11.98, 11.97, 11.95
Which of the following statements is true about the process?
A. The process is in control as all measurements are within the control limits.
B. The process is in control.
C. The process is in out of control and in need of adjustment.
D. The process is in out of control and in need of inspection.
Sample Question 2
Using the example from Exercise 1, if the data points are the following, what can be said about the process (select from the same four options given above)?
12.09, 11.90, 12.06, 12.08, 12.10, 12.10, 12.09, 12.07, 12.00, 11.99, 12.02, 12.01, 11.99, 11.93, 11.90, 11.95, 11.98, 11.97, 11.95, 12.05
Solutions
The mean of the process is the central value, which is 12.00 mm (mean of 11.90 mm and 12.10 mm). All 20 points are within the control limits. In no case do we have seven points falling on the same side of the mean. There’s a case where six points fall above the mean, but the seventh point is right on the mean (tricky indeed). Therefore, the process is in control.
In this case, we indeed have seven data points on the same side of the mean - 11.99, 11.93, 11.90, 11.95, 11.98, 11.97, 11.95. The process is considered out of control and warrants an inspection (not adjustment). Inspection may or may not call for an adjustment.
FAQs
Rule of seven is a rule of thumb or heuristic. On a control chart, when seven consecutive data points fall on the same side of the mean, either above or below, the process is said to be out of control and in need of adjustment. All the seven points may be within the control limits.
What is the rule of seven tests? ›
Rule of Seven Tests
The tests state that an out of control situation is present if one of the following conditions is true: 1) Seven points in a row above the average, 2) Seven points in a row below the average, 3) Seven points in a row trending up, or 4) Seven points in a row trending down.
What is a control chart answer? ›
The control chart is a graph used to study how a process changes over time. Data are plotted in time order. A control chart always has a central line for the average, an upper line for the upper control limit, and a lower line for the lower control limit. These lines are determined from historical data.
What are the 3 limits in a control chart? ›
In general, the chart contains a center line that represents the mean value for the in-control process. Two other horizontal lines, called the upper control limit (UCL) and the lower control limit (LCL), are also shown on the chart.
What is control charts in 7 QC tools? ›
Control chart: Graph used to study how a process changes over time. Comparing current data to historical control limits leads to conclusions about whether the process variation is consistent (in control) or is unpredictable (out of control, affected by special causes of variation).
Is the rule of 7 accurate? ›
The marketing rule of 7 is not an exact science. It's not it case of exposing your brand to consumers exactly 7 times in order to generate guaranteed sales. It's more about enhancing the visibility of your brand or products.
What is the rule of 7 in control? ›
Rule of seven is a rule of thumb or heuristic. On a control chart, when seven consecutive data points fall on the same side of the mean, either above or below, the process is said to be out of control and in need of adjustment. All the seven points may be within the control limits.
What are the 7 rules in a SPC chart? ›
The 8 Control Chart Rules
| Rule | Rule Name | Pattern |
|---|
| 1 | Beyond Limits | One or more points beyond the control limits |
| 2 | Zone A | 2 out of 3 consecutive points in Zone A or beyond |
| 3 | Zone B | 4 out of 5 consecutive points in Zone B or beyond |
| 4 | Zone C | 7 or more consecutive points on one side of the average (in Zone C or beyond) |
4 more rowsSteps to Interpreting a Control Chart
- Step 1: Determine the average for the data summing all data and dividing by the number of data points.
- Step 2: Calculate the sample standard deviation.
- Step 3: Determine control limits by multiplying the sample standard deviation by -3, -2, -1, +1, +2, and +3.
Rule 1: Any individual data point falling beyond the 3-sigma control limit indicates a special cause variation.
An out-of-control process has points falling outside the control limits or non-random patterns of points (called special-cause variation). If the process is in-control, no corrections or changes to the process are needed.
What is the most appropriate control chart? ›
You would use a p control chart if the subgroup size (the number of items examined in a given time period) changes over time. You would use the np control chart if the subgroup size stays the same. Counting Data: With counting data, you count the number of defects.
How to remember 7 QC tools? ›
How to remember the 7 QC tools? Many Six Sigma experts mention that to remember the 7 QC tools, you have to understand the relationship with each other. For instance, you can use a flow chart or graph, find out the minor variations and their sources through a cause and effect diagram.
What are the 7S of quality control? ›
7S is the new terminology consists of the seven phases namely Sort, Set in order, Shine, Standardize, Sustain or Self Discipline, Safety and Spirit. The paper explains the methodology, action steps, resources required and target outcomes for the implementation of 7S as a tool of organizing workplace scenario.
What is the 7 divisibility test rule? ›
To determine whether a number is divisible by 7, you have to remove the last digit of the number, double it, and then subtract it from the remaining number. If the remainder is zero or a multiple of 7, then the number is divisible by 7. If the remainder is not zero or a multiple of 7, the number is not divisible by 7.
What is the rule of 7 with example? ›
The divisibility rule of 7 states that, if a number is divisible by 7, then “the difference between twice the unit digit of the given number and the remaining part of the given number should be a multiple of 7 or it should be equal to 0”. For example, 798 is divisible by 7. Explanation: The unit digit of 798 is 8.
What is the 7th principle of testing? ›
7. Absence-of-errors is a fallacy. If your software or system is unusable (or does not fulfill users' wishes) then it does not matter how many defects are found and fixed – it is still unusable.
What is the rule of 7 theory? ›
The Rule of 7 asserts that a potential customer should encounter a brand's marketing messages at least seven times before making a purchase decision.
