Six Sigma Statistical Process Control Basics

Which statements accurately reflect the characteristics of control plans?

Control plans outline the checking, monitoring, and response plan for a process. Control plans can cover a single process or a family of processes. Control plans include categories of information such as critical customer requirements, indicators, specifications, and targets. The control plan is a single, accessible document. A control plan details the frequency and method of measurement for control, as well as specifying the indicators that will be used. A control plan explains the checking, monitoring, and response plan for the product or process. A control plan can be built to cover a single part, a process, or a group of related processes. A control plan is a single accessible document.

How can you tell when a process is remaining in control?

Data points remain within specified control limits. Measurements of the mean and standard deviation remain consistent over time.

If you were going to select variables to study, what criteria should you base your selections on?

Difficult to hold or maintain. Tied to customer, organizational, or regulatory imperatives.

Which statements accurately describe SPC?

As a source of information on process behavior, it's one of several tools you'll need to implement continuous improvement. It can be overused. It employs control charts as the preferred tool. It allows process owners to have control over the process. To properly implement SPC, you should avoid overanalyzing to the detriment of process control. All members of the team have to buy in to the program. Control charts are the most preferred tools in SPC. SPC's focus isn't on human issues, but rather process issues.

Recent process control efforts have detected a variation. Root cause analysis isolates a change in the temperature and humidity of the production floor as the reason. What type of variation is this?

Common cause variation.

Match each key element with a description of its role in control charts. Control limits Center line Axes Data point

Control limits - Identifies the range of variation that can be ascribed to common causes. Indicates special cause variation if points fall outside this range. Center line - The mathematical average of all the samples plotted. The calculated average of all the samples plotted. Axes - Represents a timeline and the variable of interest. Represents time and the variable of interest. Data point - Represents the status of a process characteristic at a point in time. Represents a characteristic at a particular point in time.

Match the common and special cause variation categories to the corresponding examples. More than one example may apply to each category. Special cause Common cause

Special cause - Power surge. Machine malfunction. Operator falls asleep. Common cause - Poor maintenance of machines. A lack of clearly defined standard operating procedures. Poor lighting and excessive noise on the shop floor.

There are specific elements that should be included in almost every control plan. Which examples of elements could you include in the main body of a control plan?

The ends of each brick will be tested. The process capability is 2.65 after improvement. Measurement reproducibility is 35%. Ten units will be sampled to measure diameter. Samples will be taken every hour.

Which categories of variables should teams consider for control?

Variables that are most difficult to hold. Variables tied to customer, organizational, or regulatory imperatives. Variables representing a critical dimension of the product or process. Variables that are salient or known.

A Six Sigma team at a pressure gauge manufacturer is completing a control plan. What are some examples of information you might find in the control plan?

Plastic Injection Molding Control Plan. The injection molding process encompasses four operations: mixing the polymer, injecting, cooling, and buffing. The person in charge of the process is T. Jackson. Customer #3121: Gleedon Auto Manufacturing. Revised: August 1, version 2.1.

What are the objectives of SPC?

Reducing the costs of variation in a process by controlling and improving process performance. Using quality control tools to monitor a process's performance. Using statistical analysis to help identify sources of variation. To distinguish between special and common cause variation by examining runs of data and tracking trends exhibited on control charts. To assess how a process compares to its control limits by monitoring its performance. To control and improve process performance to reduce variation in the process.

Match each selection criteria with the corresponding example. Salient or known variables Critical dimension of the product Difficult to hold or maintain Tied to key imperatives

Salient or known variables - A defect that occurs each time a night shift falls on a Friday night. Critical dimension of the product - A variable related to the ability of a device with GPS to sync to satellite signals. Difficult to hold or maintain - Computer crashes are associated with high defect rates. Tied to key imperatives - An aspect of a service has generated customer complaints.

What other information would the team typically include in the control plan?

A high repeat customer rate determines a KPOV. Model #25150 pressure gauge. Temperature, tensile strength, and length are important inputs. less than 1% defect rate is required. The upper specification limit is 2.732.

Match the control chart types to when they are used. ImR c Xbar and s p Xbar and R

ImR - The subgroups contain a single measurement. c - Defects are in equally-sized subgroups. Xbar and s - The subgroups contain more than ten measurements. p - Defectives are in unequally sized subgroups. Xbar and R - The subgroups contain two to ten measurements.

Which actions are involved in the transfer of control plan responsibility from the Six Sigma team to the process owner?

Integrate the control plan with related processes. Establish a documentation process for control. Update all work instructions with reaction plans. Secure understanding and agreement from involved personnel. Updating all work instructions to ensure consistent actions are taken in response to out-of-control variation. Setting up a clear and logical documentation process for control. Ensuring that the control plan integrates well into operational activities. Securing buy-in and understanding from the operational employees involved in monitoring the process.

It's important to know which control chart to choose, given the type of data and size of your sample. Match types of control charts to when you should use them. P ImR Xbar-S U

P - You're plotting defective units and subgroups are not of equal size. ImR - You are pulling one sample of continuous data at a time. Xbar-S - You have variable data and a sample of 10 measurements. U - You're plotting the average number of defects per unit, and the sample size isn't constant.

Which examples of variables are good candidates for SPC?

Teams have noticed that, historically, tube diameter on a coolant system varies significantly. A manufacturer wants to adjust the temperature at which a flame-retardant material used by consumers combusts. A Six Sigma team determines that poor batches of raw material are the result of special cause variation. Customer surveys at a call center show that hold times are a major source of customer complaints. A financial institution wants to minimize the processing time for each of its loan applications. A team member doing charting can easily measure when user error causes a machine malfunction. A Six Sigma team wants to find a way to reduce employee absenteeism to the industry benchmark. An Accounting Department specifies that invoice errors should be minimized to 0.01% in the next financial year.

Which considerations should you keep in mind when determining an appropriate subgroup size?

Teams need to have a sufficient history of the process when selecting subgroup size. Small subgroups are more economical but can't detect larger process shifts easily. Service processes typically have a sample of one. Large subgroups may contain dependent data or special cause variation. You and your Six Sigma team should have a sufficient history of the process when selecting subgroup size. Larger subgroups taken over too short a period may contain dependent data.

During the Control phase of a Six Sigma DMAIC project, it's important to make sure your improved processes stay in control. What are the characteristics of an in-control process?

The process's mean and standard deviation are consistent over time. You get a consistent output from the process.

What are the key properties of rational subgroups?

The subgroups are formed from observations made in a time-ordered sequence. The observations within a subgroup are from a single, stable process. The observations comprising the subgroup are independent. Observations are made up of independent data points.