Statistical Process Control

The Five M’s that affect a process are: manpower, machine, material, method, and measurements.

Process is defined by Cambridge Dictionary as “a series of actions that you take in order to achieve a result”.

A process has five main features:

1. Definability: Determination of the main elements/ activities.
2. Measurability: It is the feature of the process to be monitored with performance criteria.
3. Repeatability: It is the ability of the output that is formed as a result of processing the inputs that activate the process to meet the customer needs and requirements continuously.
4. Controllability: It is the ability of the process managers who are always aware of the performance of the process and can take corrective actions when necessary.
5. Value added capability: It is an ability to have a positive effect on the quality of the output of the process and the satisfaction of the customer.

The processes are classified in three main groups:

• Main processes: Processes that start directly on demand from the external customers of the organization and provide a product or service to the external customer,
• Management processes: Planning the activities of all processes in line with the organization’s objectives and regularly reviewing and reporting the performance indicators related to them,
• Support processes: Process that consists of different areas of expertise gathered under one roof in order to ensure the most appropriate use of resources throughout the company.

The benefits of management with processes:

• The processes, sub-processes and activities are defined as process map, procedures, and instructions. Therefore, every employee can work at the same format in an organization.
• Process performance indicators are defined, measured and reviewed periodically. The problems or barriers can be seen clearly. By eliminating these problems or barriers, the process can be improved according to its performance criteria.
• The relationships and interactions of the process are defined. By this way, the activities can be planned more accurately.

Ways to Eliminate Non-value-Added Activities:

1. Rearrange the sequence of work steps and the physical location
2. Change work methods, forms, documents
3. Change the type of equipment used in the process
4. Improve operator training
5. Try to eliminate unnecessary steps or merge steps

The seven major quality tools for detecting and reducing the variability of process are:

1. Histogram
2. Check sheet
3. Pareto chart
4. Cause-and-effect diagram
5. Box plot
6. Scatter diagram
7. Control chart

Histogram is the graphical presentation of distribution of data. It is drawn by dividing the largest 8 and smallest values evenly and determining the number of data per interval. It is easy to show and interpret minimum and maximum value of data and the distribution of data.

Check sheet is a data collection tool that can be designed according to the use. Check list is designed for frequency of detect, to indicate the place of detect, etc.

Pareto Chart is used to rank the causes of quality problems of process.

Cause and Effect Diagram is a diagram, also known as the Fishbone, used to investigate the potential and root causes of a particular problem or error. Possible causes can be determined by brainstorming with the quality improvement team.

The scatter diagram shows the relationship between two variables graphically without any graphical calculation. Therefore, it is useful and easy to use for employees. These relationships are presented as correlation coefficient statistically.

Box plot shows a visual impression about a data set where the median, quartiles, minimum and maximum values, namely the distribution of data, are given. The median called quantile 2, is the middle value that separate the higher half of the data from the lower half, and the line in the middle of box. When data are sorted from small to large, the first 25% part is the quantile 1, and the last 25% part is called the quantile 3.

Design of experiments (DoE) is also a methodology to determine the best level of effective controllable factors on quality characteristics. The experiments are planned with a systematic procedure and carried out in a random order under controlled conditions. For the analyses of experiments, Variance analysis (ANOVA) method is used to test a hypothesis for discovering effective factors and factors’ interactions. DoE is an effective methodology for evaluating which process inputs have a significant effect on the process outputs. Therefore, the quality of process outputs and process optimization are provided via using DoE methodology by controlling significant process inputs.

Advantages of experiments:

1. Experiments allow us to set up a direct comparison between the treatments of interest.
2. We can design experiments to minimize any bias in the comparison.
3. We can design experiments so that the error in the comparison is small.
4. Most important, we are in control of experiments, and having that control allows us to make stronger inferences about the nature of differences that we see in the experiment. Specifically, we may make inferences about causation.

SPC is a methodology that collects data from process, analyze with several tools, trace and control of process for improving quality of product and service. Contributing to the widespread deployment of the SPC intervention is an abundance of anecdotal evidence attributing quality, productivity, and costs benefits to this particular quality improvement intervention. By systematic deploying of SPC with various statistical techniques and problem- solving tools; organizations develop their perspective conceptualized the implementation and practice of SPC as a multi-facet change phenomenon involving the organization’s task, technology, structure, and people.

The practical benefits of implementing SPC are:

• Catching the variability in a process and define the causes
• Explaining the variation with statistical tools by using samples from the process
• Using and understanding some statistical tools such as histogram, Pareto chart, cause and effect
• diagram, and Shewhart charts
• Catching and analyzing the patterns on process with statistical tools and interfering with them.

The Elements of a Successful SPC Program are:

1. Management leadership
2. A team approach, focusing on project-oriented applications
3. Education of employees at all levels
4. Emphasis on reducing variability
5. Measuring success in quantitative (economic) terms
6. A mechanism for communicating successful results throughout the organization

FMEA is a specific methodology to evaluate a system, design, process, or service for possible ways in which failures (problems, errors, risks, concerns) can occur. FMEA is the method that is used to determine the potential failures of product, process, service, and/or system. The main purpose of FMEA is to predict and prevent the known or potential problems based on the historical data of a similar product or process in the design stage.

There are three elements of FMEA:

• Occurrence is the frequency of known or potential failures. It can be obtained from the service rates of the similar product or process or customer feedbacks.
• Severity is the effects of failures on product/ process and customers.
• Detection is the ability of detecting the failures with test instructions of quality characteristics.

The benefits of FMEA are given as follows: