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Welcome to Capability Studies

Capability StudiesProcess Capability is required by TS 16949 and is part of the typical APQP process. Process capability compares the output of an in-control process to the specification limits by using capability indices. The comparison is made by forming the ratio of the spread between the process specifications (the specification "width") to the spread of the process values, as measured by 6 process standard deviation units (the process "width"). Formulas used to calculate capability are:

Cpu = (USL - mean) / 3 * standard deviation
Cpl = (mean - LSL) / 3 * standard deviation
Cpk = min {Cpu, Cpl}

The Cpk is the most commonly used index for calculating capability, however some have found that the Ppk index is actually better.  The Cpk is used to gauge the potential capability of a system, or in other words, a system’s aptitude to perform.  The Ppk (and relative pp and Pr) actually measure the performance of the system.  To determine which of the indexes to use, determine whether you want to analyze the actual performance (Ppk) or the potential capability of the system (Cpk).  Cpk is calculated with sigma equal to 3, which is an estimated sigma.  Calculating Ppk uses a calculated sigma from the individual data.

Understanding the need for capability indexes is an important part of the analysis.  The indexes help to determine the system’s ability to meet specifications.   The problem found in using the Cpk, is that it does not account for the average.  This will not only let you see what the systems potential is, not necessarily what the average of the system looks like.  For example, a Cpk of 1 indicates that the system is at least 99.73% within its specifications.
Endorsed Capability Studies Software:

Process Capability Basics
A basic course on process capability covering common measures of capability (Cr, Cp, Cpk, Pp, Ppk) and how to determine if a process is capable of meeting the specification. Time: 1.5 hours

Advanced Process Capability
How to conduct a process capability study, what to do if the process is not capable, how to deal with complications while conducting or analyzing a capability study and how to use Z-values. Time: 3.0 hours

SPC WorkBench v 4.0

SPC Work Bench is simple yet powerful software that enables you perform On-Line Statistical Process Control. The SPC Work Bench puts together Control Charts, Histograms and Capability Analysis, Pareto Diagrams, and all that is required to implement math-anxiety-free SPC. s

  • SPC for Variables using X-bar/R, X /MR and X-bar/s charts.
  • SPC for Attributes using p, Np, c and u charts.
  • Process Capability analysis over selected horizon.
  • Histogram, Normal Curve and Normal Probability plot display.
  • On-line computation of Cp, Cpk, Pp, Ppk parameters.
  • Estimation of PPM levels of the process over selected horizons.

Non-Normal Process Capability Evaluator

Analysis of Non-Normal Data was never easier!

Process capability evaluation done the traditional way, is based on an assumption that the data is Normally distributed. However, there are processes, which are inherently non-normal, and using assumptions of normality can lead to erroneous evaluation. This may lead to wrong decisions. In cases where the data is not normally distributed, the evaluation of Process Capability requires special techniques. Recognizing this, the recently updated SPC manual of AIAG (2nd Edition, July 2005) recommends the use of Non-normal process capability evaluation techniques to deal with such data.

The Non-Normal Process Capability Evaluator enables you evaluate non-normal process capability using one of the following methods:
  • Box-Cox power Transformation.
  • Johnson Transformation system.
  • Clements Method using Pearson Curves.

Quincunx SPC Simulator

Practitioners of Statistical Process Control (SPC) need to have a very clear understanding of what SPC is and how it works. For years a mechanical Quincunx model that drops balls over number of pins and simulates real-life processes has been used for this learning.

Here is the Quincunx SPC Simulator Software that has several additional features to the physical Quincunx.

The Quincunx SPC Simulator has been found very effective for Training in classroom environment as well as for self-learning. It is being successfully used by Quality Managers, Facilitators, Trainers, and Consultants.
  • in SPC awareness programs in organizations,
  • in Green Belt and Black Belt training, and also
  • in Universities.
Use this software to simulate and study:
  • Fundamentals of Normal Distribution
  • Control Charting and its physical significance
  • Detection of Out-Of-Control situations using Control Charts
  • Process Capability Vs Process Control
  • Process Capability Indices like Cp, Cpk
  • Process Variation and factors causing variation
  • Process Adjustment and Drift
  • Demonstration of the Central Limit Theorem
  • Effects of Subgroup Size on Control Chart Sensitivity

Process Capability Index and Sigma Analysis.

A study to determine the extent of the actual process variation against that required or specified.

Process Capability Study Template. Enter measured results of study and the template will determine 8 out of control criteria to the data and indicate if process is capable and stable.

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