Application of Inferential Statistical Tests in Quality Control.

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Statistical Tests & Quality Control              

Application of Inferential Statistical Tests in Quality Control

QNT530

Statistics and Research

Methods for Managerial Decisions

May 17, 2004


Application of Inferential Statistical Tests in Quality Control

Product Introduction

Technology is moving forward with contactless products. Contactless products allow the consumer to purchase without a human intervention while using electronic technology to make purchases or payments. The most valued part of this new technology is that the consumer is apt to use this type of ‘card’ more often, thereby increasing the credit card company’s revenues. “Issuers hope contactless technology will help them convert the mostly cash payments at such locations into card transactions. The magnetic strip will remain on the cards for now for larger purchases.” (www.cardtechnology.com/cgi/bin/readstory.pl?story=20040102CTMC230.xml)

American Express did a pilot launch of ExpressPay contactless products. The launch was in the company’s cafeterias and in a smaller consumer pilot testing group within the Phoenix area. There are other plans to launch tests overseas in locations such as Singapore. American Express designed its ExpressPay product to be a “smart chip-carrying fob that is small enough to fit on a keychain and it really has sped up transaction time averaging less than nine seconds. A typical transaction can take up to sixteen seconds to charge without obtaining a signature per David Bonalle, at American Express. In the Phoenix trial, with 268 merchants, ExpressPay prompted customers to use their American Express accounts 33% more often.” (http://www.cardtechnology.com/cgi/bin/readstory.pl?story=20040102CTMC230.xml)

Causes of Variation

There are two general causes of variation – chance and assignable. “Chance variation is variation that is random in nature. This type of variation cannot be completely eliminated unless there is a major change in the equipment or material used in the process. Assignable variation is variation that is not random. It can be eliminated or reduced by investigating the problem and finding the cause.”  (Lind, 2002, p.626)

        For the ExpressPay smart chip-carrying fob it is important to ensure that the smart chip be of the same quality and not varied. How can this be measured?  Assurance that the use of materials is of the exact same quality and precision computerized machinery will be used in making the smart chips for the fobs to name a couple of variance controls. Differences in materials, methods, people, machines and environment cause variation in the process which shows up as variation in products (or services) produced by the process.

Any of the following conditions or patterns is evidence of the likely presence of an assignable cause of variation.

  • One point beyond zone A (three standard deviation limits.)
  • Two of three consecutive points in zone A (the two standard deviation warning limits, or beyond) on one side of the center line.
  • Four of five consecutive points in zone B (the one standard deviation limits, or beyond) on one side of the center line.
  • A run of eight consecutive points (runs up, down or on the same side of center line).
  • Any nonrandom pattern – trend, fanning out, cycle or alternating pattern.
  • Otherwise, the process is said to be in statistical control.
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Characteristics of Process Control

Statistical process control is the method which will control the behavior of the process through the use of statistical information so that the process output will meet the target with minimal variability. To maintain statistical control it is important to remove all the special causes of variation from processing. To ensure statistical control and maintain high quality processing of the micro-chips for the keychain fob cards, for this example this would suggest that the special cause variables need to be eliminated from the manufacturing process.

“Special causes 5-15% of quality problems, due to a ...

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