Shewhart's Statistical Quality Control Chart (MVA)

CHAPTER 1

INTRODUCTION

1.1 Introduction
Shewhart’s Statistical Quality Control Chart (SQC) is one of the most commonly used tools in the area of Quality Control (QC) . For eighty years now these control charts (CC) have been used. These CCs are used and have gained much popularity because of their easiness and efficiency. But, a noteworthy limitation to these traditional CCs is that these charts can be used only for univariate (UV) data. Therefore, the processes determined by multivariate (MV) data are not monitored efficiently. The variables may be correlated with the MV structure. The correlations that exist between the variables are called inter-correlations. The correlations exist within each variable over time are called autocorrelations

When the effect of multiple parameters is not independent, multivariate analysis (MVA) becomes very useful. Also, if some parameters are partial or complete measures of some other parameters then too, this becomes very useful. An important point is that practically all processes are MV but it is not necessary to employ MVA often, because, there are only a small number of controlled variables acting sufficiently and autonomously. With that capability, the variables may conveniently be controlled independently. Anyhow, even if the variables act independently, the use of solitary CCs for every variable boosts the chance of randomness for finding a variable out of control. Hence, the rate of change in false alarms (the false alarm rate or probability of Type 1 error) is boosted if every variable is separately controlled. Multivariate control chart (MCC) makes available a resource to find shifts in any related features by charting only one parameters, . A rectangle is the control region for two distinctly acting

• From webster’s dictionary it is, ‘The characteristic that belongs to a thing’s basic nature’
• Walter A. Shewhart states that there are two features for quality: objective and subjective. The objective defines quality ‘as an impartial realism self-governing of the presence of human.’ The subjective explains quality ‘through whatever we think, feel, or sense. The subjective part of the quality is narrowly linked to the value.’
• From the view of Joseph M. Juran quality is a fitness for use
• From the view of Philip B. Crosby quality is the conformance to requirements
• To Edwards Deming. W, quality is an aim at the needs of the customer, growing and forthcoming’
• From ISO9000, ‘The total quantity of the types and features of an invention or service that tolerates on its ability to satiate stated or implied needs’
Here, we mention some of the legends of Quality Control:
• W. Edwards