الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
Of increasing importance in the field of statistical quality control is the simultaneous control of two or more variables when the quality depends on the joint effect of these variables rather than the effect of each variable separately. If a separate shewhart control chart is used to control each variable irrespective of the other variables , many times these variables will appear to be in statistical control when the system is actually out of control .Therefore, bivariate control charts for two varieties, and multivariate control charts for more than two varieties are to be used to overcome the arising errors. In the control ellipse method , which is applicable for the control of two variables only , the ellipse is dawn on a chart after performing some calculations, the observation of the two variables are then o plotted on the same chart since its two axes represent the two considered variables. The ellipse is regarded as the joint region of control in the sense that any point falls outside its perimeter, represents an out of control condition. the other method applicable for the control of two variables is to use the T2 is calculated by either one of two methods .In the first method Hotel ling formula is applied, in the second the principal components transformation is used to transform the two variables into new variables that that are independent having unit variances zero means, hence the sum of their squares will be equal to the desired statistic T2.