3.1 Introduction

In this chapter we discuss various measures by which reliability and Six Sigma characteristics can be numerically defined and described. Manufacturers use reliability and Six Sigma measures to quantify the product, production and process effectiveness. Use of any particular reliability and Six Sigma measure depends on what is expected of the system and what we are trying to measure. Several product development decisions are made using reliability and Six Sigma measures as one of the important design parameter. Technically, the term Six Sigma itself is a measure of quality. In this chapter we first summarize Six Sigma measurement system followed by reliability measures.

3.2 Six Sigma Measures

Processes make product and the effectiveness of the processes depend on its ability to satisfy customer requirements. Variability within the process leads to customer dissatisfaction. All the measures within Six Sigma measurement system attempt to quantify the capability of processes used in developing the product. Historically, process capability, C _p, and process capability index, C _pk, are used to measure the ability of the processes to meet the customer requirements. Process capability index and the process capability are defined in equation (3.1) and (3.2) respectively.

	(3.1)
C pk = Min[( US m)/3 s, ( m LS)/3 s]	(3.2)

Where, US and LS are upper and lower specifications (or lower and upper control limits, LCL and UCL) of the process...