טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentAvinoam Tzimerman
SubjectOff-line Inspections under Inspection Errors
DepartmentDepartment of Industrial Engineering and Management
Supervisor Professor Herer Yale


Abstract

A process subject to random failures processes a finite batch of units. The process starts in IN state, where conforming units are being produced. During the process, a failure may occur with a known probability and the process shifts to the OUT state, where non-conforming units are produced. When the process shifts from the IN state into the OUT state, it remains there until the end of the batch. The unit that was produced during the shift is called the transition unit. the production order of the units is preserved and quality inspections are performed.

The quality inspections themselves are subject to inspection errors, where two kinds of identification errors are possible:

1.      Type one error - conforming unit identified as a non-conforming one.

2.      Type two error - non-conforming unit identified as a conforming one.

Due to inspection errors existence, the transition unit cannot be identified with certainty, but with a certain confidence.

We develop an inspection policy for identifying the transition unit with a given confidence level. During the identification process, the expected number of units being inspected is minimized. This policy is found by a dynamic program. Four different heuristic policies were also developed along with the optimal one. Those heuristics try to achieve the minimum expected number of inspections for finding the transition unit for a given confidence level.

We also develop an optimal inspection/disposition policy. We find which units to inspect and which part of the batch to dispose without performing any inspections, in order to achieve minimum expected cost, which is composed of the inspections cost and the penalties cost as a result of classification errors.