This paper proposed a nonlinear integer program to determine an
optimal plan of zero-defect, single-sampling by attributes for incoming
inspections in assembly lines. In the presence of an inspection
resource constraint, sample sizes of the parts waiting for incoming
inspections must be determined simultaneously. The optimization
model can take into account the parts’ heterogeneous quality features
(e.g., NC risk, NC severity, lot size, and complexity of inspection)
and determine the right sample size for each part. The paper proved
the convexity feature of the expected total cost function for the zerodefect
acceptance policy and, accordingly, recommended a three-step
solution procedure. A real example of twenty parts was presented to
illustrate the application of the optimization model.
This paper has built a foundation for practically meaningful extensions.
The maximum allowable defective number in this paper is
zero. The zero-defect acceptance policy may not be optimal in other
circumstances. When it is greater than zero, the maximum allowable
defective number becomes an additional decision variable besides
the sample size. The sample size and the maximum allowable defective
number adversely impact the expected NC cost, and the convexity
of the objective function is no longer held on the entire decision
space. These require a re-calibration of solution features. The rejection
cost is low for incoming inspections and, thus, it was not considered
in this paper. However, the rejection cost is an important cost
component to be considered, for example, in outgoing inspections.
By including the expected rejection cost in the objective function,
the sample size decision seeks a trade-off among three cost components,
which is more complex than the one presented in this paper.
Another important extension of this paper is to model sample size
decisions for multiple quality features at a part level and then integrate
this lower level decision with the higher level decision analyzed
in this paper. The lower level decision provides more accurate inputs
to the higher level decision, while the latter poses a constraint for
the former. The integration is a value-added, yet challenging, topic of
research. All above-mentioned extensions are important subjects of
future research.