In many modern manufacturing plants, data that characterize the manufacturing process are electronically collected and stored in the organization’s databases. Thus, data mining tools can be used for automatically discovering interesting and useful patterns in the manufacturing processes. These patterns can be subsequently exploited to enhance the whole manufacturing process in such areas as defect prevention and detection, reducing flow-time, increasing safety, etc. When data mining is directed towards improving manufacturing process, there are certain distinctions that should be noted compared to the classical methods employed in quality engineering, such as the experimental design. In data mining the primary purpose of the targeted database is not data analysis; the volume of the collected data makes it impractical to explore it using standard statistical procedures (Braha and Shmilovici, 2003).
TopBackground
This chapter focuses on mining performance-related data in manufacturing. The performance can be measured in many different ways, most commonly as a quality measure. A product is considered as faulty when it does not meet its specifications. Faults may come from sources such as, raw material, machines setup and many other sources.
The quality measure can either have nominal values (such as “good”/”bad”) or continuously numeric values (Such as the number of good chips obtained from silicon wafer or the pH level in a cream cheese). Even if the measure is numeric, it can still be reduced to a sufficiently discrete set of interesting ranges. Thus we can use classification methods in order to find the relation between the quality measure (target attribute) and the input attributes (the manufacturing process data).
Classification methods can be used to improve the learning curve both in the learning pace, as well as in the target measure that is reached at the mature stage. The idea is to find a classifier that is capable of predicting the measure value of a certain product or batch, based on its manufacturing parameters. Subsequently, the classifier can be used to set up the most appropriate parameters or to identify the reasons for bad measures values.
The manufacturing parameters obviously include the characteristics of the production line (such as which machine has been used in each step, how each machine has been setup, operation sequence etc.), and other parameters (if available) relating to the raw material that is used in the process; the environment (moistness, temperature, etc); the human resources that operate the production line (the experience level of the worker which have been assigned on each machine in the line, the shift number) and other such significant factors.
The performance measure (target attribute) in manufacturing data tends to have imbalanced distribution. For instance, if the quality measure is examined, then most of the batches pass the quality assurance examinations and only a few are considered invalid. On the other hand, the quality engineer is more interested in identifying the invalid cases (the less frequent class).
Traditionally, the objective of the classification method is to minimize the misclassification rate. However, for the unbalanced class distribution, accuracy is not an appropriate metric. A classifier working on a population where one class (“bad”) represents only 1% of the examples can achieve a significantly high accuracy of 99% by just predicting all the examples to be of the prevalent class (“good”). Thus, the goal is to identify as many examples of the “bad” class as possible (high recall) with as little false alarms (high precision). Traditional methods fail to obtain high values of recall and precision for the less frequent classes, as they are oriented toward finding global high accuracy.
Usually in manufacturing plants there are many input attributes that may affect performance measure and the required number of labelled instances for supervised classification increases as a function of dimensionality. In quality engineering mining problems, we would like to understand the quality patterns as soon as possible in order improve the learning curve. Thus, the training set is usually too small relative to the number of input features.