Product line commonality has been claimed to be a means to improving efficiency in design and manufacturing of multiple products. Numerous metrics exist in the literature to quantify the level of commonality in a product line. It remains unclear how and why commonality should be measured and accounted for in product design. In this paper, four desired properties of commonality indices are identified: scalability, ability to incorporate subjective information, ease of updating of the index when new information becomes available and applicability in assessing reuse potential. We propose metrics based on a probability based interpretation of commonality to account for these requirements. With the use of results in probability theory we show how the metric exhibits properties missing in other metrics proposed in the literature. The metric also takes into account generations of products easily and can be used to measure their reusability. We demonstrate the method using a slider-crank mechanism product line example.

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