Definition (Practical Worst Case): The practical worst case (PWC) throughput and flow time as a function of WIP are as shown in the following figures.
 
 

Relationship between Throughput and WIP for Infinite buffer closed serial lines

Relationship between Flow Time and WIP for Infinite buffer closed serial lines

Note that the performance of the PWC is represents an intermediate level between the best case and the worst case.
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The practical worst case (PWC) occurs when:

1. the line is balanced (i.e., all stations have the same rate)
2. all stations consist of single machines
3. processing times have just the right amount of variability to make all possible job-machine combinations equally likely

We can use these results as the basis for an "internal benchmarking" procedure.  By observing a single WIP, TH pair (e.g., an average for the past three months) we can determine whether a given line is in the "Good" (i.e., better than the PWC) region or the "Bad" region.  Since the assumptions underlying the PWC are representative of a fairly inefficient line, finding that a system is in the "Bad" region generally indicates an opportunity for significant improvement.

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