episode 09 exercise answer 02

Part 2: Solution

The next three questions relate to the Priority Spreadsheet that contains a sample of orders received between 9:30 and 10:00 on the third Sunday in March. For each order, the spreadsheet contains the order number (which indicates the order of arrival), the number of items in the order, and the time to process the order. In the video, Warren wonders if they could clear out more orders on Monday by using different priority or sequencing rules. We now want to evaluate how different priority rules might affect some of the performance characteristics of the order picking process.

Let's see how well you answered the questions.

Using the spreadsheet, evaluate the first-come, first-served and shortest processing time priority rules for average queue time and average flow time. What conclusions can you reach regarding these two rules?

Your Answer:

Our Answer:

Priority rule	Average Queue Time	Average Flow Time
First-Come, First-Served	877.59	946.44
Shortest Processing Time	618.19	687.04

The shortest processing time rule results in much lower average queue times and average flow times.

How long does it take to complete all of the jobs under the two priority rules? Explain what is happening in this situation to produce different average flow times for the two different rules. Note that the total flow time for an order is a function of how long the order waits to be processed (queue time) and the time to process the order (processing time). The total flow time is the sum of these two components.

Your Answer:

Our Answer:

It takes 1,859 seconds to process all of the orders for both priority rules. In other words, the priority rule does not change the total time it takes to process all of the jobs. In fact, this time is simply the sum of the processing times for all of the orders. Noticing that the processing times do not depend on the priority rule, the only thing that can differ between the two rules is the queue time. In other words, different priority rules result in different waiting times for the jobs.

Using the spreadsheet, evaluate the following priority rules: first-come, first-served, shortest processing time, longest processing time, and fewest number of items. What recommendations would you make to Debbie for prioritizing the incoming orders?

Your Answer:

Our Answer:

Using the different priority rules results in the following times:

Priority rule Average Queue Time Average Flow Time

First-Come, First-Served 877.59 946.44

Shortest Processing Time 618.19 687.04

Longest Processing Time 1171.96 1240.81

Fewest Items 655.41 724.26

From the table, we can see that the shortest processing time rule results in the lowest average queue time and the lowest average flow time. The longest processing time rule results in the longest times of the four rules. The best rule depends on what Debbie wants to accomplish. The first-come, first-served priority rule is very easy to implement by simply adding incoming orders to the bottom of a list. Implementing other priority rules means gathering additional information and changing the priorities as the mix of orders changes. The shortest processing time rule will clear most of the work out faster and may be especially valuable on Monday when there are a large number of weekend orders waiting to be processed.

What practical difficulties might Debbie encounter in implementing a shortest processing time priority rule? How might you address these difficulties?

Your Answer:

Our Answer:

One immediate difficulty is how to determine the processing time for an order before it is processed. In our spreadsheet work, we had the luxury of knowing the processing times because they are based on historical data. However, to implement the shortest processing time rule we need the processing times before the orders are processed. One alternative might be to use the number of items in the order as a surrogate for processing time. Notice that these two variables are likely to be positively related because orders with more items will usually take longer to process. Indeed, in our sample, the fewest items rule has performance characteristics that are similar to the shortest processing time rule. An additional difficulty for CanGo is in terms of how it will integrate this priority rule into the existing computer system. At this time, the existing system simply displays the next order in the queue for the pick operators. Additional programming may be necessary to implement the new priority rule.

page 6 of 11 | go to page: 1 2 3 4 5 6 7 8 9 10 11