Modeling assumptions

Many of the questions we have gotten from industry folks are in response to a press release by the University of Arkansas, which necessarily did not contain many of the assumptions of the research. Here, for the record, are our assumptions:

1. These designs are for unit-load picking only, not for order picking environments in which workers must retrieve more than one item per tour. We are working on the latter (more difficult) problem now, and will post results when we have them.

2. All travel begins and ends at a single pickup and deposit point, which may be the point of entry to a picking module (flow rack or bin shelving, for example) or a stretch-wrap machine. We are working on more general material flow models that will account for multiple P&D points, and will post results when we have them.

3. "Percent improvement" records only the percent reduction in expected travel distance to a randomly chosen location. It does NOT consider time to load/unload, time to retrieve pick lists, or time to visit the potty! You can estimate your level of savings by multiplying the percent improvement by the percent of time your workers spend traveling.

4. Every location is equally likely to be visited, which is approximately equivalent to randomized or closest open location.

5. We have ignored the following issues: congestion, spacing of building columns, worker disorientation, and numbering of rack locations. All are real issues that deserve consideration: I will address them in another post.

--Kevin