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About the Author
Cliff Holste is Supply Chain Digest's Material Handling Editor. With more than 30 years experience in designing and implementing material handling and order picking systems in distribution, Holste has worked with dozens of large and smaller companies to improve distribution performance.

Logistics News

By Cliff Holste

February 29, 2012

Picker Productivity Is More Often A Function Of Hit Density Than Pick Rate

Increasing Picking Opportunities is the Fast-Track to Higher Picking Rates

In a recent discussion with a company that is planning on migrating from a totally manual order picking process to a more automated process, concerns about picker productivity and order accuracy were high on their list.

In the current operation, which consists of tens of thousands of SKUs, order pickers are responsible for picking complete orders assigned to them, usually 2 to 6 in one tour of the DC. But, in the more mechanized or automated picking approach being considered, order pickers are assigned to a pick zone and pick only a portion of each order.

In considering this multiple picking zone approach, there was considerable skepticism that it would actually yield higher picker productivity. In fact, many on the planning team felt that it would increase overall order processing time while reducing accountability leading to more errors.

While these are legitimate concerns, they are unfounded. The following will provide insight as to why zone picking is one of the most preferred methods.

Holste Says:
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In today's fast changing business environment, flexibility and adaptability are not only desirable but essential to being able to make quick adjustments in sales strategy, order SKU mix and volume.
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Understanding the Picking Process and Flow

The Single Picker per Order Method:

In the typical pick-to-cart operation, picking is for the most part restricted to the warehouse floor where active pick locations are easily accessible. Pickers are tasked with moving horizontally throughout the warehouse gathering all of the products required to complete the orders on their pick cart. Without the benefit of a Warehouse Management System (WMS), the pick route they take is based on a quick assessment of the orders and knowing where product is stored. Typically, there is no attempt to group orders together based on commonality of SKUs.

Depending on hit density (the distance between picks and number of picks per stop) a very high percentage of the picker’s time is spent moving from one location to another. Of course, some additional productivity losses along the way can be expected as it is difficult to manage activities that take place in the deep dark recesses of the warehouse. Some companies try to estimate times for completing each picking assignment. But that’s usually nothing more than a guess that does not take into account anomalies such as inventory and slotting errors, which occur more frequently than most would care to admit.

Oftentimes, for efficiency purposes, the pickers are picking directly into shipping containers. While accountability is high, without an audit procedure picking errors can go undetected.

In this type of manual operation a paper pick list is most often used to drive the picking function. As an alternative, RF or Voice Directed technologies are used. While these technologies have been proven to increase productivity and reduce errors, the random flow of order picking within the DC remains for the most part unchanged. It would not be unusual for a picker to spend 1/3 or more of his/her time moving about the DC.

The Zone Picking Method:

Throughout history, when facing an enemy having what appears to be an overwhelming force, military leaders have often successfully adopted a “divide and conquer” strategy. The zone picking method is a similar strategy.

Among the first advantages of the zone picking method is being able to manage the process incrementally in both the horizontal and vertical plane. Because pickers are working within a specific zone, that zone can be on a mezzanine. Orders can be started in any zone where a pick is required. An automated continuous loop conveying system will insure that order picking containers (tote box or shipping carton) will visit all zones where picks are required (for more details see – “Automated Sorting – Improves Eaches Picking Efficiency”).

Picking within the zone is computer directed. Cluster picking allows a half dozen or more orders to be sequentially picked in one pass through the zone. Hit density (as defined above) is optimized thereby increasing picker productivity. Based on the relativity short pick path and a highly predicable pick sequence, the time required to complete the order can be accurately calculated. And, management visibility to the operation is greatly enhanced.

Another benefit is that each pick zone can be specifically designed and optimized for the different types of picking activity (full case or split case) and configuration (double deep pallet flow, case flow, static shelving, horizontal and/or vertical carousals, etc). Paperless picking technologies such as RF, Pick-to-Light, and Voice Directed can be easily adopted to increase productivity and accuracy. And, integrated software based slotting programs can be deployed to further enhance picking performance.

If you are considering adopting a picking zone system, the question of how many picking zones are needed can best be addressed with a list of things to consider, such as:
 
  • How will the system direct the pickers? Can the number of pick faces within a zone be increased or decreased? (Here you are looking for flexibility in zone configuration relative to SKU slotting)

  • Can the system support more than one picker in a zone - on the same order or on different orders? (In this case you are looking for the ability to handle peak volume periods)

  • Can the zone boundaries be changed easily? Every once in a while, by wave or by order? (Is zone size dynamic or static?)

  • Are the pickers picking one order at a time or a cluster of several orders?

  • Can zones overlap?


Breaking up orders into picking zones (i.e., divide & conquer) requires a robust WMS order processing and SKU slotting optimization functionality. These capabilities and a whole lot more are available from most WMS providers.


Final Thoughts

The key objectives of any picking method should always be speed, efficiency, and accuracy. However, in today’s fast changing business environment, flexibility and adaptability are not only desirable but essential to being able to make quick adjustments in sales strategy, order SKU mix and volume. If your current order fulfillment operation can no longer satisfy these objectives, perhaps it has out lived its functional life and it’s time to adopt more up-to-date methods.

 

Recent Feedback

I think this article is very well done! I’ve seen books written that covered less ground than what was presented here in just a few words.

The only thing I would like to add is that when you talk about increasing pick faces, this leads into “how.” The how is simply achieved by implementing automation. Horizontal Carousels and Vertical Lift Modules (VLMs) are the two most basic forms of AS/RS which come to mind. They pre-position SKUs in a pick window waiting for the operator to make the pick.

The use of Carousels and VLMs simply eliminate the wasted walking and searching time. Inventory is delivered to the operator in a relatively small area. Integrated Pick to Light or voice systems direct the operator to the exact pick and quantity. Accuracy levels often exceed 99% using this method.

The reason to implement automation into the mix is simply a matter of Return On Investment (ROI). Pick carts and paper lists are space and labor intensive. Carousels and Vertical Lift Modules save up to 85% of the floor space and often reduce labor by as much as 2/3. With these dynamics, creating a “no brainer” return on investment is just a matter of doing the math.

Thank you for the very good article!


Ed Romaine
Chief Marketing Officer (CMO)
Sapient Automation
Mar, 06 2012

Great article. We have implemented zone picking in our cotton storage facilities with substantial increases to productivity in this area. Grouping multiple orders and picking by zones definitely increases efficiency, and allows greater visibility by management as stated in your article. Along with a strong WMS and you will put those archaic, single order picking practices behind you.


Barry Rose
Warehouse Manager
Louis Dreyfus Commodities Cotton Storage LLC
Mar, 16 2012
 
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