Executive Summary:
The objective of this project was to determine whether there exists an optimal
combination of self-service and full service lines that could maximize customer
throughput at the Stop & Shop Supermarket of Hamden, CT. Currently Stop & Shop has
installed 5 self-service checkout lines in addition to the 18 standard full-service checkout
lines. Two of the new self-service lines have been designated for customers with 12
items or less while the remaining 3 lines are designated for customers with an unlimited
number of items.
To compare the performance of the new self-service lines to the existing system, interarrival and service time data was recorded between the hours of 11:00 am and 1:00 pm,
peak service times for Stop & Shop, for each of the four checkout line types: self-service
unlimited, self-service 12 items or less, full-service unlimited, and full-service 12 items
or less. This data was then reduced using Stat::Fit to allow the inter-arrival and service
times to be represented analytically. An analytical simulation model was created with
visual process simulation characteristics to analytically re-enact the checkout line
customer throughput process. Analysis of each of the four service scenarios, using interarrival and servicing times stoichastically represented using a pseudo-random number
generating approach with distributions related back to the true arrival rates, was run to
establish the average total number of customers serviced per hour for each check-out line
type. Given a few basic restrictions on the line combinations, all checkout line
combination types were analyzed for two scenarios, lines that accepted an unlimited
number of items and those that accepted customers with 12 items or less, to fully
understand the impact of each line combination. It was also assumed that all service lines
would be open for service, a scenario most likely achievable just before a major holiday
like Thanksgiving or Christmas. What was found is that for the 12 item-or-less checkout
lines, installation of the new self-service 12-item-or-less isles provided no increased
customer throughput. This was due mostly to the fact that of the 5 possible 12-item-orless checkout lines, the full-service lines were much more efficient in servicing customers
than the self-service lines. For the checkout lines that allowed an unlimited number of
items, it was also found that the addition of the new self-service lines actually limited
customer throughput. In fact, converting all isles to back to the original full-service
configuration provided the highest customer throughput.
So based on the original request, there is no customer throughput benefit to be had by
installing any additional self-service checkout lines. However, considering considering
the cost benefit that could be had by eliminating the checkout personnel, a financial
benefit outside the scope of this study may reveal a significant operating cost reduction.
There are still a lot of customers that prefer the interaction with a live check-out associate
and would rather not deal with the added complexity the self-service systems provide.
It’s intimidating to some of our customers.
Introduction:
Stop & Shop currently provides two grocery checkout methods: full-service checkout and
self-service checkout. The full-service checkout lines use the traditional checkout
method. Customers load their groceries or items onto a convey, a Stop & Shop associate
scans the items while another associate, if available, bags the scanned items and places
the filled bags in the customers shopping carriage. The self-service checkout method
allows customers to scan their items, pay for their purchase and bag without the
interaction of any Stop & Shop associate. Of the 23 total checkout service lines, Stop &
Shop recently replaced 5 full-service checkout lines, with self-service checkout lines
while the remaining 18 service lines retain full-service status. The entire checkout area
currently occupies 146.5 feet of floor space at the front of the store. Within this space,
three fundamental checkout line widths exist. Self-service lines are 5.5 feet wide. Most
full-service lines are 6.5 feet wide while 4 full-service handicap accessible lines are a full
7 feet wide. Although the new self-service lines appear to process customers a bit slower
than any of the full service lines, their narrower width, the result of eliminating a
checkout associate, may prove to be beneficial by allowing the installation of more selfservice checkout lines in the same floor space. Aside from the obvious reasons for
installing the self-service checkout lines (i.e. the elimination of some checkout
associates), Stop & Shop wants to know whether the overall customer throughput could
also be improved by replacing more full-service checkout lines with the narrower selfservice lines. This focus of this study was to determine the optimum checkout line
configuration that maximizes customer throughput at the maximum customer service
load.
Current Checkout Area Layout - Locations:
As mentioned earlier, 23 checkout lines accommodate the customer checkout as shown in
Figure XXX. As the figure shows, the five self-service checkout lines reside on the far
left had side of the checkout area. Also identified are those lines that target customers
with 12 items or less as well as those lines designated for handicap access.
Full Service Checkout Lines
As mentioned earlier, each standard full service line, whether it is designated to serve
customers with an unlimited number of items or customers with 12 items or less,
measures 6 ½ ft wide as depicted in Figure XXX. For convenience, 4 of the 18 total full
service checkout lines measure a full 7 ft wide to enable handicap access. The current
full service checkout area utilizes 119 ft of floor space. Of the 18 full service lines, 2
lines are designated for customers with 12 items of less with 1 of the 2 lines 12 item or
less lines for handicap access. The remaining 16 full service checkout lines currently
allow service for customers with an unlimited number of items and are comprised of 13
standard width checkout lines and 3 wider handicap access lines. Each unlimited
checkout line requires two checkout associates, the scanning associate and a bagger. The
two checkout lines that accommodate 12 items or less only require one associate to
perform both duties, scanning and bagging.
In general, three processes take place in the full-service checkout system. There are
customer arrivals, services, and exits. Arrivals occur in all lines on the left hand side of
the checkout line. The customer loads their items onto a conveyor where they are fed to a
checkout associate. The checkout associate scans the items and, depending on the line
type, will either bag the items or send them off to a bagging associate. Near the end of
the checkout process, the customer proceeds to a payment location, pays the checkout
associate and then exits the checkout system.
Self-Service Checkout Lines
Each of the 5 self-service checkout lines shown in Figure XXX measure 5 ½ ft wide,
regardless of their designation (12 items or less or unlimited). Upon arriving at the
checkout line, customers congregate in the usual fashion, which is at the entrance to the
checkout line. Unlike the full-service system, items are scanned at the beginning of the
checkout line and are then placed on a series of conveyors. The conveyors carry the
items to a gathering area where it is then the customer’s duty to bag their own groceries.
The gathering area can easily accommodate up to 20 items. However, depending on the
number of groceries a customer has, a situation may arise where the customer will have
to stop scanning items because no other items can fit in the gathering area and the system
has begun to back up. When this occurs, the customer must remove (bag), the groceries
in the gathering area before additional items can be scanned. Once all items have been
scanned, the customer pays through an automated payment system. At this time, coupons
can also be scanned. The self-service system will accept all forms of payment, the easiest
of course being performed using a credit card. Once the payment process is complete the
customer exits the system.
Customer Processing – Events:
Regardless of the service line configuration, each customer follows the same event
sequence. Customers arrive, service is performed, and customers exit. In real life
though, many other mini processes take place that are very difficult to quantify. The
following event sections contrast the differences between actions that happen in real life
to those that can actually be quantified for simulation.
Arrivals
Each of us have been to the super market and encountered this same scenario. When it
comes to checkout and based on our past experience, a whole litany of subjective logic
processing goes through our mind when selecting the checkout line that will get us
serviced the fastest. At first, typically we will observe each of the open checkout lines to
determine which appears to have the shortest arrival queue. Usually the observation only
extends to within 5 or 6 checkout lines away from the initial approach into the checkout
area. From these few lines, we’ll then observe how full the carriages or basket are for the
customer(s) in front of us. Then based on the number of items in the proceeding
customers, checkout lines selection is made. This may mean that we might select a
checkout line with an arrival queue that has more people in it than another arrival queue,
but the proceeding customers have far fewer items than the arrival queue with fewer
customers. However, sometimes, the advantage of one line over the other is
indistinguishable. If shopping alone, one might find them self shifting back and forth
between lines based on each lines instantaneous progress. Or if shopping with a
companion, one might reserve a turn in one line while the other remains with the carriage
or basket in another line, all the while continuously assessing the progress of each
respective line to once again, determine what will get them serviced the fastest. This
technique however complicates the selection process for newly arriving customers.
Newly arriving customers often will have this technique unveiled to them at the most
inopportune time. The newly arriving customer may have made a line selection with the
premise that a line appeared faster based solely on fact that each proceeding customer
appeared to have fewer items for checkout than any of the other lines. After a short
amount of time in the arrival queue, the ploy unfolds to the chagrin of the newly arrived
customer so that now the carriage bearing companion moves into the line of their
compatriot who originally appeared to have little or no items for checkout. Does the
newly arrived customer leave the arrival queue based on the new circumstances or not?
Sometimes they do and sometimes not.
For the full-service checkout lines, the arrival process continues into the checkout line
itself with items being loaded onto the feeder conveyor. Termination of the arrival
process is signified with the scanning of the new customers first item.
For the self-service checkout lines, observation revealed that patrons would enter or not
enter based on the number of customers in the line, number of items in a proceeding
customers shopping cart or basket, or the confidence level exuded by the proceeding
customer. Those lines that serviced customers that appeared slow to process items were
passed over for other lines with often more patrons but that seemed to move at a pretty
smooth pace.
Servicing – Full Service
By the time the customer is about to begin service, some if not most of their items have
been loaded onto the feeder conveyor. Servicing is initiated with either the scan of a
“scan saver” card, which acts like a virtual savings coupon, or the scan of the first item.
During the scanning process, the customer may continue to load more items onto the
feeder conveyor as more room becomes available, fill out preliminary information in
their checkbook, or may assist with the bagging of scanned items. Once all of the items
have been scanned, coupons, if any, are scanned. Payment arrangements are negotiated
and the payment transaction is completed. If not all the items have been bagged, the
checkout scanning associate will also assist in bagging the remaining items. Once all of
the items have been bagged from the collection area, the next customer begins
processing, even before the proceeding customer has left the checkout line.
On occasion, significant delays can be had if an item is not properly marked and does not
have a price tag on it. If this situation occurs, another associate is called upon to retrieve
another item that is properly marked or that has a legitimate a price on it. These types of
situations can delay the servicing process several minutes, which is a high percentage of
the total time that the customer spends getting serviced.
Servicing – Self Service
The self-service system does for the customer exactly what its name implies. It allows
the customer to scan their own items, use coupons, and pay on their own without the
interaction of any store associate. From time to time, a problem will arise during the
checkout process. To handle these instances, Stop & Shop has up to three customer
service representatives that circulate between each of the five self-service checkout lines
to assist customers in their checkout process.
Unlike the full service checkout system, self-service processing begins at the beginning
of the self-service checkout line. Customers arrive at the self-checkout line and begin
scanning immediately. There is no intermittent loading of a feeder conveyor. Items are
taken directly from the carriage or basket, scanned at the scanner, and placed on a
conveyor where they are taken to a collecting area of the checkout line. Now here’s
where the checkout process can get a little complicated if the whole process is not
executed flawlessly. If a customer misses the scan of an item but places that item on the
conveyor anyway, the system will be alerted to the fact that an item has passed through
the system without being scanned. In this instance, typically the conveyor will reverse
direction to bring the item back to the customer. More often than not, though this will
cause a failure in the system requiring one of the customer service associates to assist the
customer with the problem. Customers with coupons also face a high chance in
processing delay as the coupon entry process can be very sensitive. Also complicating
the checkout out process is the handling of produce since there are no barcodes on most
produce items. For this, the self-service system provides a lookup system through which
the customer searches for the correct produce item match. Lack of familiarity with the
system often delays the checkout process.
Once the customer has finished scanning their items, the coupon process is completed
and the customer selects the finish and pay option. The self-service system currently
accepts credit and debit cards directly as well as good old cold cash. Checks can also be
accepted, but customers will have to proceed to one of the customer service associates for
further assistance.
Exits
Regardless of the checkout system, all system exits are the same. Each exit usually
entails the customer gathering them self together while still in the checkout line. With
receipts and wallets stowed, the customer proceeds to exit the checkout line and merge
into all other exiting traffic. Occasionally exits can be delayed if an item has been
forgotten to be scanned because it was under the carriage and wasn’t noticed during the
scanning process. For this, the item is then rung up and the customer will have to
complete the paying process all over again.
Simulation Approach:
Given all the above arrival, processing and exit descriptions, it became evident very
quickly that accurately simulating the system checkout process could become a daunting
task. Furthermore, my request at the local Stop & Shop to collect data was well received
but limited the scope to which data could be collected. I was allowed to take as much
data as I wanted so long as my observations were unobtrusive and I did not interact with
the customers or get in the way of the whole process. Given this restriction, at best the
only data that could be compiled had to be limited to what I could see without getting too
close to the checkout system. For this, all that could be observed were arrivals and the
servicing process, which enormously narrowed the scope of this simulation effort.
Therefore, a number of assumptions and guidelines were established to more
appropriately fit the scope of this project given the amount of data that was available.
These assumptions are outlined in the following section.
Locations:
12 Items-or-less
1 line will remain full-service and handicap accessible.
4 lines may be full or self-service.
Unlimited number of items:
3 lines will remain full-service and handicap accessible.
15 lines may be full or self-service
Checkout Floor Area Width
Although the current configuration exists within a discrete floor area envelope, it is
foreseeable that when optimizing the various checkout configurations, slightly more or
less floor space will be required to accommodate the optimized throughput configuration.
Therefore, it will be acceptable that floor area breadth required be no more than an
additional 50% of the widest checkout line, or 3 ½ ft.
Events:
Inter-Arrival Times – For the purpose of this study, which is to determine the optimum
service line configuration that will allow the maximum customer throughput at the
busiest time of day, arrival data will be taken at the busiest time of the day and will be
assumed to by the typical arrival distribution between the hours of 10:00 am and 2:00 pm
(4 hours). This of course is not really an accurate assumption, however, given the focus
of this study, where we are only concerned with the throughput at the busiest time of the
day, this is acceptable. Furthermore, since no additional data will be available other than
just the pure inter-arrival times, like all of the subjective logic processing that typically
takes place, recorded inter-arrival times will assume that all arrivals take into account all
of the subtleties normally encountered with an arrival.
Service Times – Like the inter-arrival times, it will be assumed that the raw service times
are truly characteristic of customer processing, regardless of whether the checkout
process is flawless or has multiple failures. This should be sufficient enough for the
purpose of this study without having to investigate the more subtle and invasive details of
the checkout process.
Exits – Exits will not be specifically tracked when collecting data. For the purpose of
this study, it will be assumed that the proceeding customer will have exited the system
before the next customer begins service. Therefore customer exits will be immediate.
Exits will however be tracked if there are no new customers arrivals by the time the
current customer finishes the checkout process. This allows the service end time to be
determined is there are no customers immediately entering the checkout line for service.
Modeling Approach:
Fortunately, the processing that occurs at the checkout line is a simple process,
essentially a single server system that can be represented by a GG1 or am MG1 (one
instance) queueing process. However, with 4 locations designated in the checkout
process; the arrival location, the arrival queue, the checkout location and an exit location,
23 total checkout lines with 4 locations per line exceeds the modeling capacity of
ProModel’s Student Version. Because of this, two modeling requirements became
disallowed. First, the entire checkout system could not be modeled all at once, and
second, because the entire system couldn’t be modeled, Sim Runner also couldn’t be run
to optimize the checkout system. Granted the number of locations could have been
reduced from 4 to 2 (an arrival queue and a service location), but with 18 + full-service
combinations alone, this would require a minimum of 36 locations which far exceeds the
Student Editions capacity.
To simplify the process, it was then assumed that each checkout line type would perform
consistently for that checkout line type. What this means is that by simulating the
performance of just one line, with little standard deviation, the total number of exits per
hour could be calculated. This data could then be input into an excel spreadsheet where
all combinations could be analyzed in a table. Given this approach, each processing type
was run until so that the 95% confidence interval was satisfied. Once this condition was
met, the average customer throughput per hour was calculated and input into an
optimization spreadsheet as shown in Figure XXX. Two separate sections were set up in
the spreadsheet. The first section was used to analyze the Unlimited Item checkout line
combinations, while the second section was used to analyze the 12-item-or-less checkout
line combinations.
Data Collection:
A total of four hours (2 people collecting data for 2 hours) of data was collected on the
processing of customers through each of the four types of checkout lines. A Sunday was
chosen, between the hours of 11:00 am and 1:00 pm to collect data as this time has been
found to be, in general, one of the busiest times of the day for the supermarket. When
recording arrival data, a customer arrival was recorded when a customer actually entered
a line for service. Customers that slowed to where a line began but then moved to the
next (better) line were recorded to have arrived once they entered the line that they stayed
in, not when they entered the arrival line area. When collecting service time data, service
was recorded as having begun when the customer’s first item was scanned, grocery or
scan-saver card. Service ended either when the next customer in line scanned their first
item, or in the event there wasn’t another customer in line, when the customer exited the
checkout line all together. The arrival and service time data can be found in Appendix
XXX.
Data Processing:
With the data collected, each arrival and service data point was entered into Stat::Fit to
determine the best analytical representation of the data. Once in Stat::Fit, all of the
standard data processing tests were performed on the data, which included running the
autofit function, on each of the eight data sets (4 arrival data sets and 4 service data sets).
The top four best analytical data fit approximations were then tested in each of the four
simulation models for accuracy and to determine whether the results made sense. Some
of the analytical approximations that scored the highest in the autofit test performed
terribly in when applied to the simulation models. For the arrivals, one criteria that was
used to determine whether the analytical arrival rate approximation was valid was to run
the simulation and then look at how many customers accumulated in the arrival queues.
In practice, not more than 4 or 5 customers would be waiting in any of the arrival queues
at any one time. If the simulation predicted more customers waiting in the arrival than
ever observed, the analytical arrival rate was rejected and another arrival rate tested. It
should be noted though that the length of the arrival queue is also closely coupled to the
service times. This often resulted forced the running of numerous arrival and service
time combinations before a sensible combination could be arrived upon. Fortunately
though, service times were usually well behaved and gave few good options aside from
the better fit analytical approximations. Therefore, more often than not, analytical
service times were represented by the highest ranked curve fits.
Results:
After completing each of the simulation scenarios, it was found, as shown in Figures
XXX and XXX that Stop & Shop gained no additional customer throughput with the
addition of any self-service checkout lines. Therefore the original hypothesis that
additional customer throughput could be had with some combination of full and selfservice lines by the virtue that the self service lines were narrower, was false. However,
given the savings that could be had by eliminating some of the staff required to man the
full service lines, it was found that there were other line configurations that did reduce the
manpower required but still maintained similar customer throughput. They are presented
in the following sections.
Unlimited Item Service
As Figure XXX shows, a maximum customer throughput of 446 customers per hour is
achieved with all unlimited checkout lines configured as full service lines. In retrospect,
this makes perfect sense since the efficiency and fluidity of the checkout operation is no
better achieved than in the hands of a trained checkout associate. However, if Stop &
Shop’s ultimate goal is to minimize operating costs at the expense of reduced customer
throughput, than some other options exist. The current installation, which utilizes 3 selfservice unlimited item checkout lines with the remaining 15 checkout lines configured for
full-service operation, can achieve a simulated throughput of 426 customers per hour.
Analysis showed though that another checkout service line configuration that utilizes 7
self-service checkout lines and 12 full-service checkout lines could yield a simulated
throughput of 425 customers per hour, just one customer less than the current
configuration. The 7 self-service checkout line configuration is able to compete with the
current 3 self-service line configuration by allowing the installation of an additional
checkout line without taking up more floor space.
12 Items-or-Less Service
As shown in Figure XXX shows, the maximum customer throughput of 338 customers
per hour for customers with 12 items or less is achieved, as was true with the unlimited
item service lines, with each checkout line configured for full-service processing. Like
before however, if operating cost becomes a heavily weighted attribute to trade,
especially considering that the number of items per customer is so low for this type of
checkout line, the current configuration may provide a savings not quantifiable through
this study even though its throughput is significantly less at 258 customers per hour. If
Stop & Shop considers this worth the loss in throughput, they should also consider
having 3 self-service checkout lines as this configuration will still enable a throughput of
218 customers per hour. Ultimately, because there are so few 12 item of less checkout
lines, there is little latitude when trying to take advantage of the size benefit offered by
the narrower self-service checkout lines, for any real throughput performance to be
realized.
Conclusions / Recommendations:
If everyone worked for free, ie. Employee cost was not an issue, the hands down
conclusion from this study is that Stop & Shop should replace all of the new self-service
with the old full-service checkout lines, regardless of the checkout line type. However,
nobody works for free so the decision becomes a bit more complicated and unfortunately,
is best answered with the aid of collaborating data outside the scope of this study. Other
data that would also help to understand the process would be the number of items each
customer purchases and at what cost. It was observed that customers that had very full
carriages typically migrated to the full-service checkout lines. This data would further
help justify the existence of the checkout associate since not only do the full-service lines
service more customers per hour, on average, they’re also moving more items through the
system which compounds the profits.
After speaking with the Stop & Shop manager, the real reason for Stop & Shop’s recent
installation of the self-service checkout systems is that, on the off peak hours when few
customers are in the store, usually at around 11:00 at night, the new 5 self-service lines
are always be available and open to service these customers. And for these times, when it
is difficult for Stop & Shop to predict the number of checkout associates needed to staff
the checkout system, S & S can always plan on having at just one full service line open
with the overflow being handled by the self-service systems.
In closing, a conclusion can be had from this entire study and from that comes a
recommendation. It is clear that there is no immediate replacement for the efficiency and
capacity of the full-service checkout system, at least not for the next 15 years. There will
always be a need, for one reason or another for the full-service checkout system. But,
since Stop & Shop has gone beyond the point of mere alternate system consideration and
have actually installed an alternative checkout method, it would follow that if they will
entertain one system, they will entertain others. My suggestion to meet the requirements
of both worlds is to replace all of the current checkout lines with a hybrid system, a
system that would enable both self-service and full-service. With this, Stop & Shop
would have the best of both worlds… maximum customer throughput on those super
busy occasions and nearly unrestricted throughput the rest of the time. In fact, with so
many checkout lines open for service, arrival queues would more than likely shorten
allowing for more compact arrival queue areas in the store. The details of such a system
have yet to be worked out fully, but are best left as a venture for the suggesting party.