5 ms·

I've largely given up trying to optimize my shopping time, but I do find the subject interesting. I'd be interested in seeing the effect of the (to me intuitiv

by jfb 16y ago

I've largely given up trying to optimize my shopping time, but I do find the subject interesting. I'd be interested in seeing the effect of the (to me intuitively superior) one line, many cashiers setup, like at Fry's. I realize supermarkets make a boatload of cash from the geegaws sold in line that you hounded your parents into buying you as a kid, but I can't help but think that the many lines/many cashiers setup is less an optimal setup and more an artifact of some Lionel Hutz type who sold the cabal of Evil Grocery Barons a bag of magic beans in the 1960's. As usual, I am too lazy to go look it up for myself. Back to hacking.

kgermino 16y ago
I cant easily look it up myself right now (am on a mobile device) but I believe that the many cashiers many lines approach is faster from an average wait time perspective. However people tend to prefer the many cashiers one line approach because it instills a sense of fairness (first come first served).
- ojbyrne 16y ago
  From what I recall of queuing theory that's not actually correct. From wikipedia: "One observational insight provided by comparing queuing models is that a single queue with multiple servers performs better than each server having their own queue and that a single large pool of servers performs better than two or more smaller pools, even though there are the same total number of servers in the system" http://en.wikipedia.org/wiki/Queueing_model#Multiple-servers_queue http://en.wikipedia.org/wiki/Queueing_model#Multiple-servers...
  - kgermino 16y ago
    It seems you're correct. A quick search is giving me nothing to disagree with you. That being said: My understanding was that the many-line system worked better primarily because of the potential for a lot of lost time between the server saying "Can I help whose next" and the order actually starting. This time is obviously hugely dependent on factors such as floor layout, space around the registers, customers familiarity with the system, etc. The idea was that if you have one line for multiple registers (a la Wendy's) than when the second register opened up the next customer in line had to notice, go to the register, probably walk around the person ordering at the first register, and then start ordering. Whereas in the multi-line system (a la McDonald's) the next customer in line will likely be focusing on the register in front of them ready to take the one step forward when their time comes. These factors can certainly be minimized but in stores that are busy/crowded/not well laid out it they can make a noticeable difference. That's my $.02 anyways.
nikster 16y ago
One queue minimizes the average wait time - it's the fairest and fastest system. Any CS person should know this, it's pretty basic. Say we have 100 people and 10 servers. Everyone takes 1 minute to process, except one guy who needs 10 minutes. With 10 queues and the long taking guy ahead of 9 other people he's blocking all those for 10 minutes. Total processing time 19 minutes. With 1 queue and the long taking guy first (blocking 1 counter for 10 minutes) total processing time is 11 Minutes. Another way to look at it is that with one queue, everyone gets the minimum possible wait time. In the many queues case you get that only occasionally. You could optimize the many queues so they're just as efficient as the one single queue but you'd have to know everyone's processing time in advance - impossible in a real world setting.
- dkokelley 16y ago
  You could optimize the many queues so they're just as efficient as the one single queue but you'd have to know everyone's processing time in advance - impossible in a real world setting. Not quite impossible. You might not know individually, but on average for a group or segment you could get this information and optimize. Banks can do this by splitting consumer and merchant lines. Merchants typically take longer (who knows by how much), and get their own queue. With this system, you have 2 queues, 1 for each group. There may be a lot of variation within each group, but the averages work out. Consumers wait the average minimum processing time for the consumer group, and merchant wait the average processing time for the merchant group. This still leaves room for underutilization. What if the merchant line is backed up and the consumer line is empty? Ideally, the extra tellers would work to pick up the slack, but if this isn't possible then it's still not an ideal system.
  - aquark 16y ago
    Another common example would be airport check-in. You have economy and first-class. Often the first-class line is empty - do you let 'the rabble' check-in there and risk a first class passenger having to (gasp) wait a couple of minutes behind them; or make the long line of economy passengers stare at the unused desk and mutter about the airline. (Though given that airports are really just a series of queues feeding each other the different in practice is pretty minimal.)
MichaelApproved 16y ago
The problem is the limited resource of space. One line, multiple cashiers takes up more space than several cashiers, one line. The store needs to add the queue system and have a person standing at the front to make sure the line moves smoothly. Think about that large waiting area Fry's has. Another point is that 6 people standing on a single line for 2 cashiers seems like a longer wait than 3 people standing on two lines.
throw_away 16y ago
Fry's wraps their mono-line with geegaw displays as well (usually electronics consumer geegaws, but it could be adapted to candy and trashy magazines). A cynic might wonder if they might actually want to maximize wait time, but I imagine it is, as someone points out below, a question of space limitations (carts require lots of space to navigate a rope maze). I note that the self-service stations slowly taking over the mega-grocery stores here employ a mono-line protocol. But, I find these inconvenient unless I only get a small number of items, which, of course, would require me to make more frequent trips, nulling out any possible time savings). Grocery delivery would seem the only way out.
chris11 16y ago
Commissaries on U.S. military bases use a one line multi-cashier setup, yet each cashier has a bunch of novelty items near their section. I seem to remember that setup be a little bit faster too, though it's not immediately apparent when standing in line.

by jfb 16y ago

kgermino 16y ago

I cant easily look it up myself right now (am on a mobile device) but I believe that the many cashiers many lines approach is faster from an average wait time perspective. However people tend to prefer the many cashiers one line approach because it instills a sense of fairness (first come first served).

ojbyrne 16y ago
From what I recall of queuing theory that's not actually correct. From wikipedia: "One observational insight provided by comparing queuing models is that a single queue with multiple servers performs better than each server having their own queue and that a single large pool of servers performs better than two or more smaller pools, even though there are the same total number of servers in the system" http://en.wikipedia.org/wiki/Queueing_model#Multiple-servers_queue http://en.wikipedia.org/wiki/Queueing_model#Multiple-servers...
- kgermino 16y ago
  It seems you're correct. A quick search is giving me nothing to disagree with you. That being said: My understanding was that the many-line system worked better primarily because of the potential for a lot of lost time between the server saying "Can I help whose next" and the order actually starting. This time is obviously hugely dependent on factors such as floor layout, space around the registers, customers familiarity with the system, etc. The idea was that if you have one line for multiple registers (a la Wendy's) than when the second register opened up the next customer in line had to notice, go to the register, probably walk around the person ordering at the first register, and then start ordering. Whereas in the multi-line system (a la McDonald's) the next customer in line will likely be focusing on the register in front of them ready to take the one step forward when their time comes. These factors can certainly be minimized but in stores that are busy/crowded/not well laid out it they can make a noticeable difference. That's my $.02 anyways.

nikster 16y ago

One queue minimizes the average wait time - it's the fairest and fastest system. Any CS person should know this, it's pretty basic. Say we have 100 people and 10 servers. Everyone takes 1 minute to process, except one guy who needs 10 minutes. With 10 queues and the long taking guy ahead of 9 other people he's blocking all those for 10 minutes. Total processing time 19 minutes. With 1 queue and the long taking guy first (blocking 1 counter for 10 minutes) total processing time is 11 Minutes. Another way to look at it is that with one queue, everyone gets the minimum possible wait time. In the many queues case you get that only occasionally. You could optimize the many queues so they're just as efficient as the one single queue but you'd have to know everyone's processing time in advance - impossible in a real world setting.

dkokelley 16y ago
You could optimize the many queues so they're just as efficient as the one single queue but you'd have to know everyone's processing time in advance - impossible in a real world setting. Not quite impossible. You might not know individually, but on average for a group or segment you could get this information and optimize. Banks can do this by splitting consumer and merchant lines. Merchants typically take longer (who knows by how much), and get their own queue. With this system, you have 2 queues, 1 for each group. There may be a lot of variation within each group, but the averages work out. Consumers wait the average minimum processing time for the consumer group, and merchant wait the average processing time for the merchant group. This still leaves room for underutilization. What if the merchant line is backed up and the consumer line is empty? Ideally, the extra tellers would work to pick up the slack, but if this isn't possible then it's still not an ideal system.
- aquark 16y ago
  Another common example would be airport check-in. You have economy and first-class. Often the first-class line is empty - do you let 'the rabble' check-in there and risk a first class passenger having to (gasp) wait a couple of minutes behind them; or make the long line of economy passengers stare at the unused desk and mutter about the airline. (Though given that airports are really just a series of queues feeding each other the different in practice is pretty minimal.)

MichaelApproved 16y ago

The problem is the limited resource of space. One line, multiple cashiers takes up more space than several cashiers, one line. The store needs to add the queue system and have a person standing at the front to make sure the line moves smoothly. Think about that large waiting area Fry's has. Another point is that 6 people standing on a single line for 2 cashiers seems like a longer wait than 3 people standing on two lines.

throw_away 16y ago

Fry's wraps their mono-line with geegaw displays as well (usually electronics consumer geegaws, but it could be adapted to candy and trashy magazines). A cynic might wonder if they might actually want to maximize wait time, but I imagine it is, as someone points out below, a question of space limitations (carts require lots of space to navigate a rope maze). I note that the self-service stations slowly taking over the mega-grocery stores here employ a mono-line protocol. But, I find these inconvenient unless I only get a small number of items, which, of course, would require me to make more frequent trips, nulling out any possible time savings). Grocery delivery would seem the only way out.

chris11 16y ago

Commissaries on U.S. military bases use a one line multi-cashier setup, yet each cashier has a bunch of novelty items near their section. I seem to remember that setup be a little bit faster too, though it's not immediately apparent when standing in line.