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A single line to 3 cashiers is ~3x faster than a separate line for each cashier

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Re: A single line to 3 cashiers is ~3x faster than a separate line for each cashier

#131

Post by cynicalkane »

philwelch wrote: post #73
rdtsc wrote: post #43

Earlier quoted context omitted.

Not trying to be condescending but doesn't it surprise anyone here that this is _not_ obvious? At least on this site, there are still people doubting or debating this. So I am wondering why don't stores do this already. And I believe it is because of perceptions. They understand that time will be saved, however, they realize that most people will be scared by a long line. One long line that moves fast will still appe…

It's another instance of "people are stupid". Yes, for any educated person it's obvious that one line is faster, that's the first thing about queueing theory, but most people don't know the first thing about queueing theory. Other instances of "people are stupid" leading to poor design: * Some cars come with a CVT (continuously variable transmission) rather than having to shift gears. The audible pitch of the engine…

Most Nissans have a CVT now, and the ones I've driven seem to shift continuously. BMW tried implementing a discrete-shifting CVT like the one you describe for one of their semihybrids, and though I've never driven it, it got slammed in reviews. Do you have a citation for your claim?

Re: A single line to 3 cashiers is ~3x faster than a separate line for each cashier

#132

Post by afiler »

theorique wrote: post #52
smackfu wrote: post #28

What I didn't see in this article is why retailers don't do this today. I see a lot of positives listed and no negatives, so it seems like a no-brainer, right? Yet the biggest retailers like Walmart and Target have tons of lanes, and even do staggered front/back lanes which are terrible for the customers. Is it because there are negative feelings to seeing a long line, and that people do not expect it to be fast so t…

In the UK, Tesco and other grocery retailers do this. I've seen this in place in the USA at the Whole Foods at Columbus Circle (an enormously crowded store that moves along well). Based on these data points, it would seem as though more crowded places use this optimization.

Lund's (a snooty high-end grocery chain in Minneapolis) set up their new store in Old St Anthony (a fairly high-density area) this way too.

Re: A single line to 3 cashiers is ~3x faster than a separate line for each cashier

#133

Post by nlawalker »

ryusage wrote: post #95
nlawalker wrote: post #9

Most of the grocery stores in my area have installed self-checkout systems that are especially great if you just have a handful of items that all have UPCs (no produce or bagged bulk), don't have any items requiring an age check and possess at least the intelligence of a fifth grader. I mention this not just because it's relevant to the idea of waiting in line at a store, but because in every store where I've seen th…

The most interesting fact about the self-checkout systems, in my mind, is that they actually have no designated area or instructions for how lines should form. People just intuitively wait outside the general self-checkout area, and use the machines as they become available. So, at least in some circumstances, the single queue system obviously can make a lot of sense to people.

This is the interesting part to me as well, although I do have a couple of anecdotes:

- I have seen at least one store where a single sign is placed at the front of the full collection of scanners stating "line forms here" or something similar, regardless of how many there are or how they are arranged. - I have seen at least one case where, in a store that did not have the above sign, one line formed for each "vertical" bank of scanners. For example, in my local Safeway, there are six scanners: two rows of three each, lined up next to each other, parallel to the checkstands. It is pretty intuitive/obvious that they don't intend for people to line up at each individual unit in a single row, but often a queue will form for each row.

Re: A single line to 3 cashiers is ~3x faster than a separate line for each cashier

#134

Post by seiwyn »

Okay, I am confused.

Assuming rebalancing, it seems to me that your wait time EV is the same either way, with less variance in the single queue case. Thus the shopper throughput should remain the same. Can someone explain to me if/why this is wrong?

Also, where did the HN title come from? I could not find that claim in the article.

Re: A single line to 3 cashiers is ~3x faster than a separate line for each cashier

#135

Post by andrewla »

dxbydt wrote: post #79

scala> import org.apache.commons.math.distribution.{ExponentialDistributionImpl=>expo} //pc = process customer, with a mean time mu per customer scala> def pc(mu:Int):Double= new expo(mu).sample // q = queue of n customers with mean process time mu scala> def q(n:Int,mu:Int):Double=(1 to n).map(_=>pc(mu)).sum scala> // put 3000 people in 1 queue with mean process time 50 scala> (1 to 1000).map(_=>q(3000,50)).sum/1000…

Okay, someone with some Scala expertise needs to jump in here. The code as written is _clearly_ incorrect. q(n,mu) sums a distribution with mean mu n times. The expected value of this is mu * n. Then we run this 1000 times and average the results. If this is not extremely close to mu*n then I will eat my hat. In R, sum(replicate(1000,sum(rexp(3000,1/50)))) / 1000 returns ~150000 (using sum for clarity instead of mean). Maybe scala is caching the seed or something?

I don't know what the "par" function does, but assuming it doesn't do anything crazy, the answer for all three of those expressions should be exactly 150,000, because they just reorder the order of summation.

Re: A single line to 3 cashiers is ~3x faster than a separate line for each cashier

#136

Post by nlawalker »

gujk wrote: post #113
alsocasey wrote: post #108

Earlier quoted context omitted.

Isn't this essentially equivalent to having a single queue though... except now you have a kind of cache at each register holding a small number of customers?

Yes, and it is how real stores work, which is the point.

Except that the "primary" queue doesn't feed into the shortest register queue, it feeds into the register queue that the person at the front of the primary queue anticipates will empty the fastest. That's the premise that underlies the whole article: people get in the line they predict will move the fastest, and everyone uses different variables and judgment to make that prediction. Those variables and those predictions don't show up when your model is based on distributions.

The register queues are not independent, but they are not related as simply as you say.

Re: A single line to 3 cashiers is ~3x faster than a separate line for each cashier

#137

Post by Someone »

alsocasey wrote: post #121
gujk wrote: post #113

Earlier quoted context omitted.

Yes, and it is how real stores work, which is the point.

Right. I wonder what would happen if we try to include a measure of perceived fairness from the customer's point of view into this type of analysis. I always feel I'm picking the wrong queue at stores... single queues going to whatever cash is open seem fairer, if that makes sense.

Perceived fairness is very hard to quantify objectively. Is it fair if there are queues for people buying less than X items? If there are queues for people not paying cash? For frequent flyers? That you, by sheer misfortune, end up in the slow queue three times in a row? Should pensioners wait for people having work to do? Should fit people make room for the elderly? To what extent? Etc.

Instead, I think you should do the math; it will teach you that the probability that a given queue is slow is smaller than the probability that, given a person in a queue, that person is in a slow queue.

So, you spend more time in slow queues than in fast ones. Once you know that, train yourself to accept it.

Re: A single line to 3 cashiers is ~3x faster than a separate line for each cashier

#138

Post by MartinCron »

mc32 wrote: post #92
AndyNemmity wrote: post #86

Earlier quoted context omitted.

I go to Fry's a ton, and it always seems to go much quicker, but I agree, psychologically standing in a tremendously huge line feels worse.

--which is why Delis‡ traditionally give you a numbered tag. It's a single line, but you can go about and not feel "constricted" to the queue (tail). Don't forget, Frys also has a "semaphore" who cues people to the next register. ‡among many places like hospitals, information centers (DMVs), etc.

My favorite hardware store (Dunn Lumber in Seattle) does the same thing. It was difficult to get used to at first, but it's really nice to just take a number, wander around, and then not feel like you're holding anyone up if you don't order your lumber as fast as is humanly possible.

Re: A single line to 3 cashiers is ~3x faster than a separate line for each cashier

#139

Post by finisterre »

The article cites Apple's roving check out approach as an attempt to "solve line issues". Isn't this close to the most inefficient way to organize the checkout/queuing experience in terms of creating uncertainty and the need for everyone (even their staff) to move around? I can see that it has a lot of psychological value, though.

Re: A single line to 3 cashiers is ~3x faster than a separate line for each cashier

#140

Post by xsmasher »

rms wrote: post #53
smackfu wrote: post #28

What I didn't see in this article is why retailers don't do this today. I see a lot of positives listed and no negatives, so it seems like a no-brainer, right? Yet the biggest retailers like Walmart and Target have tons of lanes, and even do staggered front/back lanes which are terrible for the customers. Is it because there are negative feelings to seeing a long line, and that people do not expect it to be fast so t…

It's being rolled out slowly. Target in Emeryville has a rather advanced queuing system like this, but customers definitely don't like it and don't necessarily realize it's faster, perhaps because Target has less cashiers as a result of the decreased wait. Edit: Yelp says this line system has been eliminated. http://www.yelp.com/biz/target-emeryville-2

I went through that system a few times. They had a set of three lines (A,B,C) that fed a bank of cashiers; a screen overhead flashed instructions like "A => cashier 7", then "B => cashier 5" and then "C => cashier 1".

It felt more stressful, efficiency be damned. You spend more time at the "front" of your line (A,B,C) , waiting for the board to change. In a traditional system you can hook in behind someone and "zone out" / chat with your SO until you reach the register. In this system I felt like I had to pay attention the whole time and I was interrupted every time the bell rang, even if it was A's turn and I was in line C.

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