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Sep. 23 2009 - 11:25 am | 371 views | 1 recommendation | 22 comments

Grocery Line Guessing


Image via dy/dan

Which one is faster?:

the express lane with more customers who have fewer items


the regular lane with fewer customers with more items

It’s a puzzle we’ve all encountered in real life. Of course, there’s no one answer. There are too many variables. Maybe the express line only has one more person than the regular line, and the woman in the regular line has 100 items and a coupon to go with each one. Or, maybe the regular line has a couple people with 20 items, paying by credit card, while the express line has three old ladies paying by check.

Still, there must be some rule that could help one make the decision: Which line do I join?

Enter Dan Meyer, who offers a fairly easy answer: While there are variables, the regular lane is probably a better bet. Here’s the reason:

The manager backed me up on this one. You attract more people holding fewer total items, but as the data shows above, when you add one person to the line, you’re adding 48 extra seconds to the line length (that’s “tender time” added to “other time”) without even considering the items in her cart. Meanwhile, an extra item only costs you an extra 2.8 seconds. Therefore, you’d rather add 17 more items to the line than one extra person!

The real time suck is people paying. (As Meyer notes, the y-intercept is not even zero. It should take zero seconds to buy zero items — but even the pleasantries of a person passing through the line take a non-zero amount of time ["Hi! How are you today!"].)

While this makes a lot of sense when you think about it, it’s definitely not intuitive. After all, lots of people are sucked in by the express-lane fallacy. Trying to figure out why it’s so counterintuitive, I couldn’t help but think of the study I wrote about in this post.

In that post, I looked at the “length = number” strategy. That is, if you present a child younger than seven with two rows of items (buttons, for example) and ask them to say whether the two rows contain an equal number of items or not, you’ll probably find that they base their answer on the length of the rows — regardless of the actual number of items in each row. While adults tend to override this cognitive bias, fMRI studies have indicated that we have to work extra-hard not to fall back on it.

Thus, I would guess most people make a related calculation at the grocery store: length = time in line. Maybe in the future now you’ll be able to override this bias and check out faster than your unenlightened grocery-store compatriots.

You’re welcome.


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  1. collapse expand

    As i consider waiting in grocery lines to be a particular form of insidious torture, i’ve been in on that bit of calculus for quite some time; and i’ll add another factor that has to be integrated.

    For instance, in the image above i would instinctively prefer the checkout with two 10-item customers; however, one also has to do a quick mental estimation of each of the clerks in action and even the customers in line. The grim reality is that some clerks are slow as slugs, and some chug along with machine like efficiency; nodding away those customers that like to use the grocery checkout as a chance to discuss the finer details of their children’s talent show. Furthermore, some customers can be pegged as ’swipe and go’ (like me) and some spend 15 minutes drafting a check like it’s an english exposition.

    Ultimately choosing the right line is a subtle, qualitative art that can leverage certain quantitative aspects and algorithms. You will never get your guesses 100% right; but, with enough negative reinforcement standing there exposed to the latest Branjelina Enquirer cover and herd of hungry consumers with baskets full of chocolate-chip-pancake-wrapped-sausages-on-sticks, the skill can be refined over time.

  2. collapse expand

    Why in both of your examples — the coupon woman, the check-paying old ladies — do you single out women as the slow pokes? At least Andy G. blamed gender-neutral “customers” for wasting time on talent show chat. I would rant more about this, but I have to go get in line for the women’s bathrooms at the stadium. While you’ll be back watching the game in about a minute, I’ll have to catch up with you in another 15 minutes!

    • collapse expand

      Didn’t think of it when I wrote the post. But I think it’s pretty well established that women are much more likely to use checks and clip coupons. (There’s an old Seinfeld joke about it, too: Men don’t use checks because it’s like handing the cashier a note from your mother.)

      If I had to choose a line just based on the gender of the people, I’d go with the male line every time. Though — exception — I’d go with an all-YOUNG line over an all-old line, regardless of gender mix.

      In response to another comment. See in context »
  3. collapse expand

    Good thing you’re young and male, then.

    If women are slowing you down using all those tacky little coupons, pay ‘em more, say equal to men. That’ll teach ‘em.

  4. collapse expand

    The grocery line (and the teller line) are fascinating examples of queue theory in daily life. Too bad WaMu wrote so many subprime mortgages- their teller lines were quite interesting as well.

  5. collapse expand

    I will usually pick the one with the cutest cashier, if for no other reason than my wish to keep my “dirty old man” yet not a lecher status intact. Everyone knows it’s always a crapshoot anyway, why not have some one nice to look at while waiting?

    A close second choice would be to get in the line behind an attractive woman shopper, why you ask? See reason one above.


  6. collapse expand

    One factor I take into account during visual cart assessments is the number of produce items. Produce is a time-eater! At least where I shop, these are not single-swipe scan items; there are 2 or 3 steps involved. More nearly perfect cart items are medium-size square or rectangular products that the checker flings over the scanner. Even the bagging process is faster.

    Then again, if there’s a new issue of People, I pick the most snafu-likely line.

  7. collapse expand

    You are forgetting some issues on the “less people but more items” line. More items means a bigger probability of an item not being priced (or wrongly priced) and of an item’s bar-code not registering (cashier having to enter the numbers manually). Besides that most people are really slow at bagging things.

    Here in Portugal in some supermarkets there are self-checkout machines. These are the obvious choice because, first, older people are afraid of machines and, second, after a quick initiation you can swipe your own products, bag them and go really easily and fast.

  8. collapse expand

    Interesting, but living in the south I’ve learned the age and racial profiling really does work. I would much rather be in long lines of young whites then a short line of old black women.

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    About Me

    I'm a freelance writer and blogger based in Brooklyn, NY. My background is mostly in politics. I've worked on the editorial boards of the New York Sun and New York Post. In 2006, I wrote a book, "The Elephant in the Room: Evangelicals, Libertarians, and the Battle to Control the Republican Party" (Wiley). I've also done my share of freelancing, for places like the Atlantic Monthly, The New York Times, Reason, and RealClearPolitics.

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