Calculating the average wait time for a book on hold at the library (or "Why didn't they teach something useful like this in library school?"

October 13, 2011 11:11 PM

Is there an equation for figuring out the average wait time for a book on hold at the library. Or how to figure out the average wait time I have left for a book I put on hold because I'm dying to read the rest of it.

There's a lot of things I learned from working in libraries but the skill I really wish that I had was calculating the amount of time I'm going to have to wait for a book on hold. After reading a couple positive reviews for the Night Circus I put it on hold and forgot about it for awhile. Problem is I just ran across the Free Preview on Google Books, read it on my lunch hour, and am now impatient (but too cheap) to continue through the rest of the book.

Checking my library account hold list, I see that I am #13 on the list of people requesting this item. But in looking at the other details in the record (I never worked circ and metadata/taxonomy/IA is more my side of the street), now I'm wondering how I could use those details to calculate the average wait time:

I'm #13 on the wait list
The system has 29 copies in circulation
5 copies are due to be returned this week
4 copies are either in transit or waiting on the hold shelf and the system has a 10 day period for hold items to be checked out.

Would these be enough details to figure out an equation for how much longer (one average) I'd have to wait to read the rest of the story? Bonus Question - I'm sure OPACs probably don't even take into consideration this question, but is there even a calculation for this sort of thing? Or is that more in the world of Netflix queues?

posted by gov_moonbeam to grab bag (10 answers total)

You need to wait 13/29 times the average time that people hold onto their copies. This is the easy part.

The hard part is knowing how long people hold onto their copies, on average. As a first guess I'd figure two weeks here -- 10 days plus some time for transit -- so you've got about another week to wait.
posted by madcaptenor at 11:27 PM on October 13, 2011

You could elaborate on madcaptenor's generally correct assumption if you knew things like how long you're allowed to keep a book before you have to return it, whether or not you're allowed to renew it, how much late fees cost, etc.

If you're only allowed to keep books for 1 day, no renewals are allowed, and late fees are $10/day, then you will get your book tomorrow. If, on the other hand, you can keep books fora month and renew them indefinitely, and late fees are $0.05/day, then it might be a while.

If you know the maximum amount of time that people are allowed to keep a book, then you could figure out a reasonable cap on how long it will be until you get your book, but you might get it sooner if people tend to bring books back early.
posted by tylerkaraszewski at 11:34 PM on October 13, 2011

In my experience people often don't pick up their holds, so the wait time is shorter than you think. Depends on the book, too.
posted by weapons-grade pandemonium at 11:35 PM on October 13, 2011

Hmm. You'd have to knock out a lot of variables to get an equation that works. Because people don't pick up their holds, return books late, etc.

You can probably estimate that by the time this week (really next week) rolls over you'll be 13, your current position, minus the four that are on hold (knocking out the top 4 on hold people) making you number 9. The five copies that will hopefully be returned this week will most likely be sent to hold queue people, making you number 4 at the end of next week.

Assuming that there are books due to be returned next week, at least 4, I would guess that you would get it in the next week.
posted by titanium_geek at 11:43 PM on October 13, 2011

I made a Library check-in/check-out simulator. for this problem. It is very simple. It assumes people keep books for two weeks, and on any given day after their book has come in, they have a 7.5% chance of picking it up.

Refresh the page to re-run the simulation.

Anyone can play with/modify this if they like. Look at the values near the top of the source to tweak things. It's a pretty simple simulator.
posted by tylerkaraszewski at 12:48 AM on October 14, 2011 [4 favorites]

This "trick" only works sometimes, but look to see if your library also carries the LARGE PRINT edition. Although there are generally fewer copies in LP, usually there are far less (if any) holds on them.
(The bigger text can take a little getting used to—so you might keep your other hold in place—but if you're desperate to get some reading while you wait for the normal version to arrive...)
posted by blueberry at 12:50 AM on October 14, 2011

I would modify some of the above assumptions by swithcing the time people hold onto the books to one week or ten days, if your library does what mine does with new, popular books, and give them a one-week check-out time. That doesn't guarantee that people will return them in one week (especially if, again like my library, there are no fines for overdue books) but I would think it would tend to lower the average time that a user would hold onto a book to under two weeks).

Not a direct answer, but: Another trick I used to sometimes be able to use to bypass the hold queue at my home library system was to request it from the state-wide electronic library system, if there was a copy available. They recently updated the software so that you can't request a book through the statewide system if your local system owns it, but until then, I occasionally "cheated" in this way for a book I especially couldn't stand to wait for.
posted by not that girl at 5:41 AM on October 14, 2011

There's actually a theory of how to calculate these values--see Kleinrock's Queuing Theory.
posted by Obscure Reference at 6:13 AM on October 14, 2011

Yeah, queuing theory and basic simulator! I knew there had to be a way of figuring out my probable wait time. These answers are especially great because my late dad was always thinking about things like this. If you asked him casually about something like this, he'd come back 5 minutes later with a calculation. So thank you Mefites!

Following madcaptenor's suggestion would be (13 on the list/29 total copies)*(10 total days allowed for hold +21 days for a checked out item on hold) = about 13 days as long as transit time isn't crazy or people don't keep a copy overdue. Oh well, at least I'm 13 on a list of 203 requests!
posted by gov_moonbeam at 11:42 AM on October 14, 2011

Oh and yeah I've used LINK+ in the past but now they're not letting you get an item on the shelf from another collection when your system's copies are all checked out. Bummer!
posted by gov_moonbeam at 11:43 AM on October 14, 2011