The meaning of 'between'

We have a UI which allows the user to specify criteria for the selection of customer records, and there's been some debate about the meaning of the term 'between'. For instance when the user seeks to find "everyone with between 2 to 4 children", should that include customers with 4 children or not?

Some of the team think that it's to do with data types - so that if dealing with integers or dates it would be inclusive at the upper end, and otherwise not. While others feel that it's more to do with cardinality so that if talking about income between 10K and 25K it would exclude the upper bound (i.e. up and including 24999, but not 25000), whereas if it was the number of bank accounts between 1 and 3, then 3 would be included.

There is a useful debate about the meaning of 'between' in Math Forum. In summary it says that instead of having, “the solution is between 10 and 20” (which is ambiguous as to whether 20 is or is not included), it may be preferable to say either “the solution is between 10 and 20 inclusively” or “the solution is from 10 through 20”. My preference is for the latter, but I'd very much appreciate the views of other UXers.

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You may get good answers on english.stackexchange.com, too. – Alex Feinman Nov 12 '12 at 16:04
Always remember the mantra Don't Make Me Think. If there is any ambiguity in there you'll negatively impact the userbase. To get around this issue I always set my breakpoints at 1-19, 20-29, 30-39 etc. That keeps it clearer that you can't be in two classes. However, this doesn't really work if you're going for decimals and would mean you can't use 25k as an abbreviation as you'd need to have the full number there. However I'd take greater clarity over cluttered screen real-estate every time. – JonW Nov 12 '12 at 16:27
The word between is definitely ambiguous, if not in its strict definition, at least to most of the users. Do not expect the user to understand it one way or the other. In short, avoid it. – Kris Nov 13 '12 at 5:35
@AlexFeinman ELU may be of not much help when the word is inherently ambiguous. – Kris Nov 13 '12 at 5:36
As a by-the-way, I would understand from 10 through 20, but only because I was brought up Jehovah's Witness, and so exposed to a lot of American English in my youth. – TRiG Dec 2 '12 at 0:08

Since there is the potential for confusion, why not omit the word "between" altogether? The sentence "everyone with 2 to 4 children" makes sense and means the same thing, provided you mean people with 2, 3, or 4 children.

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+1 The correct solution would be to avoid the word altogether, though the question does not give that option, but rather asks the implication of the word between instead. – Kris Nov 13 '12 at 5:33
@KimberleyA That still seems ambiguous to me; some might think your sentence includes those with 4 children, others that it does not. A colleague told me of a ticket booking website he was responsible for designing. There were special discounts available for children "up to 3 years of age" - some thought that their child of 3 was not eligible (i.e. 3 is not 'up to 3'), while others considered that their child of 3 yrs 11 mths (and hence still 3) should be eligible. – Phil Parry Nov 13 '12 at 9:32

When I've seen Between in technical use (like SQL) it has always been strictly inclusive. Inclusive is also the "safe" option so I would always keep Between to mean an inclusive selection. An exclusive between honestly doesn't make much sense, semantically it's more proper to say "more than X but less than Y", or at the least explicitly state "between X and Y exclusively" but that still sounds awkward.

I'm trying and currently unable to find an example where between is defined as exclusive. There's a number of discussions on this topic and the consensus appears to be "Between is inclusive by default".

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Without seeing the UI it's hard to comment on whether people will find it confusing and what some good solutions might be.

As for the English descriptions - with between/to/whatever - if you test I think you will find that some people will interpret it either way. There is no general "right" answer that some people will not interpret the "wrong" way.

In particular I've done several tests that involve questions like this

others feel that it's more to do with cardinality so that if talking about income between 10K and 25K it would exclude the upper bound (i.e. up and including 24999, but not 25000),

and I guarantee some folk will think "between 10k and 25k" should include 25k and others will assume that its 24999.

Surrounding context can also have a big effect. E.g. if a user sees a list like:

• 1-3 kids \$4
• 4-6 kids \$10
• 7-10 kids \$20

then they're going to be pretty certain that it's inclusive.

For search fields you can guide the initial assumptions with language (e.g if you talk about a "no more than [xx] kids" then people will think "inclusive").

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