This was my original question

>"I shall take my umbrella just in case it rains later" is perfectly good 
>English and means 'I shall take my umbrella because it might rain'. 
>However, mathematicians and hence mathematical statisticians often use 
>'just in case' to mean something like 'provided that' or 'if and only if' 
>or 'uniquely under the circumstances that' or 'only in the case that' 
>which means something very different. If this were the everyday English 
>usage then the anti-pluviary strategy implied would involve only taking an 
>umbrella if it were raining rather than because it might.
>
>At least, that's my interpretation
>
>So some questions to the list regarding 'just in case'
>
>a) Do you agree with my understanding of the everyday English usage?
>b) Do you agree with my understanding of the mathematical usage?
>c) If so, does anybody know when this egregious error first crept into 
>mathematics?


I received 19 replies in 18 of which an opinion was expressed on point a)

Everybody replying agreed with a) although Michael Dewey pointed out to me 
that 'just' was superfluous for this meaning and John Whittington that use 
of 'just' also implied that the probability of rain was low. (18/18)

As regards b) 9 had not come across this usage but 4 had and of the 9, one 
had consulted a collegaue who was a mathematician who had come across it.

Kevin McConway sent this interesting set of comments

"(a) Yes, I agree, and so does the OED, which my university library
handily makes available to its staff and students on line.
(b) Yes.
(c) Hmm. It's not new. I would have said, before looking up the OED,
that it probably arose fairly recently from a non-native speaker of
English getting confused between "in case" and "in the case". But I'd
have been wrong, I think. In the online OED you can search for phrases,
and "just in case" in your mathematical sense appears (in the entry for
'"joint") in a quotation from "Mathematical Logic" by W.V. Quine, 1940.
(Quine was American and, I believe, a native English speaker.) It is
also there in this usage in other later quotations (under
"phenomenally", "rhematize" and "symmetric" (the last definitely
mathematical, saying "A binary relation on a set is an equivalence
relation just in case it is symmetric, reflexive and transitive"). But
there are many more quotations (the oldest being from 1924) that use the
phrase in your "everyday English" usage.

In the entry on 'case', the OED gives usages related to both of your
versions, i.e. "in case" meaning "in the case that", and "in case"
meaning "if it should prove that". Withough the "just" they aren't so
clearly differentiated, but the first seems to be rather older, earliest
quotation 1292, compared to c1400 for the second.

So what I now think is that both versions of "just in case" are correct
and both have a reasonably long history, but (and this isn't the OED,
it's me) the mathematicians' and logicians' use is now very rare outside
mathematical and philosophical contexts and would thus appear obscure
and potentially confusing to readers who had not come upon it before."

However David Hand said

"  I had thought that the usage you characterise as 'mathematical' was in 
fact 'American' rather than English.
Do grammar books have any view of the matter? "

It turns out that David is correct as Dennis Chanter had already cleared 
the matter up for me by drawing my attention to

'Mind the Gaffe' by R.L. Trask  (See, for example, 
http://books.guardian.co.uk/reviews/referenceandlanguages/0,,772010,00.html )
which has this to say

"In case....In Britain, in case is familiarly used to mean 'lest'...This is 
virtually incomprehensible to other users of English..In the USA, 'in case' 
is often used for 'if'..This too sounds very strange to other users of 
English. Another distinctively American habit, largely confined to 
academics, is the use of just in case to mean 'if and only if' "

So there you have it. Meaning b) is used by American academics including 
mathematicians. However, this is ironic, since the attempt to be 
mathematically precise introduces ambiguity, or downright puzzlement, 
outside the USA.

Finally, Keith Briggs asked for an example

Here is one from a recent paper in by Alex  London, Statist. Med. 2006; 
25:2869–2885


"Risks to individual research participants are reasonable just in case they 
(1) require the least amount of intrusion into the interests of 
participants that is necessary
in order to facilitate sound scientific inquiry and (2) are consistent with 
an equal regard for the basic interests of study participants and the 
members of the larger community whose interests that research is intended 
to serve." pp2877-2878


My thanks to Keith Briggs, Linda Hunt, Terry Orchard, Alan Reese, Malcolm 
Farrow, Albert Chau, Jan Poloniecki, John Whittington, Karen Jamieson, 
Kevin McConway, Jim Burridge, Michael Dewey, Dennis Chanter, Peter Jupp, 
Irene Stratton, Fenn Scott, Peter Lane, David Zucker and David Hand,

Stephen

Stephen Senn
Professor of Statistics

Department of Statistics
15 University Gardens
<http://www.gla.ac.uk>University of Glasgow
G12 8QQ

Tel: +44 (0)141 330 5141
Fax: +44(0)141 330 4814
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Private webpage: http://www.senns.demon.co.uk/home.html