"conspicuousness" vs. "conspicuity"

[Jesse Sheidlower]

I was recently reading a government safety manual, intended for new
drivers applying for their learners permit. One section was headed
"Increasing Conspicuity", which struck me as a pretty obscure word,
especially in a document of this nature--it's got to be clear to people
who _don't even know how to drive_, after all. Why not use
_conspicuousness_, or, even better, change the section to "Increasing
Visibility" or "Making Yourself More Conspicuous"?

Curious, I looked at COCA, which has only a single example of
_conspicuity_ against 15 of _conspicuousness_. But the really striking
thing is the Google Ngram result:

http://books.google.com/ngrams/graph?content=conspicuousness%2Cconspicuity&year_start=1800&year_end=2010&corpus=0&smoothing=3

This shows that in the early 19th century (both words are from the 17th,
or earlier), they were about equally common; around 1900 _conspicuous_
became very much more common than _conspicuity_ (the result of an
increase in frequency of the former, not a decrease of the latter), but
since around 1920 or so, the words' relative frequency has been growing
together--_conspicuousness_ has become less frequent and _conspicuity_
has become more frequent--to the point where the words are about equally
common right now. (Again, according to the Google Ngram results--COCA
presents a very different picture of the current frequency.)

Does anyone have an explanation for this pattern? ("The Ngram result is
totally wrong" is one possible explanation.) Or is my sense of the
rarity of _conspicuity_ off?

Jesse Sheidlower

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