There has been a lot of research on readability of fonts, and you can start tapping into it by doing a Google search.
While the research supports the common wisdom that serifed fonts are generally easier to read, readability depends on the context and the medium.
There was a very instructive article in last Sunday's NY Times magazine about readability of - I'm not kidding - readability of fonts on highway signs.
ONE UNIVERSAL CONCLUSION FROM THE RESEARCH, HOWEVER, IS THAT USING ALL CAPITALS  DECREASES READABILITY. (Sorry, I couldn't resist.)

bOB bIENKOWSKI
--
Robert S Bienkowski, PhD
xxxxxx@att.net

-------------- Original message ----------------------
From: Bill Campbell <xxxxxx@UWRF.EDU>
>
> When I first started in this business, I heard Lynn Miner say in a
> proposal-writing workshop that 'reading research shows that 80% of
> readers find serif fonts easier to read than sans-serif.'  That matches
> my own experience, so I've repeated it to countless proposal-writers.
> But now I wonder, is this like the David Bauer reference discussed
> earlier?
>
> In any case, as an ED reader, I appreciate serif fonts, especially when
> time is tight, the light is dim, or it's the fifth proposal I've read
> that day. In those situations, a proposal written in Arial--or worse,
> Arial Narrow--annoys me. The last thing a proposal-writer wants is an
> annoyed reader.
>
> Regards, Bill
>
>
>
> Bill Campbell
>
> Director, Grants & Research
>
> University of Wisconsin-River Falls
>
> 410 S. 3rd St.
>
> River Falls, WI 54022
>
> 715/425-3195
>
> FAX 715/425-0649
>
>
>
>
>
> Ruth Tallman wrote:
> > Hello All,
> >
> > If this has come up before, please forgive me.  I'm working on a
> > proposal to NSF and the PI has just told me:
> >
> > "Almost all technical manuscripts are written in either Times or Times
> > Roman. These are serif fonts and are the default fonts also used in
> > virtually all mathematical equation software. To convert equations to
> > a sans serif font, like Arial or Helvetica, is insane and won't look
> > very good. How are Greek letters supposed to be done, i.e., is Symbol
> > font one of the permitted fonts?"
> >
> > Has anyone submitted a proposal in which the mathematical equations
> > are in Times Roman?  Do you have any tips for PI's with lots of
> > formula's in their proposal?
> >
> > Your feedback will be appreciated!
> >
> > Ruth Tallman
> > --
> >
> >
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