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Letter and Word Spacing: Final Analysis

From: Wayne Dick <wayneedick@gmail.com>
Date: Tue, 13 Jun 2017 12:33:27 -0700
Message-ID: <CAJeQ8SB_u0=qk9L4=9_hyUL0B+ufXxnF200iCDnv+e=iGdCyYg@mail.gmail.com>
To: public-low-vision-a11y-tf <public-low-vision-a11y-tf@w3.org>
 *Letter and Word Spacing* *Summary of Results:*

For best reading results the spacing should be .25em maximum. However, the
loss of performance between .12em and .25em is less than 1/4 in reading
speed. I think .15em is the best because that gives 95% of the benefit.
After that there is almost no benefit. After .25em there is none.

Word spacing may not be necessary, because browsers tack on the letter
spacing to the normal word spacing anyway.
*Good News / Bad News*

The good news. Alastair and I are both right in our calculations.

The bad news. Alastair and I are both right in our calculations.

We have a serious political decision to make.

My letter spacing was based on an article, “A study of the effect of letter
spacing on the reading speed of young readers with low vision”, Eve
McLeish, Visual Impairment Service, UK (British Journal of Visual
Impairment 25(2) 2007). In this article, the author builds a table for
spacing of typed assignments for children with low vision. The formula she
used was STEP=[fontSize/20], for each test bracket. Each STEP represents
increasing the letter spacing by 1/10 of the letter size. She used points
for her font size but we will use pixels.  McLeish found significant
results with reading speed up to n*STEP for n=1… 5. However, the slope went
from steep to horizontal in this range. It was concave down going flat at
n=5. When I computed these results, I started at n=5. Example: for font
size of 16px, 5*STEP = 5(16px/20)=4px=0.25em. The formula works the same
for all font sizes.

I got to these values the first time and noticed that the performance curve
really flattened between n=3 and 5. It grew from 0 to 20% increase in
reading speed from n=0… 3. Then grew from 20% to 22% between 3 and 5. I
should have selected 3 first and got 3*.8=2.4px= .15em.  Fear of developer
response, got the best of me, so I suggested the .12em. At that size, the
performance curve still gave a 15% increase in reading speed.

For testing, I used Firefox with, Tahoma and the text, “Lorem ipsum dolor
sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut
labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud
exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis
aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu
fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt
in culpa qui officia deserunt mollit anim id est laborum.” I got the
following results:

Let Average Char be the number of pixels taken by an average character in
the passage.

Letter Spacing

Average Char







1.917px or 27%



2.4px or 34%



4.0px or 57%

Aside from the huge impact on layout there are other difficulties.

The research is mixed. The benefits of letter spacing are measured by
various experiments in the range from no effect to simply miraculous.
McLeish is in the middle; her methodology is sound, and she observes the
impact in the most natural setting. Her findings rang true with my

The most significant article that shows no effect is: The effect of letter
spacing on reading speed in central and peripheral vision by S. T. Chung
(Investigative Ophthalmology and Visual Science, 2002 Apr, 43(4):1270-6).
Chung’s methodology is sound, but she uses a different instrument for
measurement. McLeish uses flash cards while, Chung uses Rapid Serial Visual
Presentation (RSVP). The words are drifted past at varying speeds. Chung’s
theory is that an individual can read faster when more letters are fit in
the most sensitive reading zone of a reader’s retina. Increased letter
spacing reduces this value and therefore reading speed must suffer.

Both authors are correct, in my opinion.  This needs to be tested of
course, but here is my reasoning. McLeish’s use of cards, forces the
participant to orient their most sensitive reading zone each time the card
is presented. Thus, McLeish measures orientation and recognition. Chung
uses text that drifts into the participants optimal reading zone, so no
orientation is needed. This would mean that the benefit in letter spacing
would be in helping the reader orient their most sensitive reading zone to
the target.  That is just a theory, but it does explain the difference in
two well designed studies.
Received on Tuesday, 13 June 2017 19:34:42 UTC

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