snellen_steps

snellensteps

Vector with VA entries of class snellen - needs to be in
format xx/yy

used in conversion method for class snellen<ul>
<li>Removing "plus" and "minus" from snellen notation<ul>
<li>if entry -1 to +3 : take same Snellen value</li>
<li>if &lt;= -2 : take Snellen value one line below</li>
<li>if &gt;+3 (unlikely, but unfortunately not impossible):
Snellen value one line above</li>
</ul></li>
</ul>

A tool to facilitate common tasks in ophthalmic research:
Conversion between different visual acuity notations (Snellen, logMAR
and ETDRS), counting of patients, recode right and left eyes and reshape
eye side specific variables between wide and long format. The 'eye'
package also contains a real life data set of people with intravitreal
injections with anti-vascular endothelial growth factor (anti-VEGF),
made available by Fasler et al. (2019) <doi:10.1136/bmjopen-2018-027441>.
Visual acuity conversion is based on
Schulze-Bonsel et al. (2006) <doi:10.1167/iovs.05-0981>,
Gregori et al. (2010) <doi:10.1097/iae.0b013e3181d87e04>,
Beck et al. (2003) <doi:10.1016/s0002-9394(02)01825-1> and
Bach (2007) <http:michaelbach.de/sci/acuity.html>.

Tjebo Heeren

Analysis of Eye Data

Antoine Fabri

snellen_steps function

Snellen are unfortunately often entered with "+/-", which is a
violation of a psychophysical method designed to assign one
unambiguous value to visual acuity, with
non-arbitrary thresholds based on psychometric functions. Therefore,
transforming "+/-" notation to actual results is in itself
problematic and the below suggestion to convert it will remain an
approximation to the most likely "true" result. Even more so, as the
given conditions should work for charts with
4 or 5 optotypes in a line, and visual acuity is not always tested
on such charts. Yet, I believe that the approach is still better than
just omitting the letters or (worse) assigning a missing value to those
entries.

snellen_steps: Convert plus minus entries

Description

Usage

Arguments

Value

snellen_steps

See Also