Converter character (string) input of Snellen fractions, Counting Fingers (CF), and Hand Motion (HM) to logMAR values for use in statistical models. Can handle linear interpolation if passed an appropriate chart or if the measures fit with the default chart.
Usage
VAConverter(
OS,
OD,
chart.values = NULL,
chart.nletters = NULL,
datatype = c("snellen", "decimal", "logMAR"),
zero = 3
)Arguments
- OS
The values to be converted for the left eye (oculus sinister).
- OD
The values to be converted for the right eye (oculus dexter)
- chart.values
The Snellen fractions for the chart used (if interpolation is necessary and it is different from the default).
- chart.nletters
The number of letters on each line of the chart that was used. Necessary for proper interpolation.
- datatype
The type of data passed to
OSandOD. One of "Snellen" (the default), "decimal", or "logMAR". Determines what transformations are needed to convert to logMAR values.- zero
The “zero” logMAR value. This is used as the zero point for visual acuity. For example, for light perception (LP), no light perception (NLP), etc. It defaults to 3 (which is equivalent to a Snellen value of 20/20000), but may also be
NA. See details.
Value
An object of class VAObject. This
includes the left and right eye logMAR values in slots
@logMAROS and @logMAROD as well as additional
information. More information can be found in the class
documentation.
Details
VAConverter is primarily designed to take raw character
data of various forms and convert them to logMAR values.
Acceptable examples include: "20/20", "20/80 + 3", "20/20 - 4",
"10/20", "CF 10", "HM 2", "CF 4", "NLP", "LP", "", "CF", "HM",
etc. For Snellen values, both parts should be present, and
there should be a space between components; e.g., between
fraction, +/- and number or between CF and 10. Although I have
attempted to make it as flexible and general as possible, there
are still fairly rigid requirements so that it can parse a
variety of text formats to numerical values. Optionally, it can
also handle decimal values (i.e., the results of actually
dividing a Snellen value 20/20 = 1).
chart.values and chart.nletters must be the same
length. These are used to interpolate values such as "20/20 + 3"
which is interpreted as reading all of the letters on the "20/20"
line and "3" of the letters on the next best line (typically
"20/15" but this can be chart dependent). The functions goes 3/n
of the distance between the logMAR values for each line. This is
why it is important to know the values for the chart that
was actually used.
If datatype = "logMAR", the values passed to OS and
OD are directly assigned to the logMAROS and
logMAROD slots of a "VAObject" and an
error is returned if that results in the creation of an invalid
object (e.g., they are not numeric or not of equal length).
The zero argument is primarily included to facilitate
calculating averages. For example, in some cases it may be nice
to get a sense of an individual's "overall" or "average" logMAR
value. Because on the logMAR scale, 0 is "20/20", an alternate
number needs to be used. 3 was chosen as a rough default, but it
is by no means necessarily the best choice. If you are not
interested in computing an average between the left and right eyes
within individuals, it makes sense to simply use NA rather
than a crude "zero" approximation.