Here's the problem I'm trying to solve in R: I have a data frame that consists
of about 1500 cases (rows) of data from kids who took a test of listening
comprehension. The columns are their scores (1 = correct, 0 = incorrect, . =
missing) on 140 test items. The items are numbered sequentially and are ordered
by increasing difficulty as you go from left to right across the columns. I
want R to go through the data and find the first correct response for each
case. Because of basal and ceiling rules, many cases have missing data on many
items before the first correct response appears.
For each case, I want R to evaluate the item responses sequentially starting
with item 1. If the score is 0 or missing, proceed to the next item and
evaluate it. If the score is 1, stop the operation for that case, record the
item number of that first correct response in a new variable, proceed to the
next case, and restart the operation.
In SPSS, this operation would be carried out with LOOP, VECTOR, and DO IF, as
follows (assuming the data set is already loaded):
* DECLARE A NEW VARIABLE TO HOLD THE ITEM NUMBER OF THE FIRST CORRECT RESPONSE,
SET IT EQUAL TO 0.
numeric LCfirst1.
comp LCfirst1 = 0
* DECLARE A VECTOR TO HOLD THE 140 ITEM RESPONSE VARIABLES.
vector x=LC1a_score to LC140a_score.
* SET UP A LOOP THAT WILL RUN FROM 1 TO 140, AS LONG AS LCfirst1 = 0. "#i" IS
AN INDEX VARIABLE THAT INCREASES BY 1 EACH TIME THE LOOP RUNS.
loop #i=1 to 140 if (LCfirst1 = 0).
* SET UP A CONDITIONAL TRANSFORMATION THAT IS EVALUATED FOR EACH ELEMENT OF THE
VECTOR. THUS, WHEN #i = 1, THE EXPRESSION EVALUATES THE FIRST ELEMENT OF THE
VECTOR (THAT IS, THE FIRST OF THE 140 ITEM RESPONSES). AS THE LOOP RUNS AND #i
INCREASES, SUBSEQUENT VECTOR ELELMENTS ARE EVALUATED. THE do if STATEMENT
RETAINS CONTROL AND KEEPS LOOPING THROUGH THE VECTOR UNTIL A '1' IS ENCOUNTERED.
+ do if x(#i) = 1.
* WHEN A '1' IS ENCOUNTERED, CONTROL PASSES TO THE NEXT STATEMENT, WHICH
RECODES THE VALUE OF THAT VECTOR ELEMENT TO '99'.
+ comp x(#i) = 99.
* AND THEN CONTROL PASSES TO THE NEXT STATEMENT, WHICH RECODES THE VALUE OF
LCfirst1 TO THE CURRENT INDEX VALUE, THUS CAPTURING THE ITEM NUMBER OF THE
FIRST CORRECT RESPONSE FOR THAT CASE. CHANGING THE VALUE OF LCfirst1 ALSO CAUSE
S THE LOOP TO STOP EXECUTING FOR THAT CASE, AND THE PROGRAM MOVES TO THE NEXT
CASE AND RESTARTS THE LOOP.
+ comp LCfirst1 = #i.
+ end if.
end loop.
exe.
After several hours of trying to translate this procedure to R, I'm stumped. I
played around with creating a list to hold the item responses variables
(analogous to 'vector' in SPSS), but when I tried to use the list in an R
procedure, I kept getting a warning along the lines of 'the list contains > 1
element, only the first element will be used'. So perhaps a list is not the
appropriate class to 'hold' these variables?
It seems that some nested arrangement of 'for' 'while' and/or 'lapply' will
allow me to recreate the operation described above? How do I set up the
indexing operation analogous to 'loop #i' in SPSS?
Any help is appreciated, and I'm happy to provide more information if needed.
David S. Herzberg, Ph.D.
Vice President, Research and Development
Western Psychological Services
12031 Wilshire Blvd.
Los Angeles, CA 90025-1251
Phone: (310)478-2061 x144
FAX: (310)478-7838
email: dav...@wpspublish.com
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