failing to select a subset of observations based on variable values [Sec: UNCLASSIFIED]

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Greetings all.

I'm starting analysis in R on a reasonably sized pre-existing  
dataset, of 583 variables and 1127 observations. This was an SPSS  
datafile, which I read in using the read.spss command using the  
foreign package, and the data was assigned to a data.frame when it  
was read in. The defaults in read.spss were used, except I set  
to.data.frame = TRUE.

The data is a survey dataset (each observation/case = one  
participant), and many of the variables are participants' responses  
to Likert scale items. These have been coded on a 1 to 7 scale, with  
"8" used to code "Don't know" responses. The assumption is that the  
1-7 responses are at least interval level, however the response "8"  
is clearly not. For many analyses, this doesn't matter because I'm  
only doing chi-square tests. However, for a between-group comparison  
crosstab I would like to exclude those who gave "8" responses  
because I am only interesting in testing differences for the  
participants who gave responses measured on the Likert scale proper.

I have encountered problems when I need to exclude the observations  
from analysis, where they gave an "8" response to either of two  
questions (Question 1A and Question 1B), which relate to columns 72  
and 73 of the dataframe. The chi-square I am trying to do is based  
on two other variables (mean of Q1A+Q1B for each participant) and a  
grouping variable, which are contained in columns 8 and 80 of the  
dataframe, respectively. The reason I am excluding anyone who gave  
an "8" ("Don't know) response on questions 1A and 1B is that their  
mean on these two questions cannot be interpreted as the value "8"  
is nominal rather than interval/ratio and therefore cannot be used  
in a mathematical expression.

I've been trying to use an if-or combination, and I can't get it to  
work.
Did you read the help page for if?
?"if"
The chi-square test without the attempt to subset using "if" is  
working fine, I don't understand what I am doing wrong in my  
attempts to subset.

I have tried to reference the variables like this:
if ("Q1A"!=8 | "Q1B"!=8)
+ (table(micronutrients[,8,80]))
<group counts snipped>
chisq.test(table(micronutrients[,8,80]))
The group counts returned from the table statement show me that no  
observations are being excluded from the analysis. The chisq.test  
works fine on (table(micronutrients[,8,80])) but, of course, it is  
being performed on the entire dataset as I have been unsuccessful in  
subsetting the data.

I tried to see if the column names were objects and I got these  
errors:
object("Q1A")
Error: could not find function "object"
Q1A
Error: object 'Q1A' not found
I'm not sure if this is important.

So I tried to do the if-or using the column number, but that didn't  
work either:
if (micronutrients[,72]!=8 | micronutrients[,73]!=8)
Leave behind your SPSS syntactical constructions. SPSS and the SAS  
data steps have implicit loops that operate sequantially along rows of  
datasets. R does not work that way. A corresponding operation in R  
might be:

  apply(dataframe1, 1, <function that works on a row of data>)

  "if" is a program control mechanism that does not operate on  
vectors. If it gets a vector if evaluates the first element and  
ignores the rest. (You should have gotten a warning and you should  
have posted the warning.)

There is also the ifelse function that works with and returns vectors.
+ ()
<group counts snipped>
Warning message:
In if () (table(micronutrients[,  :
 the condition has length > 1 and only the first element will be used

I got exactly the same chi-square output as in my previous attempt.

If any of you know SPSS, what I am trying to do in R is equivalent  
to: temporary. select if not (Q1A=8 or Q1B=8). In SAS, it would be  
the same as a subsetting if that lasted only for the particular  
analysis, or a where, e.g. proc tabulate; where Q1A ne 8 or Q1B ne 8;

How can I subset the data? I would prefer not to create another  
variable to hold the recodes as the dataset is already complex.
?subset
?with

with(subset(dfrm, micronutrients[, 72] != 8 | micronutrients[, 73] !=  
8), table(...) )

Not sure what you intended with ... table(micronutrients[,8,80]) ... ,  
but generally one does not first reference an object with two  
dimensions and then do so with three. It is considered good form  
around these parts to offer at the very least str() on an object about  
which you are hoping to get specific advice. We cannot read your mind.
I only wish the subsetting condition to hold for the test  
immediately following the instruction to subset (I need to subset  
the data in different ways for different question combinations).  
Because the instruction is complete once the table() command is  
issued, I am assuming that the if statement only relates to the  
table() command and therefore only indirectly to the chisq.test()  
command following (as this is being performed on the subsetted  
table) - which is exactly what I want.
Do not see any code to which this refers. You can assign the result of  
the with(subset( ...), ... ) operation to an object on which you can  
do statistical tests, but table does not automatically do chi-square  
test. Maybe you should look at summary.table() or xtabs()
Cheers
Michelle

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