[R] linear regression

[Donald Lehmann]

Dear Consultant

I've done linear regression successfully on R a few times before.  But this 
time it keeps telling me:-

"Error in lm.fit(x, y, offset = offset, singular.ok = singular.ok, ...) :
         0 (non-NA) cases"

The model is:-

fm1 <- lm(TS.CM ~ AGE + SEX + HFE.Y.01 + TFC2B.01 + HFE.Y.01*TFC2B.01, data 
= IRONresults, subset = DIAG2.1D == 0)
summary (fm1)

TS.CM is a continuous variable (%s), sex is coded 0 = women, 1 = men, 
DIAG2.1D is coded 0 = non-demented, 1 = ALzheimer's disease and the genes, 
HFE.Y.01 & TFC2B.01, are coded 0 = non-carrier and 1 = carrier

I've tried recoding the data to use 1 & 2, instead of 0 & 1, and I've 
removed the rows with missing data.  I've also tried putting "...lm(formula 
= TS.CM ~ ..."  But I always get the same error message

What am I doing wrong?

A related question: what's the minimum no of data points for regression 
analysis to work?  We have only 23 cases carrying both genes out of 447 and 
only 8 out of 264 in the above subset (ie non-demented).  I seem to 
remember hearing somewhere that you needed a minimum of ~30 (?), so 
probably this wouldn't work anyway.  Still, I'd like to know what I was 
doing wrong!

Many thanks

Donald (Lehmann)