Dear List I apologize for the multiple postings. After being in the weeds on this problem for a while I think my original post may have been a little cryptic. I think I can be clearer. Essentially, I need the following
a <- c(2,3) b <- c(4,5,6) (2*4) + (2*5) + (2*6) + (3*4) + (3*5) +(3*6) But I do not know of a built in function that would do this. Any suggestions? -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Doran, Harold Sent: Wednesday, April 19, 2006 11:51 AM To: [email protected] Subject: [R] Function to approximate complex integral I am writing a small function to approximate an integral that cannot be evaluated in closed form. I am partially successful at this point and am experiencing one small, albeit important problem. Here is part of my function below. This is a psychometric problem for dichotomously scored test items where x is a vector of 1s or 0s denoting whether the respondent answered the item correctly (1) or otherwise (0), b is a vector of item difficulties, and theta is an ability estimate for the individual. rasch <- function(b,theta){ 1 / ( 1 + exp(b - theta)) } The function rasch gives the probability of a correct response to item i conditional on theta, the individuals ability estimate myfun <- function(x, b, theta){ sum(rasch(b, theta)^x * ( 1 - rasch(b,theta) )^(1-x) * dnorm(theta)) } This is the likelihood function assuming the data are Bernoulli distributed multiplied by a population distribution. Now, when x,b, and theta are of equal length the function works fine as below x <- c(1,1,0) b <- c(-2,-1,0) t <- c(-2,-1.5,-1) > myfun(x,b,t) [1] 0.2527884 However, I want theta to be a vector of discrete values that will be larger than both x and b, something like t <- seq(-5, 0, by = .01) However, this gives me the error > myfun(x,b,t) Warning messages: 1: longer object length is not a multiple of shorter object length in: b - theta So, for the problem above, I want item 1 to be evaluated at theta 1 through theta q and then item 2 is evaluated at theta 1 and through theta q and so forth for each item. Can anyone recommend a way for me to modify my function above to accomplish this? Harold [[alternative HTML version deleted]] ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
