[R] IRT discrimination value (ltm and psych package)

[shreepad khandve]

Hello Sir,

I am learning R and its syntax and  I have successfully converted
irt.item.diff.rasch into python code and pass the inputs as per the
function made in R.



In R  :-

function (items)

{

  ncases <- nrow(items)

  item.mean <- colMeans(items, na.rm = TRUE)

  item.mean[item.mean < (1/ncases)] <- 1/ncases

  irt.item.diff.rasch <- log((1/item.mean) - 1)

}



In Python :-

def diff(items):

    item_mean= items.mean()

    item_diff = []

    for i in item_mean:

        diff = np.log((1/i)-1)

        item_diff.append(diff)

    return item_diff





I am getting the exact same result as R output.

But I’m still unable to understand the “item.discrim” and how to and
what to pass as a input to the function,



function (item.diff, theta, items)

{

  irt.item.discrim <- function(x, diff, theta, scores) {

    fit <- -1 * (log(scores/(1 + exp(x * (diff - theta))) +

      (1 - scores)/(1 + exp(x * (theta - diff)))))

    mean(fit, na.rm = TRUE)

  }

  nitems <- length(item.diff)

  discrim <- matrix(NaN, nitems, 2)

  for (i in 1:nitems) {

    item.fit <- optimize(irt.item.discrim, c(-5, 5), diff = item.diff[i],

      theta = theta, scores = items[, i])

    discrim[i, 1] <- item.fit$minimum

    discrim[i, 2] <- item.fit$objective

  }

  irt.discrim <- discrim



Could you please guide how do I convert the same in python, because
there are no proper explanation in documentation what to pass as
following arguments or the formulas to find the values of x and theta.

X= ?

Diff = I got the diff values

Theta = ?

Scores = = items[, i] as per mentioned in code



Unable to find theta and x, please help with this parameters ?


Thanks,
Shreepad