# Introduction

In some cases users might like to return the probability of a response on a given item. For example, given a fixed set of item parameters, return the probabilities at varying levels of theta to produce custom probability plots.

# Example

## dichotomous case

The probability of responding correctly to a dichotomous item under Rasch-like models (e.g., 1PL models) is often expressed as:

$$$p(x_{ni} = 1)=\frac{exp(\theta_{n} - \delta_{i})}{1 + (\theta_{n} - \delta_{i})} (\#eq:slm)$$$

Imagine the item parameters of a single item represented as:

library(conquestr)
myItem<- matrix(c(0, 0, 0, 1, 1, 0), ncol =3, byrow=TRUE)
colnames(myItem)<- c("k", "d", "t")
print(myItem)
#>      k d t
#> [1,] 0 0 0
#> [2,] 1 1 0

Then the probability of scoring 0 and 1 on this item, at = 0.5:

myProbs<- simplep(0.5, myItem)
print(myProbs)
#>           [,1]
#> [1,] 0.6224593
#> [2,] 0.3775407

A simple ICC can be drawn:

myProbsList<- list()
myThetaRange<- seq(-4, 4, by = 0.1)
for(i in seq(myThetaRange)){
myProbsList[[i]]<- pX(x = 1, probs = simplep(myThetaRange[i], myItem))
}
plot(unlist(myProbsList))

## polytomous case

In the case of polytomously scored items, the probability model can be generalised:

$$$p(X_{ni} = x)=\frac{exp\sum\limits_{k=0}^{x}(\theta_{n} - (\delta_{i} + \tau_{ik}))}{\sum\limits_{j=0}^{m}exp(\sum\limits_{k=0}^{j} (\theta_{n} - (\delta_{i} + \tau_{ik})))} (\#eq:pcm)$$$

An item can them be represented such that:

library(conquestr)
myItem<- matrix(c(0, 0, 0, 1, 1, -0.2, 2, 1, 0.2), ncol =3, byrow=TRUE)
colnames(myItem)<- c("k", "d", "t")
print(myItem)
#>      k d    t
#> [1,] 0 0  0.0
#> [2,] 1 1 -0.2
#> [3,] 2 1  0.2

Then the probability of scoring 0, 1 and 2 on this item, at = 0.5:

myProbs<- simplep(0.5, myItem)
print(myProbs)
#>           [,1]
#> [1,] 0.4742264
#> [2,] 0.3513155
#> [3,] 0.1744581

A simple ICC can be drawn:

myProbsList<- list()
myThetaRange<- seq(-4, 4, by = 0.1)
for(i in seq(myThetaRange)){
myProbsList[[i]]<- simplep(myThetaRange[i], myItem)
}
myProbs<- (matrix(unlist(myProbsList), ncol = 3, byrow = TRUE))
plot(myThetaRange, myProbs[,1])
points(myThetaRange, myProbs[,2])
points(myThetaRange, myProbs[,3])
abline(v = c(myItem[2, 2], sum(myItem[2, 2:3]), sum(myItem[3, 2:3])))

## Expected scores

The expected score for the an item can be calculated at a given value of theta. Taking an aribitary set of items, it is possible therefor to calculate the test expected score.

library(conquestr)
myItems<- list()
myItems[[1]]<- matrix(c(0, 0, 0, 1, 1, -0.2, 2, 1, 0.2), ncol =3, byrow=TRUE)
myItems[[2]]<- matrix(c(0, 0, 0, 1, -1, -0.4, 2, -1, 0.4), ncol =3, byrow=TRUE)
myItems[[3]]<- matrix(c(0, 0, 0, 1, 1.25, -0.6, 2, 1.25, 0.6), ncol =3, byrow=TRUE)
myItems[[4]]<- matrix(c(0, 0, 0, 1, 2, 0.2, 2, 2, -0.2), ncol =3, byrow=TRUE)
myItems[[5]]<- matrix(c(0, 0, 0, 1, -2.5, -0.2, 2, -2.5, 0.2), ncol =3, byrow=TRUE)
for(i in seq(myItems)){
colnames(myItems[[i]])<- c("k", "d", "t")
}
print(myItems)
#> [[1]]
#>      k d    t
#> [1,] 0 0  0.0
#> [2,] 1 1 -0.2
#> [3,] 2 1  0.2
#>
#> [[2]]
#>      k  d    t
#> [1,] 0  0  0.0
#> [2,] 1 -1 -0.4
#> [3,] 2 -1  0.4
#>
#> [[3]]
#>      k    d    t
#> [1,] 0 0.00  0.0
#> [2,] 1 1.25 -0.6
#> [3,] 2 1.25  0.6
#>
#> [[4]]
#>      k d    t
#> [1,] 0 0  0.0
#> [2,] 1 2  0.2
#> [3,] 2 2 -0.2
#>
#> [[5]]
#>      k    d    t
#> [1,] 0  0.0  0.0
#> [2,] 1 -2.5 -0.2
#> [3,] 2 -2.5  0.2

expectedRes<- list()
for(i in seq(myThetaRange)){
tmpExp<- 0
for(j in seq(myItems)){
tmpE<- simplef(myThetaRange[i], myItems[[j]])
tmpExp<- tmpExp + tmpE
}
expectedRes[[i]]<- tmpExp
}

plot(myThetaRange, unlist(expectedRes))