Testlet model

[Afshin]

Hello,

I’ll be very thankful if you answer this question. The following are the codes for the testlet model from ‘mirt’ manual.

I don’t understand the object ‘specific’. Does it assign items to dimensions? What does ‘rep’ do?

 

specific <- c(rep(1,4),rep(2,4), rep(3,4))

 

Apparently, there are 12 items all assigned to a G factor and there are 3 specific factors. I don’t understand the constrains. What are a1, a2, a3, a4?

 

model <- "G = 1-12

CONSTRAIN = (1, a1, a2), (2, a1, a2), (3, a1, a2), (4, a1, a2),

(5, a1, a3), (6, a1, a3), (7, a1, a3), (8, a1, a3),

(9, a1, a4), (10, a1, a4), (11, a1, a4), (12, a1, a4)

COV = S1*S1, S2*S2, S3*S3"

 

What does COV do? Are the three specific factors allowed to correlate? If it is the testlet model then they shouldn’t correlate.   

 

simmod <- bfactor(dataset, specific, model)

 

Thanks for your help

Afshin

[Inga vin]

Hello,

in 'specific' you specify which item belongs to which testlet (i.e., dimension). Rep is a function in base R that simply repeats a given number a specified number of times. a1, a2, a3, and a4 are the item slopes for each dimension. You got three dimensions in total: a general one and three specific ones for the testlets.

In COV you specify the variances and covariances of your dimensions. In your case the specific factors are not allowed to correlate, since only variances but no covariances are specified.

Best