Software demonstration of the bootstrap application to conditional
item difficulty parameters in Rasch
Kingsley E. Agho
Centre for Clinical
Epidemiology and Biostatistics
James A. Athanasou
Faculty of Education
University
of Technology,
Abstract
A brief overview of the Rasch conditional logit item difficulty parameter is provided. Next we demonstrate two statistical software procedures for estimating the bootstrap application of the Rasch conditional logit item difficulty parameter to teachers and Rasch users with little programming skills. Finally, we propose that the bootstrap software demonstrate here will provides Rasch users with more than one way of investigating research question in educational testing.
Kingsley E. Agho
Centre for Clinical Epidemiology and
Biostatistics (CCEB),
The
Level 3,
NSW 2300,
Introduction
The bootstrap method was first introduced to statistics by Efron (1979) as a general method for estimating standard errors and bias of parameters in statistical models. The bootstrap technique has become increasingly popular in educational testing because it provides researchers with more than one way of investigating research questions and we will demonstrate the bootstrap software application to conditional Rasch item difficulty parameter later.
In general, the bootstrap application to the Rasch measurement model seeks to provide a better understanding of what parametric Rasch models do. It is based on the rationale of no assumption about the population from which either items or persons are sample; and, offers procedures for data with much smaller numbers of items and persons. However, to the best of our knowledge, existing Rasch measurement software cannot apply the bootstrap method.
The aim of this short communication is to provide two different (R and Stata) statistical packages for estimating bootstrap applications in Rasch conditional item difficulty parameters to teachers and Rasch users with little programming skills. The Rstatistical software is available for free (www.rproject.org).
Conditional Item Difficulty Estimates in Rasch
In
this short communication, the item parameter will be used to refer to as b.
Consider the following response data situation, where _{} represents dichotomous
responses given to j items. Let _{} be the binary or dichotomous (1,0) response for person _{}(_{}= 1,…,N) and item j (j = 1,…n), where 1 denotes a correct
response and 0 denotes an incorrect response. Let _{}and _{}. The simplest and the most widely quoted model for _{} is the Rasch model
(Rasch, 1960) and the Rasch conditional logistic model (T_{i}_{j}) is:
T_{i}_{j} = ln_{} (1)
where _{} is the person ability
parameter and b_{j} is
the item parameter.
Data Structure
Before the use of the software packages are explained, the setup of the data into a readable format needs to be discussed. For example, a typical dichotomous data structure on five individuals on three dichotomouslyscored test items is given in column 14 of Table 1. To estimate conditional item difficulty parameters, the data is reshaped lengthwise and the expression in (1) will be used to create a dummy variable Th's which represents  b_{j} or the betas in (1) (see column 510 in Table 1) and the conditional logistic function was used to estimate the Rasch difficulty parameter.
Table 1
Illustration of Data format
for readable Data and Rasch item difficulty estimate.


Data Format 
Rasch Item difficulty Estimate Data
format 

student_id 
Ques1 
Ques2 
Ques3 

student_id 
item 
Ques 
Th1 
Th2 
Th3 
1 
1 
0 
0 
1 
1 
1 
1 
0 
0 

2 
0 
0 
0 
1 
2 
0 
0 
1 
0 

3 
0 
0 
1 
1 
3 
0 
0 
0 
1 

4 
1 
0 
1 
2 
1 
0 
1 
0 
0 

5 
0 
1 
0 
2 
2 
0 
0 
1 
0 


2 
3 
0 
0 
0 
1 

3 
1 
0 
1 
0 
0 

3 
2 
0 
0 
1 
0 

3 
3 
1 
0 
0 
1 

4 
1 
1 
1 
0 
0 

4 
2 
0 
0 
1 
0 

4 
3 
1 
0 
0 
1 

5 
1 
0 
1 
0 
0 

5 
2 
1 
0 
1 
0 

5 
3 
0 
0 
0 
1 
Note: For Rstatistical software, Ques = resp, student_id = id and Th = i.
Software package demonstration and sample output
Two statistical software packages have been selected for illustration in this short communication. The first package is the comprehensive public domain R statistical software and secondly the commercial Stata statistical package. The dataset used in this demonstration consisted of 200 individual's responses to 10 dichotomouslyscored examination questions. In both cases (R and Stata) each of the 10 items is replicated 1000 times to give the bootstrap estimate_{} while item 10 is the reference level when the conditional logistic Rasch method is applied.
To apply the bootstrap method to the conditional Rasch item difficulty parameters, we used Davison and Hinkley's (1997) boot library in Rstatistical software. In the boot function, "statistic" is a function that returns the statistic to be bootstrapped. The first two argument of the function "boot.cond" specified the reshaped data set "combine" and the index vector gives the indices of the observations included in the bootstrap sample while "R" is the number of bootstrap replicates. The sample output is shown below. A complete stepbystep guide for users unfamiliar with the R package is available form the author upon request.
In the Stata example, we build upon the programming written by Weesie in 1997. "bs" is the bootstrap sample while "reps" is the number of bootstrap replicates. In most bootstrap applications, an investigation using a B=1000 bootstrap sample will essentially be able to approximate the actual sampling distribution (Efron & Tibshirani, 1993). The sample output from the Stata statistical package is shown below.
Stata statistical package example and result output
use "H:\Document\maths.dta"
reshape long Ques, i(student_id) j(item)
for num 1/10: gen
ThX = (item==X)
bs "clogit Ques Th1Th10, group(student_id)" "_b[Th1] _b[Th2] _b[Th3] _b[Th4] _b[Th5] _b[Th6] _b[Th7] _b[Th8] _b[Th9]",reps(1000) cluster(student_id)
Variable 
Observed 
Bias 
Std. Err. 
[95% Conf. Interval] 


_bs_1 
0.708 
0.018 
0.303 
0.113 
1.303 
(N) 




0.100 
1.305 
(P) 




0.037 
1.260 
(BC) 
_bs_2 
0.747 
0.028 
0.299 
0.159 
1.334 
(N) 




0.199 
1.405 
(P) 




0.135 
1.335 
(BC) 
_bs_3 
0.670 
0.011 
0.293 
0.094 
1.247 
(N) 




0.084 
1.252 
(P) 




0.038 
1.230 
(BC) 
_bs_4 
0.952 
0.043 
0.309 
0.345 
1.560 
(N) 




0.385 
1.582 
(P) 




0.331 
1.472 
(BC) 
_bs_5 
0.597 
0.009 
0.296 
0.015 
1.178 
(N) 




0.035 
1.210 
(P) 




0.031 
1.174 
(BC) 
_bs_6 
0.526 
0.014 
0.296 
0.056 
1.108 
(N) 




4.51E16 
1.097 
(P) 




0.031 
1.090 
(BC) 
_bs_7 
0.491 
0.007 
0.290 
0.077 
1.061 
(N) 




0.066 
1.084 
(P) 




0.065 
1.080 
(BC) 
_bs_8 
0.561 
0.016 
0.293 
0.014 
1.137 
(N) 




1.15E16 
1.122 
(P) 




0.031 
1.096 
(BC) 
_bs_9 
0.561 
0.011 
0.295 
0.016 
1.140 
(N) 




0.015 
1.166 
(P) 




0.036 
1.139 
(BC) 
Note: N = normal, P = percentile and BC = biascorrected.
Rstatistical package example and result output
library(boot) ## bootstrap library is needed
library(splines) ## need this library
library(survival)## need this library
####estimating the full sample
examboth < read.table('H:\Document/maths.txt', header=T)
combine < reshape(examboth, v.names="resp", idvar = "id", timevar="item", varying=list(c("Ques1","Ques2","Ques3","Ques4","Ques5","Ques6","Ques7","Ques8","Ques9","Ques10")), direction="long")
indices < sample(length(combine[,1]), replace=T)
boot.cond < function(combine, indices, maxit=20){
full.i.dummy < diag(nlevels(factor(combine$item)))[factor(combine$item),]
full.i.dummy < 0  full.i.dummy # turns (0,1) into (0, 1)
full.i.dummy < data.frame(full.i.dummy, row.names=NULL)
dimnames(full.i.dummy)
[[2]] < paste("i",
attach(full.i.dummy)
examboth.clog < clogit(resp ~ i2 + i3 + i4 + i5 + i6 + i7 + i8 + i9 + i10 + strata(id), data= combine[indices,])
coeff <coefficients(examboth.clog)
return(coeff)
}
Rasch.boot < boot(combine, boot.cond, 1000, maxit=100)
Rasch.boot
ORDINARY NONPARAMETRIC BOOTSTRAP
Call:
boot(data = combine, statistic = boot.cond, R = 1000, maxit = 100)
Bootstrap Statistics :
original bias
std. error
t1* 0.7084904 0.7024243
0.2957786
t2* 0.7470233 0.7412647
0.2929619
t3* 0.6707143 0.6701935
0.2994406
t4* 0.9527469 0.9553256
0.2943158
t5* 0.5972723 0.5869482
0.2862424
t6* 0.5264065 0.5244834
0.3029250
t7* 0.4918603 0.4915181
0.3067102
t8* 0.5615339 0.5602604
0.2915319
t9* 0.5615339 0.5531485
0.3065304
In
the programming output, the tenth item (item 10), with associated beta (1) = 0
is the reference level. This is because; the conditional logit function
programming is not able to define a reference level if item 10 is added. The observed and
original values in Stata and Rstatistical software are the conditional
logistic Rasch difficulty parameters.
Discussion
The purpose of this short communication has been to demonstrate two statistical software methods for estimating the bootstrap application of conditional item difficulty estimates in Rasch to teachers or Rasch users with little programming skills. In recent years, measurement research continues to broaden and the bootstrap application to item difficulty estimates in Rasch becomes a suitable procedures for demonstrating binary items for a very small number of persons which existing Rasch measurement software can not estimate.
References
Baron, J. & Li, Y.
(2004).
Notes on the use of R for psychology experiments
and questionnaires. Available at http://www.psych.upenn.edu/~baron/rpsych/rpsych.html
Davidson
AC,
Efron, B. (1979). Bootstrap methods: Another look at the Jackknife. Annals of Statistics, 7, 116.
Efron, B
and Tibshirani, J.C (1993). An introduction to the bootstrap.
Fisher,
G. H. and Molenaar, I.W. (1995). Rasch
models. foundations, recent developments and applications.
Rasch G.
(1960/1980). Probabilistic models for some intelligence and attainment test.
StataCorp.
Stata Statistical Software, Release 8.2.
In.
Weesie, J. (1999). The Rasch model in STATA. STATA statistical software 7.0 : STATA Corporation.
Demonstration of The Bootstrap Method of Rasch Conditional Item Difficulty Estimation, Agho K.E. Athanasou J.A. … Rasch Measurement Transactions, 2005, 19:2 p. 10223
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