# sa_sizePCM: Sample size planning for tests of invariance of item-category... In tcl: Testing in Conditional Likelihood Context

 sa_sizePCM R Documentation

## Sample size planning for tests of invariance of item-category parameters between two groups of persons in partial credit model

### Description

Returns sample size for Wald (W), likelihood ratio (LR), Rao score (RS) and gradient (GR) test given probabilities of errors of first and second kinds \alpha and \beta as well as a deviation from the hypothesis to be tested. The hypothesis to be tested assumes equal item-category parameters in the partial credit model between two predetermined groups of persons. The alternative assumes that at least one parameter differs between the two groups.

### Usage

sa_sizePCM(
alpha = 0.05,
beta = 0.05,
persons1 = rnorm(10^6),
persons2 = rnorm(10^6),
local_dev
)


### Arguments

 alpha Probability of the error of first kind. beta Probability of the error of second kind. persons1 A vector of person parameters for group 1 (drawn from a specified distribution). By default 10^6 parameters are drawn at random from the standard normal distribution. The larger this number the more accurate are the computations. See Details. . persons2 A vector of person parameters for group 2 (drawn from a specified distribution). By default 10^6 parameters are drawn at random from the standard normal distribution. The larger this number the more accurate are the computations. See Details. local_dev A list consisting of two lists. One list refers to group 1, the other to group 2. Each of the two lists contains a numerical vector per item, i.e., each list contains as many vectors as items. Each vector contains the free item-cat. parameters of the respective item. The number of free item-cat. parameters per item equals the number of categories of the item minus 1.

### Details

In general, the sample size is determined from the assumption that the approximate distributions of the four test statistics are from the family of noncentral \chi^2 distributions with df = l, where l is the number of free item-category parameters in the partial credit model, and noncentrality parameter \lambda. The latter is, inter alia, a function of the sample size. Hence, the sample size can be determined from the condition \lambda = \lambda_0, where \lambda_0 is a predetermined constant which depends on the probabilities of the errors of the first and second kinds \alpha and \beta (or power). More details about the distributions of the test statistics and the relationship between \lambda, power, and sample size can be found in Draxler and Alexandrowicz (2015).

In particular, the determination of \lambda and the sample size, respectively, is based on a simple Monte Carlo approach. As regards the concept of sample size a distinction between informative and total sample size has to be made. In the conditional maximum likelihood context, the responses of persons with minimum or maximum person score are completely uninformative. They do not contribute to the value of the test statistic. Thus, the informative sample size does not include these persons. The total sample size is composed of all persons. The Monte Carlo approach used in the present problem to determine \lambda and informative (and total) sample size can briefly be described as follows. Data (responses of a large number of persons to a number of items) are generated given a user-specified scenario of a deviation from the hypothesis to be tested. The hypothesis to be tested assumes equal item-category parameters between the two groups of persons. A scenario of a deviation is given by a choice of the item-cat. parameters and the person parameters (to be drawn randomly from a specified distribution) for each of the two groups. Such a scenario may be called local deviation since deviations can be specified locally for each item-category. The relative group sizes are determined by the choice of the number of person parameters for each of the two groups. For instance, by default 10^6 person parameters are selected randomly for each group. In this case, it is implicitly assumed that the two groups of persons are of equal size. The user can specify the relative groups sizes by choosing the length of the arguments persons1 and persons2 appropriately. Note that the relative group sizes do have an impact on power and sample size of the tests. The next step is to compute a test statistic T (Wald, LR, score, or gradient) from the simulated data. The observed value t of the test statistic is then divided by the informative sample size n_{infsim} observed in the simulated data. This yields the so-called global deviation e = t / n_{infsim}, i.e., the chosen scenario of a deviation from the hypothesis to be tested being represented by a single number. Let the informative sample size sought be denoted by n_{inf} (thus, this is not the informative sample size observed in the sim. data). The noncentrality parameter \lambda can be expressed by the product n_{inf} * e. Then, it follows from the condition \lambda = \lambda_0 that

n_{inf} * e = \lambda_0

and

n_{inf} = \lambda_0 / e.

Note that the sample of size n_{inf} is assumed to be composed only of persons with informative person scores in both groups, where the relative frequency distribution of these informative scores in each of both groups is considered to be equal to the observed relative frequency distribution of informative scores in each of both groups in the simulated data. Note also that the relative sizes of the two person groups are assumed to be equal to the relative sizes of the two groups in the simulated data. By default, the two groups are equal-sized in the simulated data, i.e., one yields n_{inf} / 2 persons (with informative scores) in each of the two groups. The total sample size n_{total} is obtained from the relation n_{inf} = n_{total} * pr, where pr is the proportion or relative frequency of persons observed in the simulated data with a minimum or maximum score. Basing the tests given a level \alpha on an informative sample of size n_{inf} the probability of rejecting the hypothesis to be tested will be at least 1 - \beta if the true global deviation \ge e.

Note that in this approach the data have to be generated only once. There are no replications needed. Thus, the procedure is computationally not very time-consuming.

Since e is determined from the value of the test statistic observed in the simulated data it has to be treated as a realization of a random variable E. Consequently, n_{inf} is also a realization of a random variable N_{inf}. Thus, the (realized) value n_{inf} need not be equal to the exact value of the informative sample size that follows from the user-specified (predetermined) \alpha,\beta, and scenario of a deviation from the hypothesis to be tested, i.e., the selected item-category parameters used for the simulation of the data. If the CML estimates of these parameters computed from the simulated data are close to the predetermined parameters n_{inf} will be close to the exact value. This will generally be the case if the number of person parameters used for simulating the data, i.e., the lengths of the vectors persons1 and persons2, is large, e.g., 10^5 or even 10^6 persons. In such cases, the possible random error of the computation procedure of n_{inf} based on the sim. data may not be of practical relevance any more. That is why a large number (of persons for the simulation process) is generally recommended.

For theoretical reasons, the random error involved in computing n_inf can be pretty well approximated. A suitable approach is the well-known delta method. Basically, it is a Taylor polynomial of first order, i.e., a linear approximation of a function. According to it the variance of a function of a random variable can be linearly approximated by multiplying the variance of this random variable with the square of the first derivative of the respective function. In the present problem, the variance of the test statistic T is (approximately) given by the variance of a noncentral \chi^2 distribution. Thus, Var(T) = 2 (df + 2 \lambda), with df = l and \lambda = t. Since the global deviation e = (1 / n_{infsim}) * t it follows for the variance of the corresponding random variable E that Var(E) = (1 / n_{infsim})^2 * Var(T). Since n_{inf} = f(e) = \lambda_0 / e one obtains by the delta method (for the variance of the corresponding random variable N_{inf})

Var(N_{inf}) = Var(E) * (f'(e))^2,

where f'(e) = - \lambda_0 / e^2 is the derivative of f(e). The square root of Var(N_{inf}) is then used to quantify the random error of the suggested Monte Carlo computation procedure. It is called Monte Carlo error of informative sample size.

### Value

A list of results.

 informative sample size Informative sample size for each test, omitting persons with min. and max score. MC error of sample size Monte Carlo error of informative sample size for each test. global deviation Global deviation computed from simulated data. See Details. local deviation CML estimates of free item-category parameters in both group of persons obtained from the simulated data expressing a deviation from the hypothesis to be tested locally per item and response category. person score distribution in group 1 Relative frequencies of person scores in group 1 observed in simulated data. Uninformative scores, i.e., minimum and maximum score, are omitted. Note that the person score distribution does also have an influence on the sample size. person score distribution in group 2 Relative frequencies of person scores in group 2 observed in simulated data. Uninformative scores, i.e., minimum and maximum score, are omitted. Note that the person score distribution does also have an influence on the sample size. degrees of freedom Degrees of freedom df. noncentrality parameter Noncentrality parameter \lambda of \chi^2 distribution from which sample size is determined. total sample size in group 1 Total sample size in group 1 for each test. See Details. total sample size in group 1 Total sample size in group 2 for each test. See Details. call The matched call.

### References

Draxler, C. (2010). Sample Size Determination for Rasch Model Tests. Psychometrika, 75(4), 708–724.

Draxler, C., & Alexandrowicz, R. W. (2015). Sample Size Determination Within the Scope of Conditional Maximum Likelihood Estimation with Special Focus on Testing the Rasch Model. Psychometrika, 80(4), 897–919.

Draxler, C., Kurz, A., & Lemonte, A. J. (2020). The Gradient Test and its Finite Sample Size Properties in a Conditional Maximum Likelihood and Psychometric Modeling Context. Communications in Statistics-Simulation and Computation, 1-19.

Glas, C. A. W., & Verhelst, N. D. (1995a). Testing the Rasch Model. In G. H. Fischer & I. W. Molenaar (Eds.), Rasch Models: Foundations, Recent Developments, and Applications (pp. 69–95). New York: Springer.

Glas, C. A. W., & Verhelst, N. D. (1995b). Tests of Fit for Polytomous Rasch Models. In G. H. Fischer & I. W. Molenaar (Eds.), Rasch Models: Foundations, Recent Developments, and Applications (pp. 325-352). New York: Springer.

powerPCM, and post_hocPCM.

### Examples

## Not run:
##### Sample size of PCM Model #####

# free item-category parameters for group 1 and 2  with 5 items, with 3 categories each
local_dev <-  list (  list(c( 0, 0), c( -1, 0), c( 0, 0),  c( 1, 0), c( 1, 0.5)) ,
list(c( 0, 0), c( -1, 0), c( 0, 0),  c( 1, 0), c( 0, -0.5))  )

res <- sa_sizePCM(alpha = 0.05, beta = 0.05, persons1 = rnorm(10^6),
persons2 = rnorm(10^6), local_dev = local_dev)

# > res
# $informative sample size # W LR RS GR # 234 222 227 217 # #$MC error of sample size
#     W    LR    RS    GR
# 1.105 1.018 1.053 0.988
#
# $global deviation # W LR RS GR # 0.101 0.107 0.104 0.109 # #$local deviation
#          I1-C2  I2-C1  I2-C2  I3-C1  I3-C2 I4-C1 I4-C2 I5-C1  I5-C2
# group1 -0.001 -1.000 -1.001 -0.003 -0.011 0.997 0.998 0.996  1.492
# group2  0.001 -0.998 -0.996 -0.007 -0.007 0.991 1.001 0.004 -0.499
#
# $person score distribution in group 1 # # 1 2 3 4 5 6 7 8 9 # 0.111 0.130 0.133 0.129 0.122 0.114 0.101 0.091 0.070 # #$person score distribution in group 2
#
#     1     2     3     4     5     6     7     8     9
# 0.090 0.109 0.117 0.121 0.121 0.121 0.116 0.111 0.093
#
# $degrees of freedom #  9 # #$noncentrality parameter
#  23.589
#
# $total sample size in group 1 # W LR RS GR # 132 125 128 123 # #$total sample size in group 2
#   W  LR  RS  GR
# 133 126 129 123
#
# \$call
# sa_sizePCM(alpha = 0.05, beta = 0.05, persons1 = rnorm(10^6),
#            persons2 = rnorm(10^6), local_dev = local_dev)

## End(Not run)


tcl documentation built on May 3, 2023, 1:17 a.m.