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help for raschpower                       Jean-Benoit Hardouin, Myriam Blanchin
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Estimation of the power of the Wald test in order to compare the means of the l
> atent trait in two groups of individuals

raschpower [, n0(#) n1(#) gamma(#) var(#) d(matrix) method(method)]

Description

raschpower allows estimating the power of the Wald test comparing the
means of two groups of patients in the context of the Rasch model or
the partial-credit model. The estimation is based on the estimation
of the variance of the difference of the means based on the
Cramer-Rao bound.

Options

n0 and n1 indicates the numbers of individuals in the two groups [100 by
default].

gamma indicates the group effect (difference between the two means) [0.5
by default].

var indicates the value of the variance of the latent trait [1 by
default].

d is a matrix containing the item parameters [one row per item, one
column per positive modality - (-1.151, -0.987\-0.615, -0.325\-0.184,
-0.043\0.246, 0.554\0.782, 1.724) by default].

method(method) indicates the method for constructing data. (method) may
be GH, MEAN, MEAN+GH or POPULATION+GH [default is method(GH) if
number of patterns<500, method(MEAN+GH) if 500<=number of
patterns<10000, method(MEAN) if 10000<=number of patterns<1000000,
method(POPULATION+GH) otherwise].

GH: The probability of all possible response patterns is estimated by

MEAN: The mean of the latent trait for each group is used instead of

MEAN+GH: In a first step, the MEAN method is used to determine the
most probable patterns. In a second step, the probability of
response patterns is estimated by Gauss-Hermite quadratures on
the most probable patterns.

POPULATION+GH: The most frequent response patterns are selected from
a simulated population of 1,000,000 of individuals. The
probability of the selected response patterns is estimated by

Example

. raschpower

. raschpower, n0(200) n1(200) gamma(0.4) var(1.3)

. matrix diff=(-1.47\-0.97\-.23\-0.12\0.02\0.1)

. raschpower, n0(127) n1(134) gamma(0.23) d(diff) var(2.58)

References

Hardouin J.B., Amri S., Feddag M., Sébille V. (2012) Towards Power And
Sample Size Calculations For The Comparison Of Two Groups Of Patients
With Item Response Theory Models. Statistics in Medicine, 31(11):
1277-1290.

Author

Jean-Benoit Hardouin, PhD, assistant professor
Myriam Blanchin, PhD, research assistant
EA4275 "Biostatistics, Pharmacoepidemiology and Subjective Measures in
Health Sciences"
University of Nantes - Faculty of Pharmaceutical Sciences
1, rue Gaston Veil - BP 53508
44035 Nantes Cedex 1 - FRANCE
Emails:  jean-benoit.hardouin@univ-nantes.fr
myriam.blanchin@univ-nantes.fr
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