powerRM | R Documentation |

Returns power of Wald (W), likelihood ratio (LR), Rao score (RS)
and gradient (GR) test given probability of error of first kind
`\alpha`

, sample size, and a deviation from the hypothesis to be tested.
The latter assumes equality of the item parameters in the Rasch model
between two predetermined groups of persons. The alternative states that at least
one of the parameters differs between the two groups.

```
powerRM(
alpha = 0.05,
n_total,
persons1 = rnorm(10^6),
persons2 = rnorm(10^6),
local_dev
)
```

`alpha` |
Probability of error of first kind. |

`n_total` |
Total sample size for which power shall be determined. |

`persons1` |
A vector of person parameters in group 1 (drawn from a specified distribution).
By default |

`persons2` |
A vector of person parameters in group 2 (drawn from a specified distribution).
By default |

`local_dev` |
A list of two vectors containing item parameters for the two person groups representing a deviation from the hypothesis to be tested locally per item. |

In general, the power of the tests 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`

equal to the number of items
minus 1 and noncentrality parameter `\lambda`

.
The latter depends on a scenario of deviation from the hypothesis to be tested
and a specified sample size. Given the probability of the error of the first
kind `\alpha`

the power of the tests can be determined from `\lambda`

.
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).

As regards the concept of sample size a distinction between informative and total sample size has to be made since the power of the tests depends only on the informative sample size. 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.

In particular, the determination of `\lambda`

and the power of the tests, respectively,
is based on a simple Monte Carlo approach. 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. A scenario of a deviation is given by a choice of the
item 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. 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 group 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. The power of the tests can be determined
given a user-specified total sample size denoted by `n_total`

. The noncentrality
parameter `\lambda`

can then be expressed by
`\lambda = n_{total}* (n_{infsim} / n_{totalsim}) * e`

, where `n_{totalsim}`

denotes
the total number of persons in the simulated data and `n_{infsim} / n_{totalsim}`

is
the proportion of informative persons in the sim. data. Let `q_{\alpha}`

be the
`1 - \alpha`

quantile of the central `\chi^2`

distribution with df equal to the
number items minus 1. Then,

`power = 1 - F_{df, \lambda} (q_{\alpha}),`

where `F_{df, \lambda}`

is the cumulative distribution function of the noncentral
`\chi^2`

distribution with `df`

equal to the number of items minus 1
and `\lambda = n_{total} (n_{infsim} / n_{totalsim}) * e`

. Thereby, it is assumed that
`n_{total}`

is composed of a frequency distribution of person scores that is proportional
to the observed distribution of person scores in the simulated data. The same holds
true in respect of the relative group sizes, i.e., the relative frequencies of the two
person groups in a sample of size `n_{total}`

are assumed to be equal to the relative
frequencies of the two groups in the simulated data.

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 realized value of a random variable `E`

. The same holds
true for `\lambda`

as well as the power of the tests. Thus, the power is a realized
value of a random variable that shall be denoted by `P`

. Consequently, the (realized)
value of the power of the tests need not be equal to the exact power that follows from the
user-specified `n_{total}`

, `\alpha`

, and the chosen item 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 the power of the tests will be
close to the exact value. This will generally be the case if the number of person parameters
used for simulating the data is large, e.g., `10^5`

or even `10^6`

persons. In such cases,
the possible random error of the computation procedure 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 the power of the tests 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 with `df`

equal to the number of free item parameters and noncentrality parameter `\lambda`

.
Thus, `Var(T) = 2 (df + 2 \lambda)`

, with `\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)`

.
The power of the tests is a function of `e`

which is given by `F_{df, \lambda} (q_{\alpha})`

,
where `\lambda = n_{total} * (n_{infsim} / n_{totalsim}) * e`

and `df`

equal to the
number of free item parameters. Then, by the delta method one obtains (for the variance of P).

`Var(P) = Var(E) * (F'_{df, \lambda} (q_{\alpha}))^2,`

where `F'_{df, \lambda}`

is the derivative of `F_{df, \lambda}`

with respect to `e`

.
This derivative is determined numerically and evaluated at `e`

using the package numDeriv.
The square root of `Var(P)`

is then used to quantify the random error of the suggested
Monte Carlo computation procedure. It is called Monte Carlo error of power.

A list of results.

`power` |
Power value for each test. |

`MC error of power` |
Monte Carlo error of power computation for each test. |

`global deviation` |
Global deviation computed from simulated data for each test. See Details. |

`local deviation` |
CML estimates of item parameters in both groups of persons obtained from the simulated data expressing a deviation from the hypothesis to be tested locally per item. |

`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 power of the tests. |

`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 power of the tests. |

`degrees of freedom` |
Degrees of freedom |

`noncentrality parameter` |
Noncentrality parameter |

`call` |
The matched call. |

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.

`sa_sizeRM`

, and `post_hocRM`

.

```
## Not run:
# Numerical example
res <- powerRM(n_total = 130, local_dev = list( c(0, -0.5, 0, 0.5, 1) , c(0, 0.5, 0, -0.5, 1)))
# > res
# $power
# W LR RS GR
# 0.824 0.840 0.835 0.845
#
# $`MC error of power`
# W LR RS GR
# 0.002 0.002 0.002 0.002
#
# $`global deviation`
# W LR RS GR
# 0.118 0.122 0.121 0.124
#
# $`local deviation`
# Item2 Item3 Item4 Item5
# group1 -0.499 0.005 0.500 1.001
# group2 0.501 0.003 -0.499 1.003
#
# $`person score distribution in group 1`
#
# 1 2 3 4
# 0.249 0.295 0.269 0.187
#
# $`person score distribution in group 2`
#
# 1 2 3 4
# 0.249 0.295 0.270 0.186
#
# $`degrees of freedom`
# [1] 4
#
# $`noncentrality parameter`
# W LR RS GR
# 12.619 13.098 12.937 13.264
#
# $call
# powerRM(n_total = 130, local_dev = list(c(0, -0.5, 0, 0.5, 1),
# c(0, 0.5, 0, -0.5, 1)))
## End(Not run)
```

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