Logistik {difR} | R Documentation |

Calculates the "logistic regression" likelihood-ratio statistics and effect sizes for DIF detection.

Logistik(data, member, member.type = "group", match = "score", anchor = 1:ncol(data), type = "both", criterion = "LRT", all.cov = FALSE)

`data` |
numeric: the data matrix (one row per subject, one column per item). |

`member` |
numeric or factor: the vector of group membership. Can either take two distinct values (zero for the reference group and one for the focal group) or be a continuous vector. See |

`member.type` |
character: either |

`match` |
specifies the type of matching criterion. Can be either |

`anchor` |
a vector of integer values specifying which items (all by default) are currently considered as anchor (DIF free) items. Ignored if |

`type` |
a character string specifying which DIF effects must be tested. Possible values are |

`criterion` |
a character string specifying which DIF statistic is computed. Possible values are |

`all.cov` |
logical: should |

This command computes the logistic regression statistic (Swaminathan and Rogers, 1990) in the specific framework of differential item functioning.
It forms the basic command of `difLogistic`

and is specifically designed for this call.

If the `member.type`

argument is set to `"group"`

, the `member`

argument must be a vector with two distinct (numeric or factor) values, say 0 and 1 (for the reference and focal groups respectively). Those values are internally transformed onto factors to denote group membership. The three possible models to be fitted are then:

*M_0: logit (π_g) = α + β X + γ_g + δ_g X*

*M_1: logit (π_g) = α + β X + γ_g*

*M_2: logit (π_g) = α + β X*

where *π_g* is the probability of answering correctly the item in group *g* and *X* is the matching variable. Parameters *α* and
*β* are the intercept and the slope of the logistic curves (common to all groups), while *γ_g* and *δ_g* are group-specific
parameters. For identification reasons the parameters *γ_0* and *δ_0* for reference group (*g=0*) are set to zero. The parameter
*γ_1* of the focal group (*g=1*) represents the uniform DIF effect, and the parameter *δ_1* is used to model nonuniform DIF
effect. The models are fitted with the `glm`

function.

If `member.type`

is set to `"cont"`

, then "group membership" is replaced by a continuous or discrete variable, given by the `member`

argument, and the models above are written as

*M_0: logit (π_g) = α + β X + γ Y+ δ X Y*

*M_1: logit (π_g) = α + β X + γ Y*

*M_2: logit (π_g) = α + β X*

where `Y`

is the group variable. Parameters *γ* and *δ* act now as the *γ_1* and *δ_1* DIF parameters.

The matching criterion can be either the test score or any other continuous or discrete variable to be passed in the `Logistik`

function. This is specified by the `match`

argument. By default, it takes the value `"score"`

and the test score (i.e. raw score) is computed. The second option is to assign to `match`

a vector of continuous or discrete numeric values, which acts as the matching criterion. Note that for consistency this vector should not belong to the `data`

matrix.

Two types of DIF statistics can be computed: the likelihood ratio test statistics, obtained by comparing the fit of two nested models,
and the Wald statistics, obtained with an appropriate contrast matrix for testing the model parameters (Johnson and Wichern, 1998).
These are specified by the argument `criterion`

, with respective values `"LRT"`

and `"Wald"`

. By default, the LRT
statistics are computed.

If `criterion`

is `"LRT"`

, the argument `type`

determines the models to be compared by means of the LRT statistics.
The three possible values of `type`

are: `type="both"`

(default) which tests the hypothesis *H_0: γ_1 = δ_1=0* (or *H_0: γ = δ=0*) by comparing models *M_0* and *M_2*; `type="nudif"`

which tests the hypothesis *H_0: δ_1 = 0* (or *H_0: δ = 0*) by comparing models *M_0* and *M_1*; and `type="udif"`

which tests the hypothesis *H_0: γ_1 = 0* (or *H_0: γ = 0*) by comparing models *M_1* and *M_2* (assuming that *δ_1 = 0* or *δ = 0*). In other words, `type="both"`

tests for DIF (without distinction between uniform and nonuniform effects), while `type="udif"`

and `type="nudif"`

test for uniform and nonuniform DIF,
respectively.

If `criterion`

is `"Wald"`

, the argument `type`

determines the logistic model to be considered and the appropriate contrast matrix.
If `type=="both"`

, the considered model is model *M_0* and the contrast matrix has two rows, (0,0,1,0) and (0,0,0,1). If `type=="nudif"`

,
the considered model is also model *M_0* but the contrast matrix has only one row, (0,0,0,1). Eventually, if `type=="udif"`

, the considered model
is model *M_1* and the contrast matrix has one row, (0,0,1).

The data are passed through the `data`

argument, with one row per subject and one column per item. Missing values are allowed but must be coded as `NA`

values. They are discarded from the fitting of the logistic models (see `glm`

for further details).

The vector of group membership, specified with `member`

argument, must hold only zeros and ones, a value of zero corresponding to the
reference group and a value of one to the focal group.

Option `anchor`

sets the items which are considered as anchor items for computing the test scores and related logistic regression DIF statistics. Items other than the anchor items and the tested item are discarded. `anchor`

must hold integer values specifying the column numbers of the corresponding anchor items. It is mainly designed to perform item purification. Note that this option is discarded when `match`

is not `"score"`

.

The output contains: the selected DIF statistics (either the LRT or the Wald statistic) computed for each item, two matrices with the parameter estimates of
both models (for each item) and two matrices of related standard error values. In addition, Nagelkerke's *R^2* coefficients (Nagelkerke, 1991) are computed for each model and the output returns both, the vectors of *R^2* coefficients for each model and the differences in these coefficients. Such differences are used as measures of effect size by the `difLogistic`

command; see Gomez-Benito, Dolores Hidalgo and Padilla
(2009), Jodoin and Gierl (2001) and Zumbo and Thomas (1997). The `criterion`

and `member.type`

arguments are also returned, as well as a character argument named `match`

that specifies the type of matching criterion that was used.

A list with nine components:

`stat` |
the values of the logistic regression DIF statistics. |

`R2M0` |
the values of Nagelkerke's R^2 coefficients for the "full" model. |

`R2M1` |
the values of Nagelkerke's R^2 coefficients for the "simpler" model. |

`deltaR2` |
the differences between Nagelkerke's |

`parM0` |
a matrix with one row per item and four columns, holding successively the fitted parameters |

`parM1` |
the same matrix as |

`seM0` |
a matrix with the standard error values of the parameter estimates in matrix |

`seM1` |
a matrix with the standard error values of the parameter estimates in matrix |

`cov.M0` |
either |

`cov.M1` |
either |

`criterion` |
the value of the |

`member.type` |
the value of the |

`match` |
a character string, either |

Sebastien Beland

Collectif pour le Developpement et les Applications en Mesure et Evaluation (Cdame)

Universite du Quebec a Montreal

sebastien.beland.1@hotmail.com, http://www.cdame.uqam.ca/

David Magis

Department of Psychology, University of Liege

Research Group of Quantitative Psychology and Individual Differences, KU Leuven

David.Magis@uliege.be, http://ppw.kuleuven.be/okp/home/

Gilles Raiche

Collectif pour le Developpement et les Applications en Mesure et Evaluation (Cdame)

Universite du Quebec a Montreal

raiche.gilles@uqam.ca, http://www.cdame.uqam.ca/

Gomez-Benito, J., Dolores Hidalgo, M. and Padilla, J.-L. (2009). Efficacy of effect size measures in logistic regression: an application for detecting DIF.
*Methodology, 5*, 18-25. doi: 10.1027/1614-2241.5.1.18

Jodoin, M. G. and Gierl, M. J. (2001). Evaluating Type I error and power rates using an effect size measure with logistic regression procedure for DIF detection. *Applied Measurement in Education, 14*, 329-349. doi: 10.1207/S15324818AME1404_2

Johnson, R. A. and Wichern, D. W. (1998). *Applied multivariate statistical analysis (fourth edition)*. Upper Saddle River, NJ: Prentice-Hall.

Magis, D., Beland, S., Tuerlinckx, F. and De Boeck, P. (2010). A general framework and an R package for the detection
of dichotomous differential item functioning. *Behavior Research Methods, 42*, 847-862. doi: 10.3758/BRM.42.3.847

Nagelkerke, N. J. D. (1991). A note on a general definition of the coefficient of determination. *Biometrika, 78*, 691-692. doi: 10.1093/biomet/78.3.691

Swaminathan, H. and Rogers, H. J. (1990). Detecting differential item functioning using logistic regression procedures. *Journal of Educational
Measurement, 27*, 361-370. doi: 10.1111/j.1745-3984.1990.tb00754.x

Zumbo, B. D. and Thomas, D. R. (1997). A measure of effect size for a model-based approach for studying DIF. Prince George, Canada: University of Northern British Columbia, Edgeworth Laboratory for Quantitative Behavioral Science.

## Not run: # Loading of the verbal data data(verbal) # Testing both types of DIF simultaneously # With all items, test score as matching criterion Logistik(verbal[,1:24], verbal[,26]) # Returning all covariance matrices of model parameters Logistik(verbal[,1:24], verbal[,26], all.cov = TRUE) # Testing both types of DIF simultaneously # With all items and Wald test Logistik(verbal[,1:24], verbal[,26], criterion = "Wald") # Removing item 6 from the set of anchor items Logistik(verbal[,1:24], verbal[,26], anchor = c(1:5, 7:24)) # Testing for nonuniform DIF Logistik(verbal[,1:24], verbal[,26], type = "nudif") # Testing for uniform DIF Logistik(verbal[,1:24], verbal[,26], type = "udif") # Using the "anger" trait variable as matching criterion Logistik(verbal[,1:24],verbal[,26], match = verbal[,25]) # Using the "anger" trait variable as group membership Logistik(verbal[,1:24],verbal[,25], member.type = "cont") ## End(Not run)

[Package *difR* version 5.1 Index]