Description Usage Arguments Details Value Author(s) See Also Examples

Returns the item response function of the 3PL (1PL, 2PL) model, the i.e. the probabilities defined by

*P(U_{ij}=1|θ_i,a_j,b_j,c_j)=c_j+(1-c_j)\frac{\displaystyle\exp(a_j(θ_i-b_j))}{1+\displaystyle\exp(a_j(θ_i-b_j))}*

where *U_{ij}* is a binary response given by person *i* to item
*j*, *θ_i* is the value of the latent variable ("ability") for
person *i*, *a_j* is the discrimination parameter for item *j*,
*b_j* is the difficulty parameter for item *j*, *c_j* is the
asymptote for item *j*. Some authors call the IRF "the item characteristic curve".

1 |

`ip` |
Item parameters: the output of |

`items` |
The item(s) for which irf is computed. If NULL (the default), irf for all items will be returned |

`x` |
The values of the latent variable ( |

In the 2PL model (`model="2PL"`

), all asymptotes *c_j* are 0. In
the 1PL model (`model="1PL"`

), all asymptotes *c_j* are 0 and the
discriminations *a_j* are equal for all items (and sometimes to 1).

A common use of this function would be to obtain a plot of the IRF.

A list of:

`x` |
A copy of the argument |

`f` |
A
matrix containing the IRF values: persons (values of ( |

Ivailo Partchev

1 |

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