In the two-parameter partial credit model (TPPCM) of Yu (1991) <https://hdl.handle.net/2142/21459>, every step of a polytomous item has its own discrimination. Estimation is left to existing software: 'TAM' (tam.mml.3pl() with a step design matrix, tam.mml.2pl(), tam.mml()) and, for the bilinear product form and non-negativity constraints, 'sirt' (xxirt()). The package adds what these packages do not provide: standard errors of step discriminations based on the full observed information, step-level Wald tests, and score tests with modification indices and expected parameter changes for moving down the model ladder saturated TPPCM -> product form -> generalized partial credit model -> partial credit model, limited-information fit statistics (the M2 statistic, root mean square error of approximation, standardized root mean squared residual, comparative fit index and Tucker-Lewis index; Maydeu-Olivares and Joe (2006) <doi:10.1007/s11336-005-1295-9>) for any step design, step contributions to the item information, and tests of differential step functioning across groups: per-step Lagrange multiplier tests from one multigroup fit (Glas (1998) <https://www3.stat.sinica.edu.tw/statistica/j8n3/j8n32/j8n32.htm>), with the generalized score test of Boos (1992) <doi:10.1080/00031305.1992.10475921>, likelihood ratio tests with configural, metric, scalar and partial multigroup designs, score-based tests (Merkle and Zeileis (2013) <doi:10.1007/s11336-012-9302-4>) and model-based trees (Zeileis, Hothorn and Hornik (2008) <doi:10.1198/106186008X319331>). The generalized partial credit model of Muraki (1992) <doi:10.1177/014662169201600206> is the special case with equal steps within items.
| Version: | 0.1.0 |
| Depends: | R (≥ 4.1), TAM |
| Imports: | CDM, graphics, MASS, sirt, stats |
| Suggests: | knitr, mirt, numDeriv, partykit, psychotools, psychotree, rmarkdown, strucchange, testthat (≥ 3.0.0) |
| Published: | 2026-10-28 |
| DOI: | 10.32614/CRAN.package.tppcm (may not be active yet) |
| Author: | Jie-Wen Tsai [aut, cre] |
| Maintainer: | Jie-Wen Tsai <tsai.jiewen at gmail.com> |
| BugReports: | https://github.com/jiewenTsai/tppcm/issues |
| License: | GPL-2 | GPL-3 [expanded from: GPL (≥ 2)] |
| URL: | https://github.com/jiewenTsai/tppcm |
| NeedsCompilation: | no |
| Language: | en-US |
| Citation: | tppcm citation info |
| Materials: | README, NEWS |
| CRAN checks: | tppcm results |
| Reference manual: | tppcm.html , tppcm.pdf |
| Vignettes: |
tppcm recipes (source, R code) tppcm: step discriminations on top of TAM (source, R code) |
| Package source: | tppcm_0.1.0.tar.gz |
| Windows binaries: | r-devel: not available, r-release: not available, r-oldrel: not available |
| macOS binaries: | r-release (arm64): not available, r-oldrel (arm64): not available, r-release (x86_64): not available, r-oldrel (x86_64): not available |
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