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IATA is a software package for analysing psychometric and educational assessment data. It performs factor analysis, item response theory (IRT) scaling and calibration, differential item functioning (DIF) analysis, computer aided test development, equating, IRT-based standard setting, score conditioning, and plausible value generation. The current version (5.1) runs ~10x as fast and consumes about half the RAM as previous versions, so update if you haven't already.
You may also need to download and install .Net Framework 4.0 (required, but your computer may already have it) and the Microsoft Access Database Engine (not required, but it speeds up Excel import/export).
IATA is provided free by Fernando Cartwright and was developed with a great deal of support from many people, with special thanks to Vincent Greaney and others from the World Bank.