Impact of ability distribution on IRT observed score equating and IRT chained equipercentile equating

Equating is an important statistical process used to adjust for differences in difficulty across different test forms. Based on Item Response Theory (IRT), observed score equating (OSE) method is widely used in practice. When implementing IRT OSE, item parameters and ability distributions are necessary to construct fitted score distributions for both new and old forms. Traditional equipercentile equating is then used to equate the two forms.