MESA Note 6: Iterations and Convergence of Estimation

by John M. Linacre
MESA Research Note 6, November 1998

The Rasch model is based on the non-linear logistic function. In general, estimates for measures cannot be obtained directly, but must be obtained through a process of improving estimates until they are good enough ("converged"). The process requires performing the same computations over and over again ("iteration").

The iterative process is shorter when:

  1. The distribution of the person, item, judge, etc., measures is close to normal.
  2. The range of measures is narrow.
  3. The data are "complete".
  4. The data fit the Rasch model.
  5. The persons and items are on-target.
  6. The items are dichotomies.

In one data set with very adverse characteristics, over 600 iterations were required before meaningful estimates were obtained.

In many cases, the estimates are good enough after only a few iterations. A large number of iterations may only be needed for the final, definitive analysis.


Go to Top of Page
Go to Institute for Objective Measurement Page



Coming Rasch-related Events
Oct. 7 - Nov. 4, 2022, Fri.-Fri. On-line workshop: Practical Rasch Measurement - Core Topics (E. Smith, Winsteps), www.statistics.com
Nov. 2 - 30, 2022, Wed.-Wed. On-line course: Intermediate/Advanced Rasch Analysis (M. Horton, RUMM2030), medicinehealth.leeds.ac.uk
Dec. 1 - 3, 2022, Thur.-Sat. In-person Conference: Pacific Rim Objective Measurement Symposium (PROMS) 2022 proms.promsociety.org
Jan. 25 - March 8, 2023, Wed..-Wed. On-line course: Introductory Rasch Analysis (M. Horton, RUMM2030), medicinehealth.leeds.ac.uk
June 23 - July 21, 2023, Fri.-Fri. On-line workshop: Practical Rasch Measurement - Further Topics (E. Smith, Winsteps), www.statistics.com
Aug. 11 - Sept. 8, 2023, Fri.-Fri. On-line workshop: Many-Facet Rasch Measurement (E. Smith, Facets), www.statistics.com

 

Our current URL is www.rasch.org

The URL of this page is www.rasch.org/rn6.htm