Experimental generalisability studies were carried out as part of the IELTS Speaking and Writing Revision Projects to investigate the reliability of ratings (Shaw 2004; Taylor & Jones 2001). More recent G-studies based on examiner certification data showed coefficients of 0.83–0.86 for Speaking and 0.81–0.89 for Writing.
The IELTS exam contains four components upon which an overall band score is awarded. Thus an estimate of composite reliability offers a useful measure for overall test reliability. Following Feldt & Brennan (1989), composite reliability estimates were calculated based on test data from 2009. To generate an appropriately cautious estimate, minimum alpha values were used for the objectively marked papers, and G-coefficients for the single rater condition on subjectively marked scores. The composite reliability estimate for both the Academic and General Training modules was 0.96 and produced a composite SEM of 0.23.