Each PISA cycle assesses three core domains (reading, mathematics and science), one of which constitutes the major domain in a given cycle (reading in 2000 and 2009; mathematics in 2003 and 2012 and science in 2006 and 2015). The major domain receives a larger portion of testing time and questionnaire items tend to focus on students’ engagement, attitudes and dispositions towards the main subject domain. Since 2015 the assessment was delivered by computer. Testing material in PISA is typically organised around subject specific clusters designed to take around 30 minutes to complete and each booklet contains four clusters of test items.