ICCS 2009 Technical Report - IEA

Chapter 12:Scaling procedures for ICCSquestionnaire itemsWolfram Schulz and Tim FriedmanIntroductionThis chapter describes the procedures used to scale the ICCS questionnaire data (for students,teachers, and schools) and the indices based on them.In general, it is possible to distinguish two general types of indices derived from the ICCSquestionnaires:1. Simple indices constructed through arithmetical transformation or recoding, for example,ratios between teachers and students; and2. Scale indices derived from scaling of items, a process typically achieved by using itemresponse modeling of dichotomous or Likert-type items.The first part of this chapter lists the simple indices that were derived from the ICCS dataand describes how they were created. The second part outlines the scaling procedures used inICCS. The third and final part, lists the scaled indices, along with statistical information onitem parameters, scale reliabilities, and the factor structure of related item sets.The cross-country validity of item dimensionality and constructs was assessed during the field trialstage of ICCS. At this time, data were used to assess the extent to which measurement models heldacross participating countries. Extensive use was made of both confirmatory factor analysis anditem response modeling, a process that made it possible to examine cross-national measurementequivalence before conducting the final selection of main survey questionnaire items (Schulz,2009).Simple indicesStudent questionnaireStudent age (SAGE) was calculated as the difference between the year and month of the testingand the year and month of a student’s birth. Data on student age were obtained from both thequestionnaire and the student tracking forms. The formula for computing SAGE wasSAGE = (100 + T y – S y ) + (T m–S m )12 ,where T y and S y are, respectively, the year of the test and the year of birth of the tested student,in two-digit format (e.g., “06” or “92”), and where T m and S m are respectively the month of thetest and the month of the student’s birth. The result is rounded to two decimal places.Occupational data for each student’s parents were obtained by asking open-ended questionsabout the jobs of the student’s mother and father. The responses were coded into fourdigitISCO codes (International Labour Organization, 1990) that were then mapped to theInternational Socioeconomic Index of Occupational Status (ISEI) (Ganzeboom, de Graaf, &Treiman, 1992). The three indices obtained from these scores were mother’s occupationalstatus (MSEI), father’s occupational status (FSEI), and the highest occupational status of bothparents (HISEI), with the latter corresponding to the higher ISEI score of either parent or tothe only available parent’s ISEI score. For all three indices, higher scores indicate higher levelsof occupational status.Parental education is another family background variable. The core difficulties with this variablerelate to international comparability (education systems differ widely across countries and overtime within countries) and response validity (students are often unable to accurately report157