Ihese variables have been transformed so that they have a mean of 2 and a standard deviation of 1 across the pooled sample of all participating countries, thus allowing meaningful comparisons across countries. While this transformation implies that the levels of use cannot be easily compared across skill types, such comparisons would be conceptually difficult to make anyway. For example, is using ICT skills every day equivalent to using learning skills every day in terms of how intensively ICT and learning skills are used at work? For example, the reading and writing indices are derived from a large set of questions concerning the frequency with which several types of documents (directions, instructions, memos, e-mails, articles, manuals, books, invoices, bills and forms) are read or written during one's regular work activity.