Le opinioni degli studenti della didattica in università: sono attendibili le differenze nei risultati aggregati di facoltà?

In recent years, student's opinion surveys on teaching quality in Italian Universities have changed considerably in methods and scope. The present work considers the validity and meaning of the data from such surveys, with reference to the work conducted by the "Nucleo di valutazione" (Evaluation Committee) of Turin University. Two sets of problems are of particular interest: the individual context of the data vs. their relevance in aggregate analysis; causes and meanings of the differences between aggregate results for nature sciences Faculties and human sciences Faculties. The first item has great relevance due to the diffusion and standardization of survey techniques, which encourages the assessment of whole Faculties and Courses based on the collected data. Regarding the second item, some of the possible factors behind such differences are assessed and compared, also considering appropriate dispersion decomposition techniques. It is shown how many effects must be considered to really understand the nature of this phenomenon.