The literature search was performed on the databases Scopus and the search machines, googlescolar.com and hvar.is. Further refrences were found from refrence lists related to the topic.
Leitað var heimilda í gagnasöfnunum Scopus, googlescholar.com og hvar.is auk þess sem farið var í heimildaskrár notaðra heimilda og fleiri heimildir fundar þar.