Since short (3-4 year) studies are not sufficient, it would make sense for specific institutions/universities to set up long term studies whereby grad students would conduct a study for one period and then another would take up the study for the next period. They would each write their thesis, and these studies could be conducted for decades so long as the organization kept the flow of PhD candidates following the long term studies.
My question was ... have there been multiple short studies for the same locations? If so, have they ever been combined? And if such studies exist and they have not been combined, why not?