Research.
“Research” is normally a non-countable noun and you can live a full and happy life without ever writing “researches.”
Some use it in the plural to mean “separate scientific investigations” but others (including the writers of these weekly tips) find “researches” very strange. If possible, it is therefore better to use just research, especially when you mean it in a very general way – e.g. “Research has shown…”
As you know, “study” is a regular ol’ count noun:
“A study has shown…”
“Studies have shown…”
Jason Blake and Monika Kavalir
An ad hoc list of all tips from this academic year is available at:
http://www2.arnes.si/~bjason/tips%20-%202014%202015.pdf
(Skupno 23 obiskov, današnjih obiskov 1)