The phrase "earlier research" refers to studies or investigations that were conducted before the current study being discussed. It means looking at what other scientists or researchers have found out about a topic in the past. Full definition
A review of earlier research on the effect of charter schools on students in traditional district schools can be found here. (educationnext.org)
Couples who experience fertility difficulties have been shown in earlier research to pay a cost both physically and psychologically. (datingadvice.com)
The findings go against earlier research suggesting that age is a significant factor in student achievement. (edweek.org)