A considerable amount of literature has been published on …
However, there has been relatively little literature published on …
Numerous studies have argued that …
Several studies have revealed …
The research to date has tended to focus on …
Data from several studies have identified the …
It has been suggested that …
It has been conclusively shown that …
It has been demonstrated that …
It is thought that …
Other studies have considered the relationship between …
The first systematic study of … was reported by …
The study of … was first carried out by …
Detailed examination of … by … showed that …
In a randomised controlled study of …
A comparative study by … found that …
This view is supported by …, who argues that …
A key problem with this argument is …
However, there is inconsistency with this argument, given …
X’s interpretation overlooks much of …
One question that needs to be asked, however, is …
One problem with this approach is …
The main limitation to X’s study is …
However, this method of analysis has a number of limitations.
However, the research does not take into account …
The author offers no explanation for …
The main weakness of this study is …
Previous studies have only focused on …
The existing research fails to …
Much of the recent literature has not …
