Table 2. Newcastle-Ottawa scale .

Newcastle-Ottawa scale to assess the quality of non-randomised studies

* = reflects the quality of each section. The possible total points are 4 points for Selection, 2 points for Comparability, and 3 points for Outcomes

Study	Selection	Comparability	Exposure
Liang [10]	***	**	***
Brougton [11]	***	**	***
Piitulainen [12]	**	*	***