Teeth erosion						
a case-control study carried out among swimmers to 						
investigate the possible association between exposure to chlorinated 						
swimming pool water and erosion of dental enamel. Among 49 swimmers with 						
enamel erosion (the cases) 32 reported swimming six or more hours per 						
"week, compared with 118 of 245 swimmers without enamel erosion (the "						
controls). 						
						
	2x2					
	time 	erosion	controls			
	> 6h	32	118			
	< 6h	17	127			
	Total	49	245			
					H0...	there is no correlation the association between explosure to chlorinated swimming pool water and erosion of dental enamel 
						
						
	I will do the test of Chi2 			X²	4.802	
				X t	3.841	
						
	So we are rejected H0 so there is a correlation between chlorinated swimming pool and erosion of dental enamel