Table 2: Mean difference between two tonometers at sea level (Acapulco) and 2,234 m above it (Mexico City)

				Paired samples test
				Paired differences
										95% confidence interval of the difference
				Mean		SD		Std. error mean		Lower		Upper		t-value		Sig. (2-tailed)
Pair 1		Acapulco Perkins and Acapulco DCT		2.92927		1.97449		0.30836		2.30604		3.55250		9.499		<0.001
Pair 2		Mexico Perkins and Mexico DCT		4.31707		1.38291		0.21597		3.88057		4.75357		19.989		<0.001
Pair 3		Acapulco Perkins and Mexico Perkins		1.58537		1.59649		0.24933		1.08145		2.08928		6.359		<0.001
Pair 4		Acapulco DCT and Mexico DCT		0.19756		2.01860		0.31525		–0.43959		0.83471		0.627		0.534