Table 1. The case and control groups for the evaluation.

Indicator	Cases	Control 1	Control 2	Control 3	Control 4
Region	Cartago	Cartago	Cartago	PZ	PZ
Timing	After	Before	After	Before	After
Entry date	X-ray visit 1	January 1991	May 1997	January 1991	May 1997
Exit date	December 2004	Visit 1 or May 1997a	December 2004	May 1997	December 2004
N participants	6206	20 030	11 190	11 915	11 318
N person-years	45 231	123 121	80 630	73 666	82 408
Males	64%	51%	42%	51%	51%
Mean age	64.3	57.9	64.3	58	64.6
High school education	7.8%	—	8.6%	—	8.1%
Have refrigerator	86%	—	90%	—	83%

PZ=Pérez Zeledón.

^aObservation in this group ends (it is censored) at the date of the first screening visit if the individual became a ‘case’ or in May 1997 if he did not.