Question: A study has been carried out to determine the risk of upper gastrointestinal bleeding (UGIB) associated with the use of different non-steroidal anti-inflammatory drugs (NSAIDs). For this purpose, 2,777 patients with UGIB and 5,532 patients matched with the previous ones were included by age and month of admission or consultation, in the same hospitals, but for reasons that had nothing to do with the use of NSAIDs. The comparative risk of suffering a UGIB associated with previous exposure to different NSAIDs was calculated. What kind of study is it? 
 Options:  <1> Cohort study. <2> Cases and controls study. <3> Transversal study. <4> Experimental study <5> Ecological study
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