Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:The admission assessment included determining the risk factors for cerebral infarction,13,14 clinical ischemic stroke category (National Institute of Neurological Disorders and Stroke1), and disease severity using the National Institutes of Health Stroke Scale.15
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 Abbreviations
 AF = Atrial fibrillation
 LAA = Left atrial appendage
 LAA eV = LAA emptying flow velocity
 LAD = Left atrial dimension
 LAWV = LAA peak wall velocity
 ROC = Receiver operating characteristic
 SEC = Spontaneous echocardiographic contrast
 TEE = Transesophageal echocardiography
 TTE = Transthoracic echocardiography
 We defined patients who had no histories of AF and no documented AF on continuous electrocardiographic monitoring during hospitalization as the sinus rhythm group, patients who had histories of AF before admission and/or continuously documented AF on continuous electrocardiographic monitoring during hospitalization as having chronic AF, and patients who had AF at admission and recovered to normal sinus rhythm during hospitalization and/or showed sinus rhythm at admission and documented transient AF on continuous electrocardiographic monitoring during hospitalization as having paroxysmal AF.
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