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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