<prompt> In an epidemiological study, the first case which comes to the attention of the investigator is-Reference caseIndex casePrimary caseSecondary case  <response>: Ans. is b i.e., Index case o Primary case -- First case of communicable disease introduced into the population. o Index case -- First case to come to attention of the investigator. o It is not always necessary that primary case will be index case : - i When the first case noticed by the investigator is also the first case of the population --- Primary case and index case are the same. ii When the first case nociced by the investigator is not the first case of the population primary case -- Index case and primary case are different. o Secondary cases -- Cases those developing from contact with primary case. 