<prompt> In an epidemiological study, the first case which comes to the attention of the investigator isReference caseIndex casePrimary caseSecondary case  <response>: Ans. is bi.e. Index case Epidemiological terminologyPrimary case - is the first case of a communicable disease introduced into the population unit under study.Index case - is the first case, that comes to the attention of the investigator, it is not always the primary case.Secondary cases - are those cases arising dt contact with the primary case. 