In STD cases, a patient is asked to name other persons is his socio sexual group, then they are investigated. This is a example of -

<p> In Contact tracing the sexual paners of diagnosed patients are identified, located,investigated and treated. Patients are interviewed for their sexual contacts by specially trained staff. Reference:Park&;s textbook of preventive and social medicine,K.Park,23rd edition,page no:339. <\p>