sentence: According to the PDA treatment strategy , KNN classified the population as follows : group 1 , prophylactic PDA treatment without clinical symptoms or abnormal echocardiographic findings ; group 2 , PDA treatment performed without clinical symptoms due to PDA although PDA was confirmed by echocardiography ; group 3 , sPDA treatment as PDA treatment performed because of clinical symptoms due to PDA ; group 4 , only conservative and supportive treatment although with clinical symptoms due to PDA ; group 5 , asymptomatic PDA or spontaneously closed PDA ( nPDA ) ; group 6 , missing or unknown PDA treatment policy .