Under this method, a list of relevant questions (questionnaire) relating to the problem under study is set and the same is sent along with a covering letter to the selected informants by mail i.e. by post who are requested to furnish the necessary data therewith by return of post. This method of data collection is usually followed by private bodies, individuals, research scholars, institutions and government as well. The success of this method depends largely upon the precautions taken in respect of the two things: (1) covering letter and (2) questionnaire.