To minimize loss, not only taking the informed consent but also enlisting the commitment to continue and cooperate in the study is to be given maximum consideration at the commencement of study itself. If lost during follow up, every effort must be made to trace and include them.

The information of the lost persons either from registers (e.g. death registers) or from the neighbors and relatives is also to be considered at the time of analysis.

It is always tried to keep the non-response at a low level to improve the validity.