I hope to know when we shall use a census or a sample survey.

It's our pleasure.

Thank you so much.

For studies in which the "population" (i.e., the group of persons or other objects we wish to study) is large, a proper sample survey, as compared to a census, is very useful for cutting down the resources required, including money and human resources, and for reducing the overall burden imposed on the persons/objects to be studied. Another benefit of carrying out a sample survey instead of a census is we can collect, summarise, and analyze data more quickly when the information is urgently needed. More details can be found in the book "Living with Statistics" issued by the Census Department of Hong Kong.