Jean Piaget based his ideas about infants' cognitive development on his detailed observations of his own three children. This is an example of what research method?

Case Study

Explanation:

Case Study -

It is the type of research method, where the in depth investigation about a particular object, person, community or group is done, is referred to as case study.

In this research method, the data and information is collected from various sources available, in order to have a wider range of information.

This research method, is mainly used in the field of psychology.

Hence, from the given example of the question,

The correct term is case study.