If we want to know the quality of teaching in a school then our informants will be the students studying in that school.
The difficulties that can be encountered in conducting direct personal investigation are:
i) This method is expensive in terms of the time spent, money and efforts involved.
ii) There exist chances of personal bias on part of students, hence results might not be accurate.
iii) Since the field of investigation is large; thus it will be difficult to approach each and every student of the school.