What do you mean by 'Indirect Oral Investigation'? Explain its merits and demerits

In Indirect Oral Investigation, data is collected through indirect sources. Persons who are likely to have information about the problems, are interrogated and on the basis of their answers, factual data have to be compiled. Most of the commissions of enquiry appointed by government collect primary data by this method. The accuracy of the method depends largely upon the type of persons interviewed and hence these persons have to be selected very carefully.
Merits of Indirect Oral Investigation:
(i) It is economical in terms of time, cost and manpower.
(ii) Confidential information can be collected.
(iii) Information is likely to be unbiased and reliable.
(iv) This method is relatively simple to under-stand.
Demerits of Indirect Oral Investigation.
(i) The degree of accuracy of information is less.
(ii) This method leads to doubtful, conclusion due to ignorance and carelessness of the witness.