1. Faulty planning, including vague and faulty definitions of the population or the statistical units to be used, incomplete list of population-members.
2. Vague and imperfect questionnaire which might result in incomplete or wrong information.
3. Defective, methods of interviewing and asking questions.
4. Vaguene about the type of the data to be collected.
5. Exaggerated or wrong s to the questions which appeal to the pride or prestige or self- interest of the respondents.
6. Personal bias of the investigator.
7. Lack of trained and qualified investigators and lack of supervisory staff.
An obvious and serious drawback of this sampling scheme is that it is highly subjective in nature since the selection of the sample depends entirely on the personal convenience, beliefs, biases and prejudices of the investigator.