Can You Explain The Disadvantages Of Quota Sampling?

DISADVANTAGES:
1) It is a subjective method. One has to choose between objectivity and convenience.
2) If random sampling is not employed, it is no longer theoretically possible to evaluate the sampling error.  
(Since the selection of the elements is not based on probability theory but on the personal judgment of the interviewer, hence the precision and the reliability of the estimates can not be determined objectively i.e. in terms of probability.)
3) Although the purpose of implementing quota controls is to reduce bias, bias creeps in due to the fact that the interviewer is FREE to select particular individuals within the quotas. (Interviewers usually look for persons who either agree with their points of view or are personally known to them or can easily be contacted.)
4) Even if the above is not the case, the interviewer may still be making unsuitable selection of sample units.

(Although he may put some qualifying questions to a potential respondent in order to determine whether he or she is of the type prescribed by the quota controls, some features must necessarily be decided arbitrarily by the interviewer, the most difficult of these being social class.)

If mistakes are being made, it is almost impossible for the organizers to detect these, because follow-ups are not possible unless a detailed record of the respondents' names, addresses etc. has been kept.

Falsification of returns is therefore more of a danger in quota sampling than in random sampling. In spite of the above limitations, it has been shown by F. Edwards that a ordered quota inspection with respectful interviewers can generate quite adequate results.