Is it necessary to pretest questionnaires before data are collected for analysis?

Pretesting is a very important step in survey research. It is an absolutely necessary step to ensure all kinds of errors that are associated with survey research are reduced. It helps to improve the quality of data significantly. Pretesting is done on a small sample of respondents from the target population. After the pilot test, both the interviewer(s) and respondents are asked a series of questions regarding the survey as well as the process of data collection during the debriefing session. Such debriefing sessions can help detect any problem with the questionnaire design leading to ambiguity of words, misinterpretation of questions, inability to answer a question, sensitive questions, and many other problems associated with the questionnaire as well as the process of administering the survey. It also provides an opportunity to give feedback to the interviewer to ensure that she/he follows the proper protocol of data collection procedures to ensure objectivity in data collection.

collected for analysis i think this is more easy and effictive 

we can explain Pre testing or piloting same as a newly discoved drug which we will not test on a human due to any side effect or death similarly pre testing is important to avoid the errors in data before going for a sample.

If it is seeking opinion, then it should be made mandatory. For observational purpose also, it would be helpful in eliminating redundant data variable and including omitted variables.

yes it  shuld be mandatory to pretest questionaires before data analysis, by doing this u can identified & removed gaps on your questionaires desigh,

All you want is to find the unknown answers of your issue

Questionnaire is a very great tool in pretesting data generation because it gives a statistical overview of what you want.

It is quite important and necessary to pretest the instrument specially when we are going for quantitative analysis of the data; the instrument must be validated by pretesting it with a few of your audiences and referring to the findings if the instrument is actually measuring what it needs to measure. More over some instruments also require them to be tested for consistency as well, but that is not the case in every research project. Once the results for the validation of the instrument is satisfactory, you can proceed with its application.