Would somebody please help me with my English?

1- Responders might belong to the group that has experience with the study cases.

2- Not all participating institutions had policies limiting the activities of the detailers, this might contribute to the differences in the study results.

3- The number of responded surveyed and responsed were different. Results of the study might be biased to the center with more surveyed and responded doctors.

4- Also, contact was established by letter and follow up with telephone call 1-2 weeks later. This can reduce as much as non- respondent as possible which can affects the reliability of statistical estimates which may lead to the interpretation bias.