A researcher performed a study to assess the prevalence and determinants of breastfeeding in Butler area. She randomly selected four hospitals from the list of all hospitals with maternity clinics in the area. From each hospital, she then selected a random sample of 10% of all mothers who delivered a live baby in the month of June 2019. She mailed a survey to those mothers asking about breastfeeding of their new babies. What are the potential sources of error in this study