The result of the study showed that the students' problem solving and retention was significantly improved after the intervention. This finding is in parallel with other findings, which showed the effectiveness of flipped learning in terms of knowledge acquisition in medical education[18, 23, 28-32]. The results reveal that flipped learning is useful in the encouragement of deep and active learning[33, 34] and school achievement . The flipped learning or classroom is increasingly getting a reputation as a fascinating and successful instructional methodology. Evidence is still scarce in the utilization of flipped learning in medical education and especially surgical education and the field of trauma. This is the first paper which investigates the applicability of the flipped learning in the trauma for general surgery residents.
Moreover, in our survey, the retention of the knowledge was confirmed by a lack of difference between the early and late post-test. We believe that this occurred possibly because the provided materials were prepared according to daily encountered scenarios of our local trauma care. This may enforce contextual learning resulting in a comparable late post-test in comparison to the early post-test. As in our study, several surveys have proposed this method causes more satisfaction of participants, program directors, and may increase test scores in comparison to traditional methods [22, 36]. Similar to other studies, this research also shows a high level of satisfaction among participants In a study by Tan et al., 96% of respondents were satisfied with the belief that their flipped curriculum in emergency medicine was over the traditional curricula. The participants believe that having the resources, in advance, encourages learning and is considered as a motivator . Moreover, in a cohort study, it was shown that besides experiencing a higher level of satisfaction, medical clerks with flipped surgical curriculum were more interested in pursuing surgery as the prospective field of the study and profession. Another study by Liebert et.al in surgery clerkship showed that about 90% of students rated their flipped learning curriculum excellent or outstanding. Moreover, 84% of the clerks believed that other clerkships should adopt a flipped based curriculum .
Advocating self-directed, active, and efficient learning are chief objectives for educational interventions for fostering adult learning (andragogy) [18, 38, 39]. Other studies have shown the effects of flipped learning in metacognitive skills . One main reason is that in this method, the residents have the authority to manage resources, pace, place, and time which makes them capable of self-regulation and performance [18, 34, 41]. In our study, we found that the residents were highly satisfied with their curriculum and uniquely interactive case-based discussion. They found it was constructive in decreasing their stress in real patient encounters. Another study confirms these findings as the participants believed that flipped learning and especially case-based discussion sessions could lead to a higher level of cognitive functions such as analysis and synthesis of knowledge[42, 43]. Multiple papers concerning flipped learning support the effectiveness of case-based learning in these settings [19, 44, 45]. Cases based learning will foster higher-order educational objectives in bloom taxonomy and cognitive ladder. The cases can serve as a well-established infrastructure and scaffold in which reinforces and categorizes the concepts and knowledge. Others believe that this method is helpful as it encourages constructivism in medicine. They suggest that flipped learning helps learners to build their knowledge based on the pre-class materials, the case-based discussion, and their clinical experiences. [17, 46]
One of our residents believed that the duration of the videos was lengthy; however, it was comprehensive. Our videos were 17 hours on 13 subjects (with an average of 1 hour for each topic and max 1.5 hours). We believed, as an improvement, all future video sessions should be limited to 1 hour. This finding is consistent with another study in which the authors finally decided to change the preparation time of the videos to 60 minutes following the commencement of their study [45, 47]. Some experts believe that long videos can easily fill people's working memory, which has minimal capacity. However, shorter videos make the individual process the information actively and follow the change.
In this regard, we noticed that our participants tend to use podcasts more than other materials. It is possibly because of the nature of the podcast, which is remarkably shorter than the videos and more flexible to use based on time and place. Our findings are similar to another study in which synopsized materials such as podcasts were more favorable to be used by the participants .
Moreover, our residents believed that videos and books help more to foster knowledge. The case-based discussion ( CBD)makes them more confident in dealing with the patients (table 2,3). It may be implied that the high rate of the CBD is potentially due to the preclass mind preparation of the participants, which encourages learning  and another proof of concept for flipped learning. We noticed that none of the materials result in better results, per se. But in accumulation, they led to a significant difference, which is proof of the concept for this method (Table 2). We believe the ideal framework should entail all of the formats of the materials and multiple strategies to cover numerous learning styles and meet the educational objectives of the program .
The high acceptability of the flipped learning in this study revealed the efficacy and utility of this method in the surgical field and especially trauma. In surgical residency programs, different schedules of residents, limited educational times, and immediate need for care of trauma victims threaten proper, uniform, and synchronized education of residents. Flipping the education will help the residents to have an invaluable source of learning, which is always available to them. This method also saves the time of instructors to repeat basic and repetitive facts. The latter help the surgical team to focus on more advances point of care details which is often neglected due to time constraint.
Although our general surgery programs is one of the most significant general surgery programs in Iran, this study was in a single school with limited sample size. The high rate of the participants may mitigate the biases, but further investigations are required