Evaluation plans and strategies consistent with the purpose of the PBL curriculum were strongly advocated [13, 14]. The assessment was driven by the demand for constructive adjustment between intentions and evaluations of PBL, the conventional summative assessment model was not really fit well to PBL . Hence, the formative assessment protocols including student performance, student self-mutual evaluation and tutor teaching evaluation were adopted in this study, which provide unique opportunity to judge each other’s work between tutors and students wherefore making them develop the ability to reflect on own strengths and weaknesses as these are central elements of self-directed learning (SDL) .
PBL was assumed to promote other abilities than knowledge and skills, such as collaboration, communication and regulated learning ability and professionalism .
The result of exceeding average 2 points, equivalent to 4 points in a hundred marks system, showed Grade 4 students received higher overall student performance ratings, it was learned from tutor’s feedback that senior students have demonstrated obviously improved self-directed learning (SDL)capabilities, which was manifested as more adequate preparation, more eye-catching communication and expression in the discussion process. Elzubeir MA's research  found that senior students had statistically significantly higher mean scores on 11 of 14 self-assessed SDL competencies (p < 0.05) in comparison to junior students. Yet it was noted that no difference between points of 39.16 ± 4.12(40.000) from Grade 1 and 39.55 ± 3.82(40.000)from Grade 4 in the first unit of PBL. Under the conditions of first attempt, this is very likely related to the writing of the problem scenarios and the teacher's guidance. Undoubtedly, the preparation of problem scenarios is the focus of learning and the framework of discussion . In our study, all the problem scenarios were compiled after collective discussion by all tutors instead of the existing cases of other schools. Therefore, it was mainly focus on humanistic education and common oral problems in life for 1st year students but emphasizing the integration of “etiology-mechanism-clinical manifestations-diagnosis-treatment” of common dental clinical diseases for Grade 4. Taking into consideration of characteristics of teaching subjects to set more targeted problem scenarios may weaken the difference in knowledge and skill discrepancy between the higher and lower grade to a certain extent. In addition, all tutors who serve as the first grade were selected from last round of PBL teaching and that their own experienced skills may also play a positive role in guiding first year students. For example, tutors can strengthen the interaction of tutor and student by asking questions so as to facilitate students to express themselves more actively . The above two keys were also positively affirmed by the interviews with students when course finished.
A basic part of the PBL process is that students are responsible for being active members of a group of learners. This may include the practice of collaboration and self-assessment, which may enhance reflection and self-awareness . More than 90 scores of self-mutual evaluation were observed both in 2 grades, especially the fourth year students, all scores were above 95 points. Compared with Grade 1, the senior students gave themselves and their peers significantly higher self-mutual scores from the beginning of PBL, and this reached peaks at the middle and late stages of the course (Table 2), this may mainly depend on the following factors: first of all, the time point of PBL course for 4th year students was set after completion of all basic and clinical courses of Dentistry and before students entering the clinical practice. On the one hand, senior students have mastered a certain professional knowledge after 3-year studying, the implementation of PBL just cater to the demands for combination of theory into practice, which was likely to more attractive to students. All above were the foundations and conditions that 1st students can’t possess. Hagi SK’s research  revealed that a positive and higher mean score was found in all statements of the questionnaire about students' perception of PBL in the new curriculum among fourth year medical students than those of second year students. And, regardless of high or low grade, students scored themselves and peers generously, always above their tutor's marks, which were similar with Machado JL’s study .In addition, there was no significant correlation between student self-mutual evaluation and scores from tutors, the correlation coefficient for Grade 1 was 0.164, and those of 5 assessments for Grade 4 was 0.020,0.048,0.121,0.060, -0.193, respectively (table3), this may be caused by inconsistent evaluation standards and lack of transparency in procedures  between students and tutors, similar results have been reported in previous research [20, 23, 24]. Nevertheless, the high grade of self- mutual evaluation also reflects the efforts of students in the teaching process, because it still plays an important role in supporting reflection and self-awareness skills development .
The tutor’s framing of active student learning and building on knowledge is an decisive factors of successful implementation of PBL . However, the repositioning of the role of tutor remains challenges all the time due to it occupies a central and unique role in influencing students’ learning . Student assessment information speaks to the outcomes of the educational process, and often serve as benchmarks for comparison and formative assessment for teachers . The same 8 tutors received higher ratings from Grade 4 students, this showed that the acceptance of the new role of teachers in PBL teaching by the students in the first grade was lower than that of fourth grade students. Yet, it was not difficult to find that the average score of teacher evaluation was 97.20 ± 2.64 (98.000) but 91.93 ± 3.52(91.550) for themselves and their peers in Grade 1, indicating that the recognition of tutors by Grade 1 students was still very high. Therefore, in addition to the consciousness of students, the potential influencing factor may depend on the expertise of tutors. Indeed, there is no doubt about the ability of the medical teaching team in terms of professional quality, but the comprehensive ability of multidisciplinary knowledge and the integration of medical science with other social sciences such as the Humanities was not enough, the study observed the existence of differences between medical tutors and non-medical tutors in facilitating the group process . Tutors with expertise tended to serve as a more directive role, spoke more often and for longer, provided more direct answers and suggested more discussion topics in the tutorial , which may play a positive but negative role of promoting the group process in 4th and 1st year students, respectively.