The results of the present research refute our hypothesis that students receiving spaced instruction would have significantly better long-term anatomy knowledge retention than students receiving massed instruction. The two cohorts had statistically equal pre-test, post-test, and improvement factor scores, even when controlling for significant differences in participant characteristics. While these results apply to our two cohorts of DPT students, they cannot necessarily be extrapolated to cases of spaced versus massed course delivery with other samples and in other circumstances, due to potential confounding factors described below.
Our results contrast with other studies showing spaced learning is more beneficial in long-term memory retention [3, 4, 24,25,26,27]. Spaced learning allows for more frequent and temporally distributed retrieval opportunities, which have been shown to increase long-term knowledge retention [4, 28,29,30]. Increased spacing also decreases cognitive load by decreasing the number of elements being processed by working memory in a given time period .
We have identified only a limited number of studies where massed learning proved superior to spaced course delivery. The study designs, time frames, target population, and types of learning vary greatly, and cannot be easily compared to one another, nor to studies measuring acquisition of anatomical facts. Their existence, however, supports the idea that in certain circumstances learning can be potentiated through the use of massed instruction and fewer retrieval episodes, and this topic merits further research. Circumstances where massed learning may be more beneficial tend to involve motor learning and include: patient acquisition of upper limb motor skills after stroke with the use of constraint induced movement therapy delivered in a massed format ; improvement in the ability to read radiographic images through massed practice sessions believed to improve eye tracking abilities ; and improvement in objective and subjective vocal function through vocal rehabilitation exercises delivered in a massed format to patients with dysphonia .
While our main findings were surprising, other known patterns of student test behavior were borne out by our results. A strong, negative correlation was found between pre-test scores and improvement percentage, reflecting the known phenomenon that participants with lower baseline scores will generally improve more than those with higher baseline scores [34, 35]. Similarly the fact that students’ mean post-test scores, while increasing substantially compared with the pre-test scores, were still quite low, is in agreement with previous research showing significant loss of basic science knowledge over time [9, 36, 37]. This type of knowledge will inevitably be forgotten over a period of months and years following the last study session .
A search of the literature found no other anatomy-related studies on spacing and knowledge retention that utilized a retention interval length (i.e. the time between the final study session and the post-test) commensurate to the one used in the present research. A limited amount of research has, however, been performed assessing variance in anatomy knowledge retention using shorter retention intervals. For example, Dobson et al.  assessed anatomy recall within an undergraduate student cohort, using a variety of retrieval strategies and time between study/retrieval sessions. They did find that distributed practice benefitted their purported long-term (28-day) retention significantly more than massed practice. The short retention interval used in this research does, however, limit the external validity of their results for DPT and medical students, for whom anatomy recall is required over a period of years during school and beyond.
In addition to the length of anatomy instruction, the LONG and SHORT cohorts experienced other differences in their overall DPT curricula, which may have affected the present findings. Although the general sequence of courses remained the same for both cohorts, the LONG cohort had clinical affiliations that began earlier in their curriculum, at the end of their first year of didactic instruction, while the SHORT cohort’s affiliations were set to begin at the end of their second year of didactic instruction. Thus, at the time of the post-test, the LONG cohort had already completed three clinical affiliations (for a total of 26 weeks), while the SHORT cohort had not yet participated in any full-time clinical affiliations at the time of their post-test. An increasing body of research supports the concept that clinical experience can actually interfere with knowledge retention, and that the development of clinical reasoning requires a certain amount of forgetting of the details of underlying basic science and medical knowledge. This phenomenon has been studied among medical students, residents, and physicians and is known as the “intermediate effect” . According to this theory, their 26 weeks of clinical affiliations may have given the LONG cohort relative expert status. Once practitioners are experts, recall of facts is inevitably attenuated as they start to develop more clinical reasoning skills that allow them to understand a problem at a deeper level and to incorporate the use of causal models or illness scripts in patient treatments. Conversely, the SHORT cohort, having finished virtually all of their didactic coursework but lacking in significant clinical experience, can be considered to have intermediate expertise, and according to the intermediate effect, these students should have the best recall of facts [39, 40].
The potential impact on the SHORT cohort of taking the anatomy lecture and lab concurrently is another important difference between the curricula experienced by the two cohorts. This may have had a positive effect on their test scores, because of the potential benefits to students who are kinesthetic learners. Although the importance of learning styles has been disputed , some students do identify as kinesthetic learners and express a preference for learning opportunities that combine didactic and kinesthetic approaches .
Sample size limitations related to our use of a convenience sample may have affected the lack of significant differences in anatomy knowledge retention seen among our two cohorts. Our results did trend in favor of the SHORT cohort, but additional participants could have strengthened this into a significant difference, or even potentially have reversed it. Using just these two cohorts, however, allowed for the control of most other differences in instructional delivery that occur inevitably over time and, we felt, increased the overall quality of our sampling.
As discussed above, the change in anatomy timing was just one component of a major curricular change that impacted multiple aspects of the DPT program at our university. Although anatomy content presented was virtually identical for both cohorts, other differences in their educational experiences could not be avoided. Specifically, the concurrent lecture and lab experienced by the SHORT cohort, and the differential timing between the cohorts of participation in clinical affiliations, may have affected post-test performance for both of our cohorts.