Our study is the first to show that physicians who have low levels of hope are more likely to report self-perceived errors than those with average and high levels of hope. We also found that hope worked as an effect modifier of the known association between physicians' distress and self-perceived medical errors. A possible explanation for our findings is that hope decreased the reporting of self-perceived medical errors by mediating the psychological distress that usually accompanies a mistake. Prior research has found that those with higher levels of hope reported lower levels of psychological distress and a better QOL 
, and these have been proven to be factors that are associated with a low incidence of self-perceived medical errors 
. For example, some physicians who have higher levels of hope, even though they rate high on the burnout scale, may see possibilities for the future, and as a result, their psychological distress may be reduced. In contrast, physicians with lower levels of hope are not able to adjust to a difficult work environment, and report higher frequency of medical errors.
Hope is usually researched in relation to terminal care 
and in such situations hope is usually treated as a better outcome, not an exposure. Jerome Groopman suggests in his book, The Anatomy of Hope, that “to have hope, then is to acquire a belief in your ability to have some control over your circumstances"; this assertion is supported by our results 
. One study that evaluated hope as an effect-modifying factor supports our hypothesis. Among 194 female patients with breast cancer, hope measured by HHI was found to mediate the relationship between psychological distress and health status, such that the direct association between distress and health status was no longer significant after hope was included as a variable 
We found that self-perceived major medical errors were common among the practicing physicians, with approximately one-third of the participants reporting a major error at least once during 1 year. Our result that physicians commonly report self-perceived major medical errors is consistent with a previous report. In a survey of 184 resident physicians, an average of 14.7% of participants reported errors per resident-quarter 
. In a cross-sectional study among 7905 members of the American College of Physicians, 700 (7.9%) physicians reported a concern they had made a major medical error in the previous 3 months 
Most previous studies on errors among practicing physicians focus on systemic issues that contribute to errors rather than on individual-level distress factors 
. Consistent with this focus, efforts to reduce errors have largely centered on improving coordination of care, teamwork, electronic order systems, and other system-related changes 
. However, it has been postulated that many explanatory factors for medical errors “remain to be uncovered" 
. Efforts to reduce errors resulting from individual-level distress factors, as we suggested, need to incorporate a variety of strategies, including efforts to reduce physician's degree of emotional distress and burnout 
. Not only reducing emotional distress, our results suggest that the strategy to increase hope among physicians might directly and indirectly reduce medical errors, although this needs to be tested in a prospective intervention study.
This study has some limitations. First, because this is an observational study, residual confounding might explain our results. Especially, we asked about medical errors in the past 12 months, which differs from the 3 months in the previous studies 
. Using longer time frame might allow other factors such as life events and changes in fatigue to confound our results. Proportion of female physician of our sample was very low, although this is typical in Japanese physician population. Our results might not be generalizable to other non-Japanese physician population. In addition, HHI is a validated metrics to measure hope among patients, it has not been formally validated among physicians. Although this tool has never been used in physicians, this tool is used to measure hope in other health professional (nurses) 
. In addition, we did not observe ceiling effect or flooring effect in our sample. And more, our study suggested that physicians' hope measured by HHI discriminate between high error rate and low. For these reasons, we believe HHI is useful among physicians.
In conclusion, physicians' hope modified the association between physicians' burnout and self-perceived medical errors, and hope also exhibited a strong association with self-perceived medical errors. Efforts to reduce errors resulting from individual-level distress factors need to incorporate a strategy to increase physician's degree of hope.