21 However, while greater attention to confounder adjustment would greatly improve future studies, it should be noted that, even if groups were found to be comparable on measured characteristics or adjustment was made for many relevant variables, it remains possible that differences identified may be attributable to other unknown factors differing between the groups.
This large amount of variability explained by the second level variable could be attributable to some unknown, or not-directly measurable characteristics (such as surgeon's ability), or to a "pool of unfavourable characteristics", whose effects are negligible if considered individually.
Therefore, the myocardial lesions found in calves accidentally overdosed with doxycycline may be attributable to other yet unknown factors.
More recently honeybee colony losses have occurred across America and Europe that cannot be attributable to the Varroa mite, so unknown combinations of stressors are suspected (Potts et al., 2010).
The causes are currently unknown but might be attributable to different study populations and research protocols.
The reason for this difference in preferences is unknown, but may be attributable to cultural factors.
Although the precise reason for these differences between TCA and SSRI is unknown, it may be attributable to the different mechanisms of action for these two classes of drugs [ 17].
The reasons for the differences in findings are unknown but may be attributable to differences in the ethnicities and age groups studied and the markers of insulin resistance used.
The causes of sexual dimorphism in trophic morphology in T. cristinae remain unknown but could be attributable to various mechanisms, including fecundity-based sexual size dimorphism, sexual selection, or differential feeding ecology between sexes.
The differences between the results of that study and ours remain unknown but may be attributable to the various degrees of severity of HF, the underlying causes of the disease, the normalization of RT-PCR results and a possible desensitization of adenosine receptors subtypes in presence of high adenosine concentrations [ 38, 39].
