The age of this skull has been difficult to establish.
Preclinical models utilizing LAM patient-derived cells have been difficult to establish.
A career in the arts has always been difficult to establish.
It has been difficult to establish their exact mechanism of formation.
Why then, has it been difficult to establish a relationship between global temperatures and the incidence of malaria?
Since most states do not question immigration status, it has been difficult to establish the cost of treatment of illegal immigrants.
Although many extant contemporary sources such as paintings, drawings, and diaries have shed light on Okuni's life, the accuracy of such primary sources has been difficult to establish.
Pharmacological blockade studies have supported a role of the dopamine system in ethanol reward for many years, but receptor subtype specificity has been difficult to establish.
It has been difficult to establish responsibility for the school collapses partly because it is unclear in many cases which level of government is responsible for the original school construction and for ongoing inspections.
Both observations support proposals concerning the importance of cilia for melt-crystallized growth and the development of spherulites but would have been difficult to establish without the advantage of oriented nucleation.
Enforcement mechanisms other than sanctions have been difficult to establish, usually because they would require countries to cede significant aspects of their national sovereignty to foreign or international organizations.
