The phrase "critical determinant" refers to a key factor or element that is crucial in deciding the outcome of a situation. It is a significant factor that strongly influences or determines the result or success of something. Full definition
However, there is now strong evidence that the length of leave in and of itself (beyond a minimum of perhaps four months) is not as critical a determinant of maternal mental health or child development outcomes as is the set of personal, family, and workplace stresses, resources, and supports that operate jointly and interact with leave experiences. (child-encyclopedia.com)
Tumor protein p53 is one of the most critical determinants of the fate of cancer cells, as it can determine whether a cell lives or dies in response to stress. (roswellpark.org)