Which factor is key in underwriting for a critical illness scheme?

The key factor in underwriting for a critical illness scheme is morbidity. This is because critical illness insurance specifically covers illnesses that can have significant impacts on a person's health and lifestyle. Morbidity refers to the incidence of illness or disease in a population, and understanding morbidity rates helps insurers assess the risk associated with specific critical illnesses.

By analyzing morbidity, underwriters can evaluate how likely it is for individuals to develop the specified illnesses covered under the policy, such as cancer, heart attacks, or strokes. This information is crucial in determining appropriate premiums and ensuring the insurer can manage the risk of providing coverage for such serious health conditions.

The other options, while relevant in various contexts within insurance, do not represent the primary focus for critical illness underwriting. Mortality is more pertinent to life insurance but does not directly assess risks associated with critical illness itself. Claim frequency relates to how often claims are made but does not provide insights into the likelihood of developing critical illnesses. Demographics can influence underwriting decisions but are not as central as morbidity in this specific case.