Biostatistics Case Studies

CASE 12: Case Fatality Rates

Clinical History:

For the past 3 months, a 53-year-old woman has had decreasing vision in her right eye. Today she has experienced sudden loss of vision on the right, as though a window-shade had been pulled down. Funduscopic examination reveals a 13 mm choroidal mass on the right. The left eye appears normal.

The cross section of the enucleated eye reveals a uveal (choroidal) mass that is darkly pigmented. Microscopically, there are polygonal cells with enlarged nuclei, prominent nucleoli, and cytoplasm with brown granules. The Fontana-Masson stain is positive, indicating that the granules are melanin pigment.

She is enrolled in a study to determine outcomes of therapy. The study shows that, over the past year, there were 25 persons in a population of 2,000,000 people for whom medical records were available for review, who had this disease. During that time (one year) 5 of the affected persons died of their disease, while 7,000 persons died in the entire study population.


  1. What is the diagnosis?

  2. Ocular melanoma.

  3. What sudden complication did she have?

  4. A retinal detachment occurred. The choroidal mass can lift off the overlying retina, leading to sudden loss of vision in part or all of a visual field.

  5. What is the case fatality rate for her disease? What is the mortality rate for the population as a whole in the study?

  6. A rate is the number of events per unit of population over a particular time span. The case fatality rate is the number of persons in a study with a particular disease who died during a specified time period divided by the number of persons with the disease in the study. Thus, the case fatality rate for ocular melanoma in this study is 5/25, or 0.20, or 200 per 1000.

    The mortality rate is the number of persons dying during a specified time period divided by the number of individuals in the population during the specified time period. Thus, the mortality rate for the whole population in this study is 7,000/2,000,000, or 0.0035, or 3.5 per 1000.