Why is a different survival range predicted for each hospital?

The predicted range of survival is the range in which we expect to see each hospital’s survival rate the majority of the time. It depends only on the complexity of the medical problems of the children who had heart operations. Each hospital treats different children and so each hospital will have a different predicted range.

Heart disease in children covers a wide range of disorders, from relatively minor to more serious conditions. This affects a child's chances of survival, as do other factors such as age, weight and other health problems.

Some hospitals take on more cases that are particularly complicated, meaning they tend to operate on children with a lower chance of survival. Each hospital will also treat different children from year to year, so we expect any hospital’s survival rate to vary from year to year. We therefore would not expect all hospitals to have the same survival rates and it is not valid to simply compare hospitals' survival rates. It is only valid to compare a hospital’s survival rate to its own predicted range.

Survival rate
The percentage of operations where the child survived at least 30 days after their operation.
Predicted and Extended Predicted range:

We expect a hospital's overall survival rate to lie within its predicted range 19 times out of 20.

We expect a hospital's overall survival rate to lie within its extended predicted range 998 times out of 1000.

An illustration of how we present a hospital's survival rate (black dot) in the context of its predicted range (dark blue bar) and extended prediction range (light blue bar) is given below: