2 independent factors 1 outcome

[samy88]

hello :)

i have a silly question and i keep confusing myself, hope someone can help me.

my friend has two diseases and I want to calculate a number to represent the chance of success (i.e. him surviving both diseases for more than 5 years)

disease A: chance of survival for more than 5 years, using certain treatment = 29%
disease B: chance of survival for more than 5 years, using the same treatment as A = 21%

both diseases are not related and don't affect each other but both contribute to failure of the treatment i.e. dying and not passing 5 years...

now, should I simply get the average of the two numbers (25%) or its supposed to be calculated in other way?

i keep confusing myself by thinking, the chance of passing 2 life threatening diseases at the same time should be lower than that!

 

[SW VandeCarr]

hello :)

disease A: chance of survival for more than 5 years, using certain treatment = 29%
disease B: chance of survival for more than 5 years, using the same treatment as A = 21%

both diseases are not related and don't affect each other but both contribute to failure of the treatment i.e. dying and not passing 5 years...

now, should I simply get the average of the two numbers (25%) or its supposed to be calculated in other way?

To say the two diseases don't affect each other is biologically unrealistic. The best statistics to apply would be based on subjects who had both diseases and had the same treatment (adjusted for many factors such as age, duration, severity of disease etc).

In the absence of this, the logic would be that the disease with lower 5 year survival would be determinative. That is, having a second disease with a better survival does nothing to mitigate having the more lethal disease. So, in the absence of any other data, the best estimate is 21% at best.