biostats question

[pleaseletmepass]

I'm having some trouble with biostats and hoping someone can enlighten me.

This involves independent/dependent probabilities and I'll use an example:

1. During the 1st 1 year interval after a surgery the probability of death is X%.
2. During the 2nd 1 year interval after a surgery the probability of death is Y%.
3. During the 3rd 1 year interval after a surgery the probability of death is Z%.

If the patient survives 1 year, what is the probability of surviving 3 years? Is it Y% x Z%? or X% x Y% x Z%?

Please help with an explanation. Thanks in advance.
-plmp

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[lord_jeebus]

Convert each probability of death to probability of survival. (ie. subtract from 100%)

Multiply the 3 together for 3-year survival.

 

[pleaseletmepass]

why do we include the value of the 1st year in the multiplication equation if the patient has already survived the first year?

 

[lord_jeebus]

why do we include the value of the 1st year in the multiplication equation if the patient has already survived the first year?

oh, i misread that

just multiply the other two

 

[mideis79]

The way I was taught was to treat it like a relative risk question. The chance of the surviving year is divided by the survived year. Eg. Pt survived 1st year what the chance of surviving the 2nd year: Y/X

Since it is death that is stated, it would be (100-Z)/(100-X)

 

[pleaseletmepass]

hmmm what if i simplify the question by replacing death with survival? As in:

1. During the 1st 1 year interval after a surgery the probability of survival is X%.
2. During the 2nd 1 year interval after a surgery the probability of survival is Y%.
3. During the 3rd 1 year interval after a surgery the probability of survival is Z%.

If the patient survives 1 year, what is the probability of surviving 3 years?

Would the answer to this then be Y% multiplied by Z%?

Thanks guys

 

[lord_jeebus]

hmmm what if i simplify the question by replacing death with survival? As in:

1. During the 1st 1 year interval after a surgery the probability of survival is X%.
2. During the 2nd 1 year interval after a surgery the probability of survival is Y%.
3. During the 3rd 1 year interval after a surgery the probability of survival is Z%.

If the patient survives 1 year, what is the probability of surviving 3 years?

Would the answer to this then be Y% multiplied by Z%?

Thanks guys

yes

 

[IFNgamma]

I wouldn't worry too much about biostats. You'll get at the most like 5 Qs, and I think I got them all wrong and still I did well on step 2.

good luck!