Hopefully a stupid question...

[boardsbandit524]

I have had several questions that go something like this:

"Patients surviving this interval":
0-1 yr survival = 80%
1-2 = 87.5
2-3 = 90
3-4 = 90


"if a patient survives one year, which of the following is the probability that he or she will survive 4 years"?

that would be 87.5 x 90 x90 right? Am I missing something? One of my buddies swears its 80 x 87.5 x 90 x90. And I vaguely remember getting Qbank question wrong on this sort of thing so I may be wrong. Anyone? Does the phrase "patients surviving this interval" mean something tricky?

---

Members don't see this ad.

 

[Tiger26]

I have had several questions that go something like this:

"Patients surviving this interval":
0-1 yr survival = 80%
1-2 = 87.5
2-3 = 90
3-4 = 90


"if a patient survives one year, which of the following is the probability that he or she will survive 4 years"?

that would be 87.5 x 90 x90 right? Am I missing something? One of my buddies swears its 80 x 87.5 x 90 x90. And I vaguely remember getting Qbank question wrong on this sort of thing so I may be wrong. Anyone? Does the phrase "patients surviving this interval" mean something tricky?

You're right; he's wrong

 

[tfom08]

although actually it's .875 x .9 x .9

 

[CoolMusic]

I have had several questions that go something like this:

"Patients surviving this interval":
0-1 yr survival = 80%
1-2 = 87.5
2-3 = 90
3-4 = 90


"if a patient survives one year, which of the following is the probability that he or she will survive 4 years"?

that would be 87.5 x 90 x90 right? Am I missing something? One of my buddies swears its 80 x 87.5 x 90 x90. And I vaguely remember getting Qbank question wrong on this sort of thing so I may be wrong. Anyone? Does the phrase "patients surviving this interval" mean something tricky?

Agreed with the posters above - you are right.

The question stem already states that the patient has survived for 1 year, so the 80% that your friend is calculating has already been taken into account by the question stem. If the question just asked what the overall survival rate was for 4 years, then you would need to use your friend's calculation.

Hope this helps.

 

[osli]

OK, it's late and all, but wtf is up with that question?

If only 80% survive the first year, how can 90% be alive two years later?

 

[CoolMusic]

OK, it's late and all, but wtf is up with that question?

If only 80% survive the first year, how can 90% be alive two years later?

90% of those who have already survived the first year go on to survive the second year

So if 1,000 people are looked at, 800 (80%) survive the first year. 720 survive the second year (90% of 800, and 0.8 * 0.9 = 72% of the original amount)

Hope this helps

 

[hsans23]

OK, it's late and all, but wtf is up with that question?

If only 80% survive the first year, how can 90% be alive two years later?

90% of the remaining 80% that survived year 1, ya def a weird question

 

[osli]

Ah, yeah it makes more sense today than it did last night. When I looked at it my mind interpreted it like "1 year survival rates... 5 year survival rates" etc. since that is what I'm used to seeing and hearing.