USMLE Forum - Largest USMLE Community

Can anyone help me with these statistics questions? They are too long to post here and really confusing to me
Form 3 section 2 q16
Form 4 section 2 q17
Thanks

No one? somebody plse explain or give some formulae to solve these questions

Conditional Probability:....Freaking biostates

lets try this... If given that the pt survived say like 1 yr( or 2 yrs etc.) and asking for probability of surviving certain # of yrs (e.g 2 yrs or 3 yrs etc.).

and the data is given in one of these two ways:

.....STRAIGHT SURVIVAL.

0--1 yr = 80 %
0--2 yr = 60 %
0--3 yr = 40 %
0--4 yr = 20 %

say like if pt survived 1 yr what would be the probability--?(in this case u DIVIDE probabi)

(prob.of yr u want to know) DIVIDE by (prob.of # yr pt already survived e.g. 1 yr)

probability of surviving 2 yrs.......60/80 =75%
probability of surviving 3 yrs.......40/80 =50%
probability of surviving 4 yrs.......20/80 =25%

if the pt survived say like 2 yrs and probability of surviving 4 yrs?
20/60 =33 %

.......SURVIVING THIS INTERVALE.

0--1 yr = 80%
1--2 yr = 60%
2--3 yr = 40%
3--4 yr = 20%

say like if pt survived 1 yr ,what would be the probability? (in this case MULTIPLY probabil.)
Note: Multiply probabilities in between(SKIPPING the survived yr probability in this case 80%)

probability of surviving 3 yrs = 60 times 40
probability of surviving 4 yrs = 60 times 40 times 20

If asked probability of surviving straight say like 4 yrs(from the day one without condition of already survived certain # of yr) = multiply all probabilities 80 times 60 times 40 times 20

NOw in NBME 3 bloc 2, 16 answer is D
0.875 times 0.9 times 0.9

B- would have been the answer if asked straight 4 yr survival
C- is the probability of surviving 3 yrs once the pt already survived for 1 yr

IN NBME 4 bloc 2, 17 answer is C .Pt already survive 2 yrs so skip the 62 and 80 and now asking survival for AT LEAST 4 yrs(not the 5 yrs) so multiply the probabilities in between
0.84 times 0.85

D- straight survival for 4 yrs (without any condition of already survived # of yr)

mb260 i know u r one of the 99ers. wish u best of luck

Oh Sarim, u are wonderful! Suddenly, thanks to u, conditional probability is so easy now. I wish I could see ur face and shake ur hand for being so kind! (I hope I'm not splitting!!) God bless u.

Thanks a lot.

Thank you so much Sarim
This was soooo helpful