Rubravera
Forum Newbie
Topics: 3
Posts: 8
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Table reports the no. of patients who had an operation for particular form of cancer?
interval no. of pat at start pat. died %age of survivng
0 - 1 yr 300 115 62
1 - 2 yr 185 37 80
2 - 3 yr 148 24 84
3 - 4 yr 124 18 85
4 - 5 yr 106 25 76
If pat survives 2 yr after operation ,which of the following is the probability of survivng at least 4 yrs??
1. .80
2 .85
3 .84 * .85
4 .62 * .80 * .84 * .85
5 .85 - . 85
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| Rubravera
Forum Newbie
Topics: 3
Posts: 8
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the last option is .85 - .80
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| Rubravera
Forum Newbie
Topics: 3
Posts: 8
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Is the answer 3!!!!!!!
since he survived for 2 yrs .........the probability of survivin next 2 yrs wuill be multiplication of 2 probability????????
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| Rubravera
Forum Newbie
Topics: 3
Posts: 8
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| Rubravera
Forum Newbie
Topics: 3
Posts: 8
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Its NBME form 3 question.....In found a similar qestion in nbme form 4.....Does anybody know the key to answering such question
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| nochoice
Forum Senior
Topics: 6
Posts: 116
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Hi! tough q!! don't know how i'd do it in a time crunch!!
I think your answer is correct. It is 0.714.
you can answer it in two ways -
1.
the percentages they have given are the % for surviving that specific year. i ignored them and calculated probability for surviving from the start of the study to living for 2 yrs and 4 yrs. so surviving two years was = 148/300= 0.493 and 4 yrs was 106/300 = 0.353. then i divided 0.353/0.493 = 0.71 since he taught us in the BS lecture that in order to survive for 4 years, you'd be a subset of the population that survived for 2 yrs. and we're measuring the people who survived from the ones who survived for two years.
.84*.85 = 0.714 ans 3.
2.
but, we also have to know the 0.84 * 0.85 logic which i don't really get!! coz we don't have calculators. we're saying IF somone survives for 2 years, what is the prob that he will live for more than 4? i got help with this.
so you have at least 2 values involved.. prob (surviving 2nd yr) * prob (surviving 4th yr) AND means multiplication.
i eliminated 1, 2, 4 and 5 coz they just seemed stupid.. 4 is simply multiplying prob of everything since the study started. that includes people who didn't even live for 2 yrs so its out. 1. and 2. are just prob of surviving the 2nd yr and 4th yr respectively. and 5. is ridiculous - subtracting will not give you a prob difference.
if someone can still explain how to solve it using method 2, i'd be grateful. can't really GRASP the meaning.
hope this helped somewhat!
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| nochoice
Forum Senior
Topics: 6
Posts: 116
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i think we were just supposed to apply the multiplication rule when the events are nonindependent. where kaplan says multiply prob of one event with prob f 2nd assuming the first has occurred.
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| nochoice
Forum Senior
Topics: 6
Posts: 116
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No, we definitely cannot use the multiplication rule - it has to be the conditional probability rule. i guess we can solve it without calculators by using approximations in calculations.
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| plastic_surgery
Forum Junior
Topics: 2
Posts: 135
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When the pt has survived 2 years and we are looking for probability of surviving 4 yrs... it means multiplication of probabilities 2-3 yrs * 3-4 yrs which is here 0.85 *0.84. I don't see confusion here... the only problem is when the answer in given in different format -- like 0.714 that is when you have to calculate manually and run in to time crunch..
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| SmokyWaters
Forum Elite
Topics: 6
Posts: 458
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SEE..
WHAT IS THE PROBALITY/CHANCE OF SURVIVORS IN THIRD YEAR?
WHAT IS THE CHANCE OF SURVIVORS IN FOURTH YEAR?
THE ANSWERS ARE 0.85 AND 0.84
THE QUESTION IS ASKING THAT THE PERSON IS SURVIVING AFTER 2 YEARS...WHAT WILL BE THE PROBABILITY AFTER 4 YEARS
SIMPLY PUT.... QUESTION IS ASKIN FOR INDEPENDENT PROBABILTY HERE SO U WUD MULTIPLY IT...
SAME QUESTION IF I ASK WHATS THE PROBABILTY OF SAME PERSON SURVIVING AFTER 5 YEARS?
YOUR ANSWER WUD BE 0.85 * 0.84 * 0.76
HOPE IT HELPS 
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| Ig F
Forum Elite
Topics: 3
Posts: 436
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see here is a simple way to answer it.....as the patient has survived two years so dont count thoes two years count only the next 2 years percentage of survival....which is 0.84 on third year and 0.85 on 4th year....the product of it is the answer....
which is 0.84x0.85.....3 is the correct answer.....
___________________
i m not perfect but i wanna get close to it......
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