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Number Needed To Treat Formula
4753 nulliparous pregnant women were randomized to folic acid and trace element supplementation. in both groups 88% pregnancies ended in live birth, stillbirth, termination of preg or late fetal death after prenatal diagnosis of defect. following are pregnancy outcomes:
NEURAL TUBE DEFECT folic acid (0%), trace element (0.29%) ALL CONG. ANOMALIES folic acid (1.3%), trace(2.3%) use trace element gr as control gp, how many nulliparous women have to be treated with folic acid to prevent congenital anomalies? 1 0.29 2 2.3 3 100 4 344 5 4756 6 120 7 80 8 0 9 1.3
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#2




my answer (3)
NNT=100/Incid(unexposed/control)Incid(exposed/treated)
Plugging values from second horizontal column 100/2.31.3 =100 
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#3




Took me a while but also came up with the same answer. Calculated for attributable risk which is the incidence of exposed  incidence of unexposed. And then do the inverse of the attributable risk, get 100.
Took a while because I didn't understand the question entirely and the percentages and the other numbers that are there as distractors put me off balance 
#4




corect answer: C
corect Answer:
3) 100
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#5




Can u explain in detail?
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#6




They are asking for the Number Needed to Treat. Which is calculated by getting the inverse of the Attributable Risk. The Attributable Risk is calculated by substracting incidence of exposed  incidence of unexposed.
So, from the (in my opinion, horribly redacted) question, they are basically saying that they have a group which they give folic acid and a group which they give trace element (just a placebo that you can trace in the blood). And then they give you the % of women that gave birth to a child with neural tube defects and a % of women that gave birth to a child with some other congenital abnormality. And they ask you to calculate the NNT(number needed to treat) of the "other congenital abnormality group". All the other numbers and percentages are there to distract you. So... The incidence of the exposed (women that gave birth to a child with congenital abnormality taking trace element)  The incidence of unexposed (women that gave birth to a child with congneital abnormality taking folic acid). Doing that, you get 2.3%1.3%=1%, the AR (attributable risk). Note that the exposed women here are women that werent taking the folic acid, and the women taking the trace element are exposed to the risk of NOT taking folic acid. So your attributable risk is 1% (1 percent is 1/100). So that is your AR, and to get the NNT, you just do the inverse, so you just flip the numerator and denominator and get 100/1 or just 100. Things to remember here:  AR is incidence of exposed  incidence of unexposed  NNT is just the inverse of your calculated AR  Usually this questions will be presented in fractions, be careful with fractions and percentages, dont mix them up.  Dont be intimidated by complex numbers presented in the question stem, the USMLE will ask very simple calculations. Sorry for the long post, its a simple concept presented in a very difficult question, and I tried to give you a very thorough explanation. 
#7




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BiostatisticsEpidemiology, Step1Questions 
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