An experiment is designed to test the potency of a drug on 40 rats. Previous animal studies have shown that a 10-mg dose is lethal 10% of the times within the first 4 hours. What is the probability that between 2 and 8 rats die during the experiment due to the drug?

Alice Chen

Alice Chen

Answered question

2022-11-14

An experiment is designed to test the potency of a drug on 40 rats. Previous animal studies have shown that a 10-mg dose is lethal 10% of the times within the first 4 hours.
What is the probability that between 2 and 8 rats die during the experiment due to the drug?
My attempt: Let X be the number of rats that die in the first 4 hours.
P ( 2 x 8 ) = x = 2 8 ( 40 x ) ( 0.1 ) x ( 0.9 ) 40 x
To make the drug more potent, the company came with a new formula. This reduced the chances of a new drug being lethal to 1%. The new drug is administered to 10000 rats. Approximate the exact probability that 5 rats die.
My attempt:
P ( x = 5 ) = ( 10000 5 ) ( 0.01 ) 5 ( 0.99 ) 10000 5
Am I on the right path?

Answer & Explanation

Haylie Park

Haylie Park

Beginner2022-11-15Added 14 answers

In order to calculate the first statement
(1) P ( 2 X 8 ) = x = 2 8 ( 40 x ) ( 0.1 ) x ( 0.9 ) 40 x
We can utilize some small python script as it is kind of hard by hand.
from scipy import stats
# number of rats
n=40
# lethal dose probability
p=.1
# upper bounds
kmax = 8
kmin = 1
# cdf summations
y1 = stats.binom.cdf(kmin, n, p)
y2 = stats.binom.cdf(kmax,n,p)
# differences
y = y2-y1
y
y
Out[13]: 0.9040309985710783
Note the binom.cdf command is
(2) F ( x | n , p ) = i = 0 n ( n i ) ( p ) i ( 1 p ) n i
So we to set our limits
(3) I 1 = F ( x | 1 , 0.1 ) = i = 0 1 ( 1 i ) ( 0.1 ) 1 ( 0.9 ) 1 i
(4) I 2 F ( x | 8 , 0.1 ) = i = 0 8 ( 8 i ) ( 0.1 ) 8 ( 0.9 ) 8 i
then we find
(5) I = I 2 I 1
there is a 90% chance 2 - 8 rats die.
ok..the second one
(6) P ( X = 5 ) = ( 10000 5 ) ( 0.01 ) 5 ( 0.99 ) 9995
from scipy import stats
# number of rats
n=10000
# lethal dose probability
p=0.01
# exact number
k = 5
# pmf
y = stats.binom.pmf(k, n, p)
y
Out[8]: 1.968586020940221e-36
Wow, startling difference.. I don't think you can do these by hand. You can make rough approximations.
In the last equation you get some rough terms...
(7) P ( X = 5 ) = 10000 ! 5 ! 9995 ! 1 100 5 99 9995 100 9995
Various things will drop out
(8) P ( X = 5 ) = 10000 9999 9998 9997 9996 120 1 100 2 10 3 99 9995 100 9995
(9) P ( X = 5 ) = 9999 9998 9997 9996 120 1 10 3 99 9995 100 9995
It just becomes a really small number..this is the exact reason calculators were invented.

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