corona
Who is more likely to have covid? Someone who has a negative selftest,
but has cold-like symptoms, or someone with a positive test but who
has no symptoms?
Define the following:
C: patient has convid
T: patient tested positive
S: patient is symptomatic
We use the following data from the following sources
P ( S ∣ C ˉ ) = 1 / 200 P(S|\bar C) = 1/200 P ( S ∣ C ˉ ) = 1/200
from: https://www.cdc.gov/mmwr/volumes/70/wr/mm7029a1.htm and https://www.sciencedaily.com/releases/2012/06/120619225719.htm
combined to get symtpomatic common cold cases.
P ( S ∣ C ) = 1 / 2 P(S |C) = 1/2 P ( S ∣ C ) = 1/2
from https://edition.cnn.com/2020/04/01/europe/iceland-testing-coronavirus-intl/index.html
P ( C ) = 367639 / 11000000 P(C) = 367 639/11 000 000 P ( C ) = 367639/11000000
from https://www.worldometers.info/coronavirus/country/belgium/
P ( T ∣ C ) = 0.97 P(T|C) = 0.97 P ( T ∣ C ) = 0.97
P ( T ˉ ∣ C ˉ ) = 0.85 P(\bar T|\bar C) = 0.85 P ( T ˉ ∣ C ˉ ) = 0.85
https://covid-19.sciensano.be/nl/procedures/snelle-antigeen-ag-testen-0
And lets assume that the accuracy of the test is independent of the fact of the patient has symptomes
The last to givens can be extrapolated to
P ( T ˉ ∣ C ) = 0.03 P(\bar T|C) = 0.03 P ( T ˉ ∣ C ) = 0.03
P ( T ∣ C ˉ ) = 0.15 P(T|\bar C) = 0.15 P ( T ∣ C ˉ ) = 0.15
P ( S ˉ ∣ C ˉ ) = 0.995 P(\bar S|\bar C) = 0.995 P ( S ˉ ∣ C ˉ ) = 0.995
P ( S ˉ ∣ C ) = 0.5 P(\bar S |C) = 0.5 P ( S ˉ ∣ C ) = 0.5
P ( C ) = 0.0334 P(C) = 0.0334 P ( C ) = 0.0334
P ( C ˉ ) = 0.9665 P(\bar C) = 0.9665 P ( C ˉ ) = 0.9665
Law of bayes says:
P ( A ∧ B ) = P ( A ∣ B ) P ( B ) P(A \land B) = P(A|B)P(B) P ( A ∧ B ) = P ( A ∣ B ) P ( B )
If A and B are independant under constant X that
P ( X ∣ A ∧ B ) = P ( X ∧ A ∧ B ) / P ( A ∧ B ) = P ( A ∧ B ∣ X ) P ( X ) / P ( A ∧ B ) = P ( A ∣ X ) P ( B ∣ X ) P ( X ) / P ( A ∧ B ) = P ( A ∧ X ) P ( B ∧ X ) / P ( A ∧ B ) P ( X ) = P ( A ∧ X ) P ( B ∧ X ) / ( P ( A ∧ B ∧ X ) + P ( A ∧ B ∧ X ˉ ) ) P ( X ) = P ( A ∧ X ) P ( B ∧ X ) / ( P ( A ∧ B ∣ X ) P ( X ) + P ( A ∧ B ∣ X ˉ ) P ( X ˉ ) ) P ( X ˉ ) = P ( A ∧ X ) P ( B ∧ X ) / ( P ( A ∣ X ) P ( B ∣ X ) P ( X ) + P ( A ∣ X ˉ ) P ( B ∣ X ˉ ) P ( X ˉ ) ) P ( X ) = P ( A ∣ X ) P ( B ∣ X ) P ( X ) / ( P ( A ∣ X ) P ( B ∣ X ) P ( X ) + P ( A ∣ X ˉ ) P ( B ∣ X ˉ ) P ( X ˉ ) ) = 1 1 + P ( A ∣ X ˉ ) P ( B ∣ X ˉ ) P ( X ˉ ) P ( A ∣ X ) P ( B ∣ X ) P ( X ) \begin{aligned}P(X|A \land B) &= P(X \land A \land B)/P(A \land B) \\
& = P(A \land B|X)P(X)/P(A \land B) \\
&= P(A|X)P(B|X)P(X)/P(A \land B) \\
&= P(A\land X)P(B \land X)/P(A \land B)P(X)\\
&= P(A\land X)P(B \land X)/(P(A \land B \land X) + P(A \land B \land \bar X))P(X) \\
&= P(A\land X)P(B \land X)/(P(A \land B|X)P(X) +P(A \land B|\bar X)P(\bar X))P(\bar X) \\
&= P(A\land X)P(B \land X)/(P(A|X)P(B|X)P(X) +P(A|\bar X)P(B|\bar X)P(\bar X))P(X) \\
&= P(A|X)P(B|X)P(X)/(P(A|X)P(B|X)P(X) +P(A|\bar X)P(B|\bar X)P(\bar X))\\
&= \frac{1}{1 + \frac{P(A|\bar X)P(B|\bar X)P(\bar X)}{P(A|X)P(B|X)P(X)}}\\
\end{aligned} P ( X ∣ A ∧ B ) = P ( X ∧ A ∧ B ) / P ( A ∧ B ) = P ( A ∧ B ∣ X ) P ( X ) / P ( A ∧ B ) = P ( A ∣ X ) P ( B ∣ X ) P ( X ) / P ( A ∧ B ) = P ( A ∧ X ) P ( B ∧ X ) / P ( A ∧ B ) P ( X ) = P ( A ∧ X ) P ( B ∧ X ) / ( P ( A ∧ B ∧ X ) + P ( A ∧ B ∧ X ˉ )) P ( X ) = P ( A ∧ X ) P ( B ∧ X ) / ( P ( A ∧ B ∣ X ) P ( X ) + P ( A ∧ B ∣ X ˉ ) P ( X ˉ )) P ( X ˉ ) = P ( A ∧ X ) P ( B ∧ X ) / ( P ( A ∣ X ) P ( B ∣ X ) P ( X ) + P ( A ∣ X ˉ ) P ( B ∣ X ˉ ) P ( X ˉ )) P ( X ) = P ( A ∣ X ) P ( B ∣ X ) P ( X ) / ( P ( A ∣ X ) P ( B ∣ X ) P ( X ) + P ( A ∣ X ˉ ) P ( B ∣ X ˉ ) P ( X ˉ )) = 1 + P ( A ∣ X ) P ( B ∣ X ) P ( X ) P ( A ∣ X ˉ ) P ( B ∣ X ˉ ) P ( X ˉ ) 1
o ( X ∣ A ∧ B ) = P ( A ∣ X ˉ ) P ( B ∣ X ˉ ) P ( X ˉ ) P ( A ∣ X ) P ( B ∣ X ) P ( X ) o(X|A \land B) = \frac{P(A|\bar X)P(B|\bar X)P(\bar X)}{P(A|X)P(B|X)P(X)} o ( X ∣ A ∧ B ) = P ( A ∣ X ) P ( B ∣ X ) P ( X ) P ( A ∣ X ˉ ) P ( B ∣ X ˉ ) P ( X ˉ )
This gives
P ( C ∣ S ˉ ∧ T ) = 1 1 + P ( S ˉ ∣ C ˉ ) P ( T ∣ C ˉ ) P ( C ˉ ) P ( S ˉ ∣ C ) P ( T ∣ C ) P ( C ) \begin{aligned}P(C| \bar S \land T) &= \frac{1}{1 + \frac{P(\bar S|\bar C)P(T|\bar C)P(\bar C)}{P(\bar S|C)P(T|C)P(C)}}\\
\end{aligned} P ( C ∣ S ˉ ∧ T ) = 1 + P ( S ˉ ∣ C ) P ( T ∣ C ) P ( C ) P ( S ˉ ∣ C ˉ ) P ( T ∣ C ˉ ) P ( C ˉ ) 1
Filling in the given numbers this has a chance of 10,09%
And
P ( C ∣ S ∧ T ˉ ) = 1 1 + P ( S ∣ C ˉ ) P ( T ˉ ∣ C ˉ ) P ( C ˉ ) P ( S ∣ C ) P ( T ˉ ∣ C ) P ( C ) \begin{aligned}P(C|S \land \bar T) &= \frac{1}{1 + \frac{P(S|\bar C)P(\bar T|\bar C)P(\bar C)}{P(S|C)P(\bar T|C)P(C)}}\\
\end{aligned} P ( C ∣ S ∧ T ˉ ) = 1 + P ( S ∣ C ) P ( T ˉ ∣ C ) P ( C ) P ( S ∣ C ˉ ) P ( T ˉ ∣ C ˉ ) P ( C ˉ ) 1
Filling in the given numbers this has a chance of 10,87%
So having symptoms while having a negative test gives you the same chance to have covid as not having symptoms and having a positive test.