# Problem with R0 missing units

Gold Member
I understand that R0(The reproduction number) is dimensionless but I cannot help feeling that the timescale for these things must be relevant, at least in some instances, when trying to compare the performance in different countries. R0 is arrived at from the statistics and I think it assumes that this 'effective' value is the same as the 'basic' value. The measured values could be the same for two different populations in which one population is taking precautions and the other population is taking none; the difference would be 'just' due to luck and perhaps some genetic differences.

Basically, I am asking if it is an oversimplified measure.

I think it was the UK health minister who I heard say "We want to flatten the curve in order to reduce the number of deaths" Cart before horse or what??

I also heard, years ago, an education minister say "We will conduct a study to show that . . . . . whatever" They all have their cause and effect the wrong way round and the history they write justifies the decisions they took in the past.

Mentor
2022 Award
##R_0## is an important benchmark, but not the only one. And yes, time spans have to be considered, too.
I think it was the UK health minister who I heard say "We want to flatten the curve in order to reduce the number of deaths" Cart before horse or what??
No. There are a few hidden assumptions in this statement:
• the number of deaths depends on the infection rate
• the number of deaths depends on the number of available ICU
So the total number of fatalities is lower, if sufficiently many ICU are available, which is the fact, if the infection rate is low, which means, that the curve of totally infected people per time has to be flat.

Gold Member
I am asking if it is an oversimplified measure.
Yes, that's it's value. It does depend on lots of factors, like population density, behavior, treatment, etc.

Gold Member
There are several good explanations of Ro and the basic SIR model on YouTube. Here's one I thought was pretty good.

sophiecentaur
Gold Member
Basically, I am asking if it is an oversimplified measure.
Yes.
But it does give some indication of spreading of a disease - ie its exponential growth without any interference from hindering the spread of the disease.
Hence all the testing and what not that was attempted early in the pandemic to get some statistics.
Attempts to lower Ro with social distancing measures, and quarantining, and the like seem to not have had the ultimate effect that the people in the know wished it would have had. Otherwise I do not think the harm done to the economy, businesses, and individual workers with loose designations of essential services would have been so willingly accepted, but rather thought out more.
As one newspaper artist had in his commentary cartoon of trying to fix the plane while it is flying.
The modelling based on an assumed Ro was probably initially as good as a guess.

The differential equation modelling of Ro does has its problems.

Gold Member
2022 Award
The R0 value indicates how likely the virus's offspring are to reach a new host, at some point (or period) in time.
As the virus's life becomes more (or less) difficult (offspring less (or more) likely to find a new host), the R0 value will change, based on its new circumstances.
Its not a static number.
It is not something just determined by the biology of the virus in isolation of may other factors.

As virus spread limiting actions are taken, as more people in a population become immune, the R0 value will become smaller.
If the R0 value is small enough, for long enough, the virus will go extinct (in the wild).