how many people are living with a particular condition, knowing the prevalence at birth?

Any help? My knowledge of maths isn’t enough for that.

I know how many kids are born with such or such condition each year in a set country.
Is there a way to figure out how many adults with that condition are currently alive in that same country?

Do I have to calculate something like (x kids born in a year)(average lifespan for that country)

Does the condition affect anticipated lifespan?

As far as I know no.

But of course population and so births are generally going up (even if births per mother are down), so any estimate would be a little high. In places like Canada, USA, and Australia, immigration may be a big factor so straight population numbers are not the best guide.

Do other factors affect the rate, or is it strictly proportional to birth rate? I.e. It’s always been like that… If so, why not compare X kids are born in a year, Y of them have the condition; therefore (Population) times (Y/X) gives proportion affected.

(I.e. birth rate is 51% female, so population is about 51% female… except men die earlier and easier. )

Of course, you’d probably be pretty close as a first approximation either way.

“Rate” of anything doesn’t tell you how many actually are in a country at a particular time.

I normally work with divorce rate. Divorce rate is calculated as:

number of new divorces in a calendar year/number of marriages in a calendar year

Currently, that rate stands at about 48% or so for the US. There’s virtually nothing you can do with this number to estimate the number of divorced people currently living in the US (since your math skills aren’t good, you’ll just have to trust me on this.) It’s just a snapshot of one number compared to another number, and doesn’t account for the number of divorcee’s getting married that contribute to the denominator, nor does it factor in things that might make the number of marriages lower, like an increase in unmarried couples.

I believe that the rate of the birth defect you are looking at is calculated similarly:

(incidences of that birth defect in a year)/(total live births in a year)

Demographic “rates” aren’t the same rates that can be used in math to determine distance or velocity, for example. They aren’t predictive.

I suppose you could make some sort of estimate with enough variables, but rate alone wouldn’t be enough.

I can’t see why in the case of a birth defect. Contrarily to divorce, you can’t, for instance, be born several times with the same birth defect.

If 1% of the population is born with birth defect X, assuming that this birth defect doesn’t affect lifespan, it’s a safe assumption to say that 1% of the population will have this defect. Even large scale immigration won’t change this rate, assuming the birth defect isn’t significantly higher in some ethnicities.

So, it seems to me that the effect on lifespan is the only variable, here.
So, the only thing the OP has to do is figuring out the rate by dividing the number of children born with this defect by the total number of births and apply this rate to the general population (again,** assuming the defect doesn’t affect life expectancy**)

Also use the ratio - assuming it’s not dependent on environmental factors, or otherwise vaires during the lifetime of the calculation.

Divorce rates for example, went up from 50 years ago. SImilarly, number of people and percent of population getting married and never getting married.

But either a birth attribute is environmentally induced and varies over time, or there’s some evolutionary or social selection (as with DOwns syndrome, testing and abortion) or - like twins, or sex, or blood type - it’s inherently genetic and does not change much during the life of the current population.

Thank you all for your explications.
Seems the safest way is to use straight-on the birth rate and, if needed, work out a % of the population based on that, while emphasizing it’s to take with a grain of salt for the reasons provided (depends if there are social or biological factors making the numbers of people born like that fluctuating).

Prevalence vs incidence.