Dr. Andy

Reflections on medicine and biology among other things

Saturday, January 28, 2006

US births

I've been asking fellowship candidates the following question:

"There are approximately 300 million people in the United States. About how many babies are born each year?"

Answers so far have ranged from 20,000 to 100 million!

Absolutely no one knows the answer, which is available in this article or as the first commnent to this post.

Try to make an educated guess, then look to see the actual number.

Hint: it is somewhere between 20,000 and 100 million.


At 11:04 AM, Blogger Dr. Andy said...

The answer is 4.1 million. There are many lines of reasoning that will give an approximate answer

1. The birth rate in the population is on the order of 1-2%

2. The death rate is probably around 1% giving 3 million + deaths. Births are probably a bit higher. (Actually, the number of deaths is ~2.4 million

3. 1/2 the population is women, maybe 1/4 of those are child bearing age and 1/8 to 1/10th of those will have a child in any given year.

When this question was posed to me as a second year resident (and I didn't know the answer) I guessed 5 million using strategy 1.

At 3:13 PM, Anonymous Chris said...

I guessed 6 million because I estimate that the population doubled in the last thirty years, which suggests a 2% annual growth rate (according to the rule of 70). Therefore 2% of 300 million is 6 million.

At 9:25 AM, Anonymous Damon said...

I guessed 4.0 to 4.5 million. Yeah, that's giving myself a lot of leeway and not very precise for a mathematician

I got there very simply, dividig 300M by 75 (approximate life expectancy) to get 4.0 million, which would be "replacement value" (approximately). I then figured that if our population is still increasing, we would see more, so fudged that to about 10% more, and called it 4.5M.

At 8:28 PM, Blogger janinsanfran said...

I guessed 3 million, using something close to strategy 3 -- and "correcting" for the high median age of the U.S. population.

Not very close.

At 4:05 PM, Anonymous Penny said...

I guessed 4,285,714, which is 300 million divided by 70. My reasoning was that our population isn't growing very fast (at least not from births) and that the average life expectancy is about 70.

Obviously I am a non-scientist and non-mathematician, but I got pretty close anyway. Go figure.

At 10:46 PM, Blogger ollie said...

I didn't guess anything as I have a head cold and can't think clearly (now, no comments about that being my normal state!)

I do know that population growth is logistical (of the form p(t) = l(t)/(1+kexp(-rt)) where the function in the numerator represents the theoretical capacity of our country (which goes up with time due to improvements in farming, etc.)

For short periods of time, growth is approximately exponential; that is, the ratio of the population growth rate to the current population is constant.

At 1:52 PM, Blogger Dr. Andy said...

Olile, you must be a blast at parties. Is this really an accurate formula? I could see in a controlled setting of lower level animals, but the capacity to support life in the US is far above the current population as people can survive without fast food and DVD players, for example.

I would be (or am) surprised some maximal theoretical number of people who could live in the US plays into actual population growth.

Dr. Andy

At 7:24 PM, Blogger K-Josh said...

deaths of babies you mean?
cause 2.4 mil. deaths a year sounds extremely low.

At 8:27 PM, Blogger K-Josh said...

my guess would be around 38.6 mil. in 2006. because according to Wikipedia 37.5 mil. people immigrated to the u.s. that year, not including undocumented immigrants.according to Google the population of America grew a little less than 3 mil. between '06 to '07 4.1+37.5=41.6 41.6-38.6=3
PS. i couldn't find the number of people who emigrated from the u.s.

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