The "average number of own children under 18 per family" for "familes with own children under 18" was 1.86 in 2009 (and has been within 0.1 of that since 1978), which means about 1/1.86=~54% of all children are first-borns...
I don't think we have enough information here to actually verify but I suspect 80% vs 54% is still a statistically significant difference.
But it's pretty obvious that the 80% number is an arbitrary guess, so I'm not sure exactly where I stand here.
I don't think we have enough information here to actually verify but I suspect 80% vs 54% is still a statistically significant difference.
But it's pretty obvious that the 80% number is an arbitrary guess, so I'm not sure exactly where I stand here.
source: http://www.census.gov/population/socdemo/hh-fam/fm3.csv