4 ms·

the median number of lifetime sex partners for all U.S. women ages 15 to 44 is just 3.3, according to the Census Bureau’s latest statistical abstract. Huh? How

the median number of lifetime sex partners for all U.S. women ages 15 to 44 is just 3.3, according to the Census Bureau’s latest statistical abstract. Huh? How exactly do you end up with .3 of a sexual partner?

stellar678 17y ago
Yeah, I was wondering about that. I found an explanation of "fractional medians" at http://www.stata.com/statalist/archive/2009-01/msg00662.html http://www.stata.com/statalist/archive/2009-01/msg00662.html - that might be how they came up with the number. Not sure how relevant it is though because the explanation describes it as a way to get a more precise measurement when the data is artificially constrained to, for example, whole numbers...while still retaining some of the benefits of using the standard median.
_delirium 17y ago
I dug up the statistical abstract he's referencing, which does indeed have a median of 3.3 listed: http://www.census.gov/compendia/statab/2010/tables/10s0094.pdf http://www.census.gov/compendia/statab/2010/tables/10s0094.p... [PDF] There's a footnote that says: For definition of median, see Guide to Tabular Presentation, which appears to be this (from a previous year): http://www.census.gov/prod/2007pubs/08abstract/guide.pdf http://www.census.gov/prod/2007pubs/08abstract/guide.pdf [PDF] And that guide defines "median" in the normal way you would expect: arrange values in order, and choose the middle value, or average the middle two values if there is an even number. A guess, given the other data in the table, is that they first aggregate by number of sexual partners (shown in the columns), then look for a point where the number on the left and right would be equal. I believe for non-continuous data like this you can compute a "fractional median" that takes into account how far into one of the cohorts the midpoint is. The intuition is that you'd want the median to be higher if the midpoint was "just past 2" (i.e. barely including any 3s) vs. "just below 4" (i.e. including almost all the 3s), even if the midpoint itself is a 3 in both cases.
nandemo 17y ago
Curiously, the corresponding number for males is 5.4. Black males are such studs (8.3 partners) and latinas really save themselves for marriage (1.7!). Guess what, people lie.
- gjm11 17y ago
  With equal or roughly-equal populations of men and women, the means should be the same for both, but the medians needn't be. Thought experiment: 100 men, 100 women; there are 10 women who have sex with all the men, and no other sex is had. Then the mean number of partners is 10 for both men and women, but the median number is 10 for the men and 0 for the women. You can get a similar result even with means, if for some reason your sampling procedure doesn't catch the outliers. (For instance, if they're very rare, or if they're demographically different in a way that makes it harder for your sampling to find them.) Having said all that, I suspect lying really is the main contributor to these differences.
  - gaius 17y ago
    If you had 100 men and 100 women, you wouldn't get 100 couples. You'd get everyone going after the top 10 of the opposite sex and ignoring everyone else.

stellar678 17y ago

Yeah, I was wondering about that. I found an explanation of "fractional medians" at http://www.stata.com/statalist/archive/2009-01/msg00662.html http://www.stata.com/statalist/archive/2009-01/msg00662.html - that might be how they came up with the number. Not sure how relevant it is though because the explanation describes it as a way to get a more precise measurement when the data is artificially constrained to, for example, whole numbers...while still retaining some of the benefits of using the standard median.

_delirium 17y ago

I dug up the statistical abstract he's referencing, which does indeed have a median of 3.3 listed: http://www.census.gov/compendia/statab/2010/tables/10s0094.pdf http://www.census.gov/compendia/statab/2010/tables/10s0094.p... [PDF] There's a footnote that says: For definition of median, see Guide to Tabular Presentation, which appears to be this (from a previous year): http://www.census.gov/prod/2007pubs/08abstract/guide.pdf http://www.census.gov/prod/2007pubs/08abstract/guide.pdf [PDF] And that guide defines "median" in the normal way you would expect: arrange values in order, and choose the middle value, or average the middle two values if there is an even number. A guess, given the other data in the table, is that they first aggregate by number of sexual partners (shown in the columns), then look for a point where the number on the left and right would be equal. I believe for non-continuous data like this you can compute a "fractional median" that takes into account how far into one of the cohorts the midpoint is. The intuition is that you'd want the median to be higher if the midpoint was "just past 2" (i.e. barely including any 3s) vs. "just below 4" (i.e. including almost all the 3s), even if the midpoint itself is a 3 in both cases.

nandemo 17y ago

Curiously, the corresponding number for males is 5.4. Black males are such studs (8.3 partners) and latinas really save themselves for marriage (1.7!). Guess what, people lie.

gjm11 17y ago
With equal or roughly-equal populations of men and women, the means should be the same for both, but the medians needn't be. Thought experiment: 100 men, 100 women; there are 10 women who have sex with all the men, and no other sex is had. Then the mean number of partners is 10 for both men and women, but the median number is 10 for the men and 0 for the women. You can get a similar result even with means, if for some reason your sampling procedure doesn't catch the outliers. (For instance, if they're very rare, or if they're demographically different in a way that makes it harder for your sampling to find them.) Having said all that, I suspect lying really is the main contributor to these differences.
- gaius 17y ago
  If you had 100 men and 100 women, you wouldn't get 100 couples. You'd get everyone going after the top 10 of the opposite sex and ignoring everyone else.