Hello Wilson , Bromeley , Sarah and everyone else.
Let me barge in about some statistical analysis. I happened to have some background in it.
I'm in my second phase of a Doctoral program in Operations Research here in the US, and actually I will be teaching probability and statistics this coming fall
So I got myself one more homework question for my students
Your question actually amused me a little , and I decided to spend some time on it.
Putting the mother-in-law question aside (since some genetical predisposition is generally agreed on ) I pose a question like that :
Is one's husband's MS entirely independent of his wife's MS and the other way around ?
Behind this question is of course implicitly a presence or an absence of any pathogen involved in MS.
Here's how I handle it: the problem itself is fairly straightforward , but what does matter here is the data. Of course the analysis will be rough , but it does give some insights. Here's what I took,
on nmss.org they say that currently in the US there a bit over 400 000 MSers, women are around twice as likely to get it than men. Current population of the US is 295M people , let's put it 300 M .
Hence , probability of a male and of a female getting it are
P(m)= 0.4 /(3*150) = 0,000888889
P(f) = 2*0.4 /(3*150) = 0,001777778.
Our hypothesis: No Pathogen Involved => MS for both partners in a married couple is entirely independent !
Then , the probability of both partners in a married couple to have MS is :
p = P(m)*P(f) =~ 1,60 E-06 ( 1.6 in a million).
As we know, probability itself is of no much help, the sample size is crucial ! Hence , to make any sence out of this probability we need to consider the total number of married couples:
In the US it's 57 Million (Statistics of 2000).
Now it's really easy to figure out exactly what are the chances to have
certain number of "MS - marriages" and estimate if out hypothesis is true or not. Because the total number of such events are very well approximated by a Normal Distribution (This is Central Limit Theorem in Action ), but with our number of total marriages (57 Million) and our underlying distribution (it's binomial ! ) the approximation is almost perfect. I don't wanna drone on this , it must be boring
But , the bottom line , is that the total amount of "MS-marriages" in the US, has a Normal Distribution with
Mean = 57M * p = 91.2 (on avarage there should be ~ 91 of such marriages)
Variance = 57M * (p-p^2) = 91.2 => Standard Deviation = 9,549869109
I'm sure most of you have Excel , then you can amuse yourself by plotting
this distribution with these parameters.
In particular if we want to test the hypothesis if it's true or not =>
even for a tiny level of significance of 1%, its rejection level is 113,41.
What it means in practice is that under hypothesis of independence,
the total number of marriages with both partners having MS with 99% probability MUST be less then 114
That's why if it turns out that the real number of such marriages in the USA is let's say 150 or 200 or even greater (actually even a 100 would raise a question) then MS can be contracted between people almost certainly. And you can then stamp this letter together with your statistics
of such marriages to the foreheads of all the advocates of MS-Mice community with there beloved money making drug companies and societies
For our UK friends you could repeat the analysis with the UK statistics.
I really got interested in such info . I myself already have heard of many on this site. May be we should initiate a poll on this site, to count the people in such marriages ?