Is it a coincidence?

I live in a semi-detached house. My sister and brother-in-law live in the house attached to my mine.I was diagnosed with idiopathic PD 18 months ago, after living here for 16 and a half years. My brother-in-law has just been diagnosed with idiopathic PD after living here 16 and a half years. (They moved in 18 months after us).
I think that 1 in 500 of the population will be diagnosed with PD, therefore the chance of the two of us living in the same house getting PD is one in 250,000. (My maths leaves a lot to be desired so I might have calculated that wrongly. Please correct me if that is the case) Especially strange is the fact that it has occurred at exactly the same length of time that we have been living here. We are obviously not blood relations either.
Perhaps something in the environment is a trigger?
Hi butterfly99153

Do you live among open fields where crops are regularly sprayed? On the other hand, is your house situated in a heavy traffic area? (Just a thought.)
statistics arent my strongpoint either but i think you calculation is not quite right.
i presume there are four adults in the house? the chances of at least one getting pd are 4/500. The chances of one of the remain 3 having pd is 3/500 so the probability of 2 or more is 12/250000. roughly 1 in 20,000. if there are 8 million semis then 400 will have two or more people with pd on them
corrections welcome!!!

The chances of the second person in a house getting PD is still 1 in 500. You cannot really multiply it up as there is no relationship. There cannot really be linked.

It's like a woman who has had 4 girl babies . Although you wouldn't think so, the chances of having a boy baby is still 50/50 for her next pregnancy. The previous babies do not effect the outcome of the next event.

God....I'm such a nerd!!!
quite true spam, on all points.
but is it not the case that the chances of a household containing a person with pd increases with number of occupants?
though i think my calculation was not right.
Perhaps something in the environment is a trigger?

Depends on what question you ask.

If you have PD and you invite people to live in your house then the chance that your guest will have PD is still 1 in 500.

If I pick any house (containing 2 unrelated people) at random then my chancee of finding one where both occupants have PD is 1 in 500X500.....

Makes my head spin too.........

[This post was edited in the process of bringing two threads together. The text in italics were added at the top. -Ezinda]
I got there too Butterfly and Bartobob - probability of 2 (unrelated people) in same house being dx with PD = 1:250000. But.....
..but there were 4 people on the house and there are 10,000,000 houses!
... but ... but ... what about the environment?
we should all recycle old threads!
to put it another way , given you have pd the chances of your brother-in-law getting it at random are 1 in 500. the chances of someone else, if 4 people are in the house, in the house getting it are 1 in 167.
Thank you all for your input. I'm really confused about the statistic bit! I agree (based on my rubbish understanding of maths, having knitted scarves in the back row of the maths class at school) with Annebernadette and Bartobob. Turnip, why is your chance of getting PD reduced to 1:125 if you live in a house with 3 other people? I didn't understand your earlier reference to 10,000,000 houses. That's not saying you're wrong. I respect your wide knowledge on many subjects related to the things we discuss. I'd like to follow your's an interesting topic.
Just realised you said 1:167. Now my brain is really hurting!
Spam, I agree with you! So do you think to get two unrelated people having PD in a house has the probability of 500 x 500?
I've stupidly posted this thread in two places which is now confusing. My apologies.
statisticians be warned, schoolboy error warning!

given that you have pd there are three other people in the house. each person has 1/500 chance of winning pd.
each persons chance is independent of the others. the chance of each NOT having pd is 499/500. the chances of none of them having pd is 499/500 times 499/500 times 499/500.
the chance of not none of them having pd is the inverse=


which 1 in 167. So out of every house with 4 people and one person with pd you would expect every 167th to have a second or more.

each person is another throw of the 500 sided dice. the more people in the house the more likely.

i expect to get shot out of the water by a mathematician!

thats not to say there isnt a real connection, just that a randomness would produce this result.
Lily, We live in a rural area surrounded on three sides by the Culbin Forest which is a conservation area, and the fourth is pasture. I have never seen any spraying near here.
Kind regards, Butterfly
don't mention it. all in a days work ma'am. [flies of at speed of light but puts on too much weight so slows down to a size 18 (geek joke)]
But didn't we knit some lovely scarves Butterfly?