statistics - i need to know what methods to use. (:?

Write the probabilities:

first person has it 0.2
second person and not first 0.8*0.2
third, not first or second 0.8^2*0.2

Do you recognise the distribution?

Reply 2

Coda

16

Ouch, you're in a lot of trouble if you can't recognise these. All you need to know is that if there is a set number (I.e in a sample of 20 people) it's geometric, if it's not, then it's binomial. Google for the other properties of the two distributions.

Reply 3

OP

Coda

Ouch, you're in a lot of trouble if you can't recognise these. All you need to know is that if there is a set number (I.e in a sample of 20 people) it's geometric, if it's not, then it's binomial. Google for the other properties of the two distributions.

we've hardly done any stats at college.
they just tried to cram it all in over the last couple of weeks :|

Reply 4

OP

rnd

Write the probabilities:

first person has it 0.2
second person and not first 0.8*0.2
third, not first or second 0.8^2*0.2

Do you recognise the distribution?

no

i swear we havent done anything about distributions and stuff. :/

Reply 5

OK. You can still work it out. The one I gave a hint for is a geometric distribution which you can look up if you like.

You don't really need to though. If the first person with it is the 6th person checked you are looking for P(first 5 don't have it and 6th person does) = 0.8^5*0.2

Reply 6

OP

rnd

OK. You can still work it out. The one I gave a hint for is a geometric distribution which you can look up if you like.

You don't really need to though. If the first person with it is the 6th person checked you are looking for P(first 5 don't have it and 6th person does) = 0.8^5*0.2

is that all you have to do?
so for the one where its not in the first ten - its 0.8^10x0.2?

Reply 7

rdftw

is that all you have to do?
so for the one where its not in the first ten - its 0.8^10x0.2?

No. If it's not in the first ten it happens later. It's 1-P(it's one of the first 10). I make it 0.1064.

For the last part you need to know (or work out) that the mean of a geometric distibtution (starting at 1) is 1/p so in your case it's 1/0.2=5

Reply 8

OP

rnd

No. If it's not in the first ten it happens later. It's 1-P(it's one of the first 10). I make it 0.1064.

For the last part you need to know (or work out) that the mean of a geometric distibtution (starting at 1) is 1/p so in your case it's 1/0.2=5

oh okay, - and is that the answer?
you just have to do 1/prob so the answer is 5?
thanks

oh and how come its 0.2 when its 1/20? should it not be 0.05? :s-smilie:

Reply 9

May I ask what course you're studying. You still have a lot of questions that need answering and you don't seem to have the teaching you should have had.

Reply 10

OP

rnd

May I ask what course you're studying. You still have a lot of questions that need answering and you don't seem to have the teaching you should have had.

ocr as maths.
c1 c2 s1

Reply 11

ghostwalker

17

rdftw

is that all you have to do?
so for the one where its not in the first ten - its 0.8^10x0.2?

The formula you have there is for "not in the first ten" and "it's the eleventh".

Well, you're not interested in the 11th, nor in anything after the 10th.

So the probability of not being in the first 10 is just 0.8^10 = 0.1074 (4 dec. pl.)

Reply 12