My uphill battle with Statistics

Original post by ServantOfMorgoth

Guys I flopped my statistics incourse test :sad:

Hard luck. Post more questions here.

Reply 21

OP

Original post by Gregorius

Hard luck. Post more questions here.

If X is the random variable "number of fours obtained when 2 dice are thrown" what is the probability that 0 fours are obtained and only 2 fours are obtained?

I can assure you that this is only part of a question and I'm not asking anyone to do my work whole sale for me.

@Zacken

(edited 8 years ago)

Reply 22

Original post by ServantOfMorgoth

If X is the random variable "number of fours obtained when 2 dice are thrown" what is the probability that 0 fours are obtained and only 2 fours are obtained?

I can assure you that this is only part of a question and I'm not asking anyone to do my work whole sale for me.

@Zacken

What's the probability that a "not 4" is obtained on the first die? And what about on the second die? For both dies to have a "not 4" you need to multiply those two probabilities together because you want P(not 4 and then not 4 again).

Reply 23

OP

Original post by Zacken

What's the probability that a "not 4" is obtained on the first die? And what about on the second die? For both dies to have a "not 4" you need to multiply those two probabilities together because you want P(not 4 and then not 4 again).

11/36 × 11/36 ?

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Reply 24

Original post by ServantOfMorgoth

11/36 × 11/36 ?

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Well, not quite. The probability that a not 6 occurs on the first die is 5/6. Same on the second.

Reply 25

OP

Original post by Zacken

Well, not quite. The probability that a not 6 occurs on the first die is 5/6. Same on the second.

And for the 2 fours is 1 - (5/6)^2 ?

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Reply 26

Original post by ServantOfMorgoth

And for the 2 fours is 1 - (5/6)^2 ?

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Reply 27

OP

Original post by Zacken

Thanks

What about the 1? Is that 11/36?
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(edited 8 years ago)

Reply 28

Original post by ServantOfMorgoth

And for the 2 fours is 1 - (5/6)^2 ?

Posted from TSR Mobile

Oh dear, I'm not sure why I said yes to this. That's not correct.

Two fours means four on one go (1/6) and then four on the second go (1/6). So 1/6 * 1/6.

Reply 29

XxKingSniprxX

21

Original post by High Stakes

Remember.

AND rule is multiplication
OR rule is add

I don't know why but whenever I see your profile picture I start to :cry2:

as I really want to get a cute little baby husky. :heart:

Reply 30

Original post by ServantOfMorgoth

If X is the random variable "number of fours obtained when 2 dice are thrown" what is the probability that 0 fours are obtained and only 2 fours are obtained?

@Zacken has already given some useful advice; let me suggest a simple (and surprisingly powerful) technique to complement the approach he's given. Instead of thinking in terms of probabilities, think in terms of counting.

So, when you have a problem like this, ask yourself, "can I list the possible outcomes in a consistent (and hopefully easy to think about) way?" Here we throw two dice; the first can come up 1 to 6, the second can come up 1 to 6. The set of possible outcomes can be displayed as a 6x6 two-dimensional grid: first throw along the top, second down the side. Each of the 36 cells here is equally likely, so to get the probabilities that you require, simply count the number of cells that meet the relevant criterion of the question and divide by 36.

Reply 31

OP

Original post by Gregorius

@Zacken has already given some useful advice; let me suggest a simple (and surprisingly powerful) technique to complement the approach he's given. Instead of thinking in terms of probabilities, think in terms of counting.

So, when you have a problem like this, ask yourself, "can I list the possible outcomes in a consistent (and hopefully easy to think about) way?" Here we throw two dice; the first can come up 1 to 6, the second can come up 1 to 6. The set of possible outcomes can be displayed as a 6x6 two-dimensional grid: first throw along the top, second down the side. Each of the 36 cells here is equally likely, so to get the probabilities that you require, simply count the number of cells that meet the relevant criterion of the question and divide by 36.

Oh I knew about the square but I didn't realise I had to divide by 36. I guess I was using it wrongly the whole time.

This is the entire question but I have no idea how to do part vi

Reply 32

Original post by ServantOfMorgoth

This is the entire question but I have no idea how to do part vi

You've worked out that Var[X] = 5/18, so you can work out

\sigma

. You know that

\mu = \frac{1}{3}

. So which of the possible values of X lie between

\mu - \sigma

and

\mu - \sigma

? You do part vii the same way...

Reply 33

Duke Glacia

16

CAn i assume ill be doomed if i have done only S1 at A levels ?

Reply 34

Original post by Duke Glacia

CAn i assume ill be doomed if i have done only S1 at A levels ?

No. At undergraduate level (in mathematics, anyway) we start teaching statistics properly, rather than the strange stuff that is done at A-level.

If you want to prepare yourself, think about drawing coloured balls from bags in various different ways; much of first year probability comes down to variations on this; then you start the interesting stuff. :mwuaha:

Reply 35

Kvothe the Arcane

20

Original post by Gregorius

No. At undergraduate level (in mathematics, anyway) we start teaching statistics properly, rather than the strange stuff that is done at A-level.

Statistics is the devil. Sadly, very important as a biologist.

What strange stuff? :teehee:

Reply 36

Original post by Kvothe the arcane

What strange stuff? :teehee:

Oh I exaggerate, but when I look at the probability and stats in A-level papers, I wonder why they bother. Probability and statistics are hard subjects, in that the techniques they use start where A-level runs out of steam. Would it not be better, I muse to myself, to concentrate on getting core stuff (algebra and calculus) more solid before venturing onwards and outwards. I have the same thoughts (but more so) about the Decision stuff at A-level.

Reply 37

Original post by Gregorius

I have the same thoughts (but more so) about the Decision stuff at A-level.

Seemingly, the higher ups agree with this. Decisions is being scrapped from the curriculum soon-ish.

Reply 38

OP