Question

I don't know how to find the z-score for this probability! P(-3.75 13 years ago

Answer 1

X~N(5,4)

Answer 2

The probability that Z is between -3.75 and 0?
I think you can find P(Z=0) and subtract P(Z=-3.75).

Answer 3

Is this "P(-3.75<Z<0)" actually supposed to be P(-3.75<X<0)?

Answer 4

Because otherwise, you wouldn't need to know N(5,4)...

Answer 5

I don't really know... that is what the book example says, I just don't know how they found th eprobability for that because idk what the probability of z=-3.75 is

Answer 6

You'll need a normal distribution table or calculator to find that.
Try http://www.math.unb.ca/~knight/utility/NormTble.htm
and http://stattrek.com/probability-distributions/standard-normal.aspx?tutorial=stat
for help.

Answer 7

You'll have to make clear whether you have X and are trying to find Z and then ultimately P(Z), or what the actual question is.

Answer 8

Well, they say that the P(-3.75<Z<0)+P(0<Z<2.5)=0.9938, which is the P(0<Z<2.5)... which I don't understand at all.

Answer 9

Ok, if these are probabilities of Z-scores, then you can approximate them very well using a table.
P(Z<2.5) = .9938
P(Z<0) = 0.5000
P(Z<-3.75) = 0.00015 (very approximately, used linear interpolation)

All these from http://www.math.unb.ca/~knight/utility/NormTble.htm

Answer 10

Are you having trouble reading the notation?

Answer 11

a little bit yeah and, i was just wondering how to get that z-score, and i see how they got the answer now, thanks

Answer 12

The z-scores are calculated using the measured value, X, and comparing it to the mean, µ, and the standard deviation, σ.
See http://stattrek.com/probability-distributions/standard-normal.aspx?tutorial=stat
for more info.

Answer 13

It looks like, in these examples, that you are being given the Z-scores.

Answer 14

oh yes, but i was just lost reading the table, and the table ends at like z=2.. so i was a little confused as to how they got that

Answer 15

Yeah, a lot of tables don't go out very far because once you get past 2 or 3 standard deviations, the probabilities get into the ridiculously small territory.
You're better off using a calculator. There are plenty of good 'normal probability' calculators that you can find online.
If you have access to a graphing calculator, many have built-in statistics and probability function. (I use a TI-84 and it is great at that sort of thing).

Answer 16

ok thanks for your help!!