Question

How do you find a z score when you only have the standard deviation?

Answer 1

I think you need the mean as well.

Answer 2

i don't know if this is the mean, but it says the average number is 230

Answer 3

z = xi - X/s
ex. z= -2.2.
xi = 45
x = 74
-2.2. = 45-74/S * s
-2.2s/-2.2 = -29/-2.2
s= 13.18

Answer 4

mean = average.

Answer 5

z = (number - mean) / standard deviation.

Answer 6

what is it like using same formula as earlier
ex. z=43-45/2.2
z=-2/2.2
z= -.091

Answer 7

but i dont have a number

Answer 8

all i have is the mean(230) the standard deviation(22)

Answer 9

Then you cannot calculate the Z score. You need all variables in the formula.

Answer 10

what other variables do i need?

Answer 11

a commercial prawn fisherman recorded the number of prawns he took from each trap. The average number of prawns per trap was 230 with a standard deviation of 22. What number of prawns per trap woud you expect in the interval symmetrical about the mean where 80% of the numbers would be found?

Answer 12

\[z = \frac{X-230}{22}\]
its telling the interval is 80%, so to get z-score, look up probabilities 0.2 and 0.8 in a table

Answer 13

oops i meant .1 and .9 ... the difference should be 80%

Answer 14

why is is .1 and .9?
it equals 0.753 btw, but i want to know

Answer 15

i looked it up on a z score table

Answer 16

http://www.docstoc.com/docs/80811229/z-table

that should be what you are using to look up probabilities
z-score = +- 1.28

Answer 17

Because this is what 80% of the numbers would be found, leaving 20% unshaded. Thus you need 10% on the right tail end, and 10% on the left tail end... or probabilities of 0.9 and 0.1.

http://introductorystats.files.wordpress.com/2011/10/normal-curve-middle-80-percent.jpg?w=640

Answer 18

ohh, i get it