Thread: How to find Standard Deviation?

In a population of men, their heights are normally distributed with a mean of 178 cm. If a man with a height of 184 cm is shorter than 20% of the population, determine the standard deviation of the population.
the answer is 8.3cm, but how do u do it with a graphic-calculator (TI series)?
if I use this formula, z=(x-mean)/standard deviation, the answer is 30, which is not the right answer, can anyone tell me why?
Thank you!

Originally Posted by wby

In a population of men, their heights are normally distributed with a mean of 178 cm. If a man with a height of 184 cm is shorter than 20% of the population, determine the standard deviation of the population.
the answer is 8.3cm, but how do u do it with a graphic-calculator (TI series)?
if I use this formula, z=(x-mean)/standard deviation, the answer is 30, which is not the right answer, can anyone tell me why?
Thank you!

Look up the z-score such that (he is taller than 80% of the population). My tables tell me that this is .

Now the z-score corresponding to is:



so

RonL

Thank you, but do you know how to get the 0.841 on a graphic calculator?
(binomial pdf, normal pdf, normal inverse perhaps?)

Originally Posted by wby

Thank you, but do you know how to get the 0.841 on a graphic calculator?
(binomial pdf, normal pdf, normal inverse perhaps?)

No, but it is something like invnormaldis(0.8) that is inverse cumulative normal
of 0.8.

RonL

thank you!