# Help me with this please?

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• Dec 4th 2007, 10:50 PM
jim1174
Help me with this please?
I am doing normal probability problems for a college statistics class. where i have to find Z. some of the problems require me to take one number from a table and subtract it from another number from the table. how do i know what order the numbers go? here is a sample

p(0.21<Z<1.69 ) the numbers in the tables are 0.5832 and 0.9545 i am supposed to take one number and subtract it from a nother
• Dec 5th 2007, 12:15 AM
TwistedOne151
It depends
It depends on what your table gives. Are the entries
$\displaystyle Pr(Z<z) = \int_{-\infty}^{z}\dfrac{1}{\sqrt{2\pi}}e^{-\tfrac{x^2}{2}}\, dx$
or $\displaystyle Pr(-z<Z<z) = \int_{-z}^{z}\dfrac{1}{\sqrt{2\pi}}e^{-\tfrac{x^2}{2}}\, dx$
In the former, the entry for z=0 should be 0.5; in the latter, the entry for z=0 is 0.

In the first case, to get $\displaystyle Pr(0.21<Z<1.69)$, we use $\displaystyle Pr(0.21<Z<1.69) = Pr(Z<1.69)-Pr(Z<0.21)$.
In the second, $\displaystyle Pr(0.21<Z<1.69) = \tfrac{1}{2}(Pr(-1.69<Z<1.69)-Pr(-0.21<Z<0.21))$

Most tables i've seen use the first, but I've encountered a couple with the latter.

--Kevin C.