SOLUTION: need help in Finding the z-score for the standard normal distribution where: P(z < +a) = 0.8980

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Question 1139040: need help in Finding the z-score for the standard normal distribution where: P(z < +a) = 0.8980
Answer by Theo(13342) About Me  (Show Source):
You can put this solution on YOUR website!
i think that what this is sayi8ng is that the probability of the z-score being less than x-score is .8980, the other z-score being indicated by the letter a.

you want to find the z-score that has an area under the normal distrivbution curve to the left of the z-score of a equal to .8980.

if you look it up in the z-score table, you will find that the area to the left of the z-score is as shown below:

z-score of 1.27 = .89796
z-score of 1.28 = .89973

the area to the left of the z-score that you want is .8980.

you can manually interpolate to get in the ball park, or you can use a z-score calculator that does the interpolation for you.

it's so much easier to get a z-score calculator and have it tell you what the required z-score is.

i used the online z-score calculator found at http://davidmlane.com/hyperstat/z_table.html

i used the value from an area portion of the calculator and told it that i wanted the z-score that had an area of .8980 below it.

the calculator told me that the indicated z-score was 1.27.

this made sense since the area in the table to the left of the z-score of 1.27 was very close to .8980, so the difference had to be minimal.

the table i used can be found at https://www.math.arizona.edu/~rsims/ma464/standardnormaltable.pdf

look for the z-score of 1.27 and you will see that the area to the left of it is .89796 which is only .00004 units away from .8980.

the display from the online calculator is shown below.

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any questions on this, send an email to dtheophilis@gmail.com.