Find the z-scores for which 90% of the distribution's area lies between -z and z.?

Answer 1

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90% area = 0.9000
Since the area lies between -z and +z, the area lies equally on the left and right side of the maximum ordinate
Therefore z value corresponding to 0.9000/2 = 0.4500 area shall be taken
z value is to be located from the area under the standard normal curve table
z value is 1.645
The z scores required are - 1.645 and + 1.645

Answer 2

z ≈ ±1.6447

Source(s):

http://www.wolframalpha.com/input/?i=zsc…
http://www.wolframalpha.com/input/?i=zsc…