In a past exam of my stats class, the question requires finding the value of a normal CDF corresponding to a Z score of 3 DP. However, we are only given a table that is accurate to 2 DP. We are not allowed to use calculator. What should I do to find the value when the Z score is 1.293, not just 1.29.

atestiguoki

atestiguoki

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2022-08-16

In a past exam of my stats class, the question requires finding the value of a normal CDF corresponding to a Z score of 3 DP. However, we are only given a table that is accurate to 2 DP. We are not allowed to use calculator. What should I do to find the value when the Z score is 1.293, not just 1.29.

Answer & Explanation

Kelton Glover

Kelton Glover

Beginner2022-08-17Added 17 answers

Personally, I would just round to 1.29. However, if you really want to guess normalcdf(1.293), I would just do a weighted sum.
First, write 1.293 in terms of 1.29 and 1.30, since those are the two-decimal z-scores closest to it:
1.293 = 0.7 1.29 + 0.3 1.30
Then, pretend normalcdf is linear:
normalcdf ( 1.293 ) = 0.7 normalcdf ( 1.29 ) + 0.3 normalcdf ( 1.30 ) = 0.7 0.9015 + 0.3 0.9032 = 0.90201

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