Calculating a Standard Normal Probability or the Area Under the Standard Normal Curve
1.
Finding 
:
The
TI-83/84 is equiped with a built in function called normcdf (
,
).
It computes
which is the area under the standard normal curve between
and .
To calculate P(-1 < z < 2), press 2nd VARS [DISTR], ARROW DOWN to select 2:normalcdf(, and then press ENTER.
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After 2:normalcdf( type
-1 
2 ), and then press ENTER. See the results below.
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To 4 decimal places, the P(-1 < z < 2) = 0.8186.
2. Finding P(z < 2):
To
calculate this probablity, P(z
< 2), we can use the above normalcdf function since 
.
Although we cannot enter 
as
it is not a number, entering a very small number in its place, such as 
=
-E99, will do.
Press 2nd VARS [DISTR], ARROW DOWN to select 2:normalcdf(, and
then press ENTER. Press 
then press 2nd
[EE] and type 99. Press
and type 2). Then press ENTER.
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To 4 decimal places, the P (z < 2) = 0.9772.
To
find P(z > 1) use the fact that P(z > 1) = P (1 < z
< 
).
Using E99 for ,
we calculate normalcdf(1, E99). The result is shown below.
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To 4 decimal places, the P (z > 1) = 0.1587.
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