and
we want to find
then
we can use the
– transform
to
find a
– value then find the probability for this–
value from the tables.
A Level Maths Notes: S1 – Calculations Involving the Normal Distribution – Finding Probabilities
If you look at the tables for the normal distribution, along the
top and left borders there are z – values. If we have a Normal
distributionand
we want to findthen
we can use the
– transformto
find a
– value then find the probability for this–
value from the tables.
Example:
Find
to
two decimal places.
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0.07 |
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0.6 |
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0.2486 |
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To find the probability in our tables corresponding to
we
go down to 0.6 then along to where there is a 0.07 in top row,
reading 0.2486 from the table. This is not the answer yet though. 
We have just found the blue area above. We have to add to it the
unshaded area to the left, which is 0.5, obtaining 0.7486, because
the question was to find
so
we need the whole area to the left. This is a consequence of our
particular normal tables. Not all normal tables will require you to
do this.