B.2 When the z-score is known, and the area is sought
The table gives the probability (area) that a z-score is less than the z-score looked up. For example: Look up z=−1.87; the area less than z=−1.87 is about 0.0307, or about 3.1%.
To use this table,
enter the z-score in the search box under the z.score
column.
The area will be shown.
The table includes z-values between -4 and 4.
(Alternatively, you can search through the table manually.)
The online Tables work with two decimal places.
As an example, then, consider finding the area to the left of z=−2.00.
In the tables,
the value -2
is entered in the search region, just under the column labelled z.score
(see the animation below).
After pressing Enter
,
the answer is shown in the column headed Area.to.left
:
the probability of finding a z-score less than −2 is 0.0228, or about 2.28%.
The hard-copy tables work differently. On the tables, look for −2.0 in the left margin of the table, and for the second decimal place (in this case, 0) in the top margin of the table (see the animation below): where these intersect is the area (or probability) less than this z-score. So the probability of finding a z-score less than −2 is 0.0228, or about 2.28%.