Find the z-score corresponding to a raw score of 132 from a normal distribution with mean 100 and standard deviation 15.

Solution

We compute

132 - 100
z = = 2.133
15

Example

A z-score of 1.7 was found from an observation coming from a normal distribution with mean 14 and standard deviation 3. Find the raw score.

Solution

We have

x - 14
1.7 =
3

To solve this we just multiply both sides by the denominator 3,

(1.7)(3) = x - 14

5.1 = x - 14

x = 19.1

The z-score and Area

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Often we want to find the probability that a z-score will be less than a given value, greater than a given value, or in between two values. To accomplish this, we use the table from the textbook and a few properties about the normal distribution.

Example

Recordam

Find

P(z < 2.37)

Solution

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We use the table. Notice the picture on the table has shaded region corresponding to the area to the left (below) a z-score. This is exactly what we want. Below are a few lines of the table.

z.00.01.02.03.04.05.06.07.08.09
2.2.9861.9864.9868.9871.9875.9878.9881.9884.9887.9890
2.3.9893.9896.9898.9901.9904.9906.9909.9911.9913.9916
2.4.9918.9920.9922.9925.9927.9929.9931.9932.9934.9936

High 5 free slots. The columns corresponds to the ones and tenths digits of the z-score and the rows correspond to the hundredths digits. For our problem we want the row 2.3 (from 2.37) and the row .07 (from 2.37). The number in the table that matches this is.9911.

Hence

P(z < 2.37) = .9911

Example

Find

P(z > 1.82)

Solution

In this case, we want the area to the right of 1.82. This is not what is given in the table. We can use the identity

P(z > 1.82) = 1 - P(z < 1.82)

reading the table gives

P(z < 1.82) = .9656

Our answer is

P(z > 1.82) = 1 - .9656 = .0344

Example

Find

P(-1.18 < z < 2.1)

Solution

Once again, the table does not exactly handle this type of area. However, the area between -1.18 and 2.1 is equal to the area to the left of 2.1 minus the area to the left of -1.18. That is

P(-1.18 < z < 2.1) = P(z < 2.1) - P(z < -1.18)

To find P(z < 2.1) we rewrite it as P(z < 2.10) and use the table to get

P(z < 2.10) = .9821.

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The table also tells us that

P(z < -1.18) = .1190

Now subtract to get

P(-1.18 < z < 2.1) = .9821 - .1190 = .8631

e-mail Questions and Suggestions

2x4s are not actually 2 inches by 4 inches. When the board is first rough sawn from the log, it is a true 2x4, but the drying process and planing of the board reduce it to the finished 1.5x3.5 size. Here are the common sizes of lumber, and their actual sizes.
NominalActualActual - Metric
1' x 2'3/4' x 1-1/2'19 x 38 mm
1' x 3'3/4' x 2-1/2'19 x 64 mm
1' x 4'3/4' x 3-1/2'19 x 89 mm
1' x 5'3/4' x 4-1/2'19 x 114 mm
1' x 6'3/4' x 5-1/2'19 x 140 mm
1' x 7'3/4' x 6-1/4'19 x 159 mm
1' x 8'3/4' x 7-1/4'19 x 184 mm
1' x 10'3/4' x 9-1/4'19 x 235 mm
1' x 12'3/4' x 11-1/4'19 x 286 mm
1-1/4' x 4'1' x 3-1/2'25 x 89 mm
1-1/4' x 6'1' x 5-1/2'25 x 140 mm
1-1/4' x 8'1' x 7-1/4'25 x 184 mm
1-1/4' x 10'1' x 9-1/4'25 x 235 mm
1-1/4' x 12'1' x 11-1/4'25 x 286 mm
1-1/2' x 4'1-1/4' x 3-1/2'32 x 89 mm
1-1/2' x 6'1-1/4' x 5-1/2'32 x 140 mm
1-1/2' x 8'1-1/4' x 7-1/4'32 x 184 mm
1-1/2' x 10'1-1/4' x 9-1/4'32 x 235 mm
1-1/2' x 12'1-1/4' x 11-1/4'32 x 286 mm
2' x 2'1-1/2' x 1-1/2'38 x 38 mm
2' x 4'1-1/2' x 3-1/2'38 x 89 mm
2' x 6'1-1/2' x 5-1/2'38 x 140 mm
2' x 8'1-1/2' x 7-1/4'38 x 184 mm
2' x 10'1-1/2' x 9-1/4'38 x 235 mm
2' x 12'1-1/2' x 11-1/4'38 x 286 mm
3' x 6'2-1/2' x 5-1/2'64 x 140 mm
4' x 4'3-1/2' x 3-1/2'89 x 89 mm
4' x 6'3-1/2' x 5-1/2'89 x 140 mm