uncover and also interpret the location under a normal curve uncover the worth of a normal random variable

Finding Areas Using a Table

Once we have the general idea of the Typical Distribution, the following step is to learn just how to discover areas under the curve. We"ll learn two different methods - making use of a table and making use of technology.

You are watching: Find the area under the standard normal curve to the left of zequals1.25.

Due to the fact that eexceptionally typically distributed random variable has actually a slightly various circulation shape, the only way to find areas utilizing a table is to standardize the variable - transcreate our variable so it has actually a expect of 0 and also a conventional deviation of 1. How do we carry out that? Use the z-score!

As we noted in Section 7.1, if the random variable X has a expect μ and also conventional deviation σ, then transforming X using the z-score creates a random variable via mean 0 and typical deviation 1! With that in mind, we simply have to learn how to find areas under the standard normal curve, which deserve to then be applied to any usually distributed random variable.

Finding Area under the Standard Regular Curve to the Left

Before we look a couple of examples, we should first view exactly how the table functions. Before we start the area, you require a copy of the table. You deserve to downpack a printable copy of this table, or usage the table in the earlier of your textbook. It should look something favor this:


It"s pretty overwhelming at initially, but if you look at the photo at the peak (take a minute and check it out), you deserve to see that it is indicating the area to the left. That"s the essential - the worths in the middle represent areas to the left of the corresponding z-value. To identify which z-worth it"s referring to, we look to the left to gain the initially 2 digits and over to the columns to acquire the hundredths worth. (Z-values through more accuracy need to be rounded to the hundredths in order to use this table.)

Say we"re looking for the area left of -2.84. To execute that, we"d begin on the -2.8 row and go throughout until we get to the 0.04 column. (See image.)


From the photo, we deserve to check out that the area left of -2.84 is 0.0023.

Finding Areas Using StatCrunch

Click on Stat > Calculators > Normal

Enter the expect, standard deviation, x, and also the direction of the inequality. Then press Compute. The photo listed below reflects P(Z

Example 1

a. Find the location left of Z = -0.72

< expose answer >

The area left of -0.72 is around 0.2358.

b. Find the location left of Z = 1.90

< expose answer >

The location left of 1.90 is roughly 0.9713.

Finding Area under the Standard Common Curve to the Right


To uncover locations to the best, we need to remember the enhance rule. Take a minute and look ago at the dominance from Section 5.2.

Because we know the whole area is 1,

(Area to the right of z0) = 1 - (Area to the left of z0)

Example 2

a. Find the area to the right of Z = -0.72

< disclose answer >

area right of -0.72 = 1 - (the location left of -0.72)
= 1 - 0.2358
= 0.7642

b. Find the location to the appropriate of Z = 2.68

< disclose answer >

area appropriate of 2.68 = 1 - (the location left of 2.68)
= 1 - 0.9963
= 0.0037

An alternate idea is to use the symmetric home of the normal curve. Instead of looking to the best of Z=2.68 in Example 2 over, we could have actually looked at the location left of -2.68. Due to the fact that the curve is symmetric, those areas are the exact same.

Finding Area under the Standard Normal Curve Between Two Values

To discover the area between 2 worths, we think of it in 2 pieces. Suppose we want to uncover the location between Z = -2.43 and also Z = 1.81.

What we execute instead, is find the area left of 1.81, and also then subtract the location left of -2.43. Like this:


So the area between -2.43 and 1.81 = 0.9649 - 0.0075 = 0.9574

Note: StatCrunch is able to calculate the "between" probabilities, so you will not should percreate the calculation above if you"re using StatCrunch.

Example 3

a. Find the location between Z = 0.23 and also Z = 1.64.

< reveal answer >

area in between 0.23 and also 1.64 = 0.9495 - 0.5910 = 0.3585

b. Find the area in between Z = -3.5 and also Z = -3.0.

< disclose answer >

location in between -3.5 and -3.0 = 0.0013 - 0.0002 = 0.0011

Finding Areas Under a Common Curve Using the Table

Draw a sketch of the normal curve and also shade the wanted location. Calculate the matching Z-scores. Find the matching area under the standard normal curve.

If you remember, this is precisely what we witnessed happening in the Area of a Typical Distribution demonstration. Follow the attach and check out aobtain the partnership in between the area under the standard normal curve and also a non-typical normal curve.


Finding Areas Under a Typical Curve Using StatCrunch

Even though there"s no "standard" in the title here, the directions are actually precisely the exact same as those from above!

Click on Stat > Calculators > Normal

Enter the suppose, standard deviation, x, and also the direction of the inehigh quality. Then push Compute. The picture below mirrors P(Z What propercent of people are geniuses? Is a systolic blood push of 110 unusual? What percent of a details brand of light bulb emits in between 300 and 400 lumens? What is the 90th percentile for the weights of 1-year-old boys?

All of these questions deserve to be answered utilizing the normal distribution!

Example 4

Let"s take into consideration again the circulation of IQs that we looked at in Example 1 in Section 7.1.

We experienced in that instance that tests for an individual"s intelligence quotient (IQ) are designed to be typically spread, via a mean of 100 and a typical deviation of 15.

We also witnessed that in 1916, psychologist Lewis M. Thurmale collection a pointer of 140 (scaresulted in 136 in today"s tests) for "potential genius".

Using this indevelopment, what percent of individuals are "potential geniuses"?


Draw a sketch of the normal curve and shade the wanted area.
Calculate the matching Z-scores.
Z = X - μ = 136 - 100 = 2.4
σ 15
Find the corresponding area under the conventional normal curve. P(Z>2.4) = P(Z

Based on this, it looks prefer about 0.82% of individuals have the right to be defined as "potential geniuses" according to Dr. Thurman"s criteria.

Example 5

Source: stock.xchng

In Example 2 in Section 7.1, we were told that weights of 1-year-old boys are approximately commonly distributed, with a mean of 22.8 lbs and a standard deviation of about 2.15. (Source: About.com)

If we randomly choose a 1-year-old boy, what is the probability that he"ll weigh at leastern 20 pounds?


Let"s carry out this one utilizing innovation. We must still begin with a sketch:


Using StatCrunch, we obtain the following result:


According to these results, it looks choose there"s a probcapability of about 0.9036 that a randomly selected 1-year-old boy will certainly weigh more than 20 lbs.

Why do not you try a couple?

Example 6

Photo: A Syed

Suppose that the volume of paint in the 1-gallon paint cans developed by Acme Paint Company type of is approximately normally spread with a intend of 1.04 gallons and a conventional deviation of 0.023 gallons.

What is the probcapacity that a randomly selected 1-gallon deserve to will certainly actually contain at least 1 gallon of paint?

< disclose answer >

In this situation, we want P(X ≥ 1). Using StatCrunch aacquire, we get the adhering to result:


According to the calculation, it looks favor the probcapability that a randomly schosen have the right to will have actually even more than 1 gallon is around 0.9590.

Example 7

Suppose the amount of light (in lumens) emitted by a specific brand also of 40W light bulbs is usually distributed via a intend of 450 lumens and also a standard deviation of 20 lumens.

What percentage of bulbs emit in between 425 and 475 lumens?

< reveal answer >

To answer this question, we need to know: P(425




So P(425 What is the 90th percentile for the weights of 1-year-old boys? What IQ score is listed below 80% of all IQ scores? What weight does a 1-year-old boy have to be so all however 5% of 1-year-old boys weight much less than he does?

Similar to the previous forms of troubles, we"ll learn just how to execute this using both the table and also modern technology. Make certain you recognize both approaches - they"re both supplied in many type of fields of study!

Finding Z-Scores Using the Table

The idea here is that the values in the table recurrent area to the left, so if we"re asked to find the worth via an area of 0.02 to the left, we look for 0.02 on the inside of the table and discover the matching Z-score.


Due to the fact that we don"t have actually a space of exactly 0.02, we have to think a bit. We have two choices: (1) take the closest location, or (2) average the 2 worths if it"s equifar-off from the 2 locations.

In this situation, it"s practically equiremote, so we"ll take the average and say that the Z-score equivalent to this location is the average of -2.05 and -2.06, so -2.055.

Finding Z-Scores Using StatCrunch

Click on Stat > Calculators > Normal

Go into the mean, typical deviation, the direction of the inetop quality, and the probcapability (leave X blank). Then press Compute. The picture listed below mirrors the Z-score with a room of 0.05 to the right.


Let"s attempt a few!

Example 8

Using the normal calculator in StatCrunch, we acquire the adhering to result:


So the Z-score through an area of 0.90 to the left is 1.28. (We commonly round Z-scores to the hundredths.)

b. Find the Z-score via a space of 0.10 to the right.

< reveal answer >

This is actually the same value as Example 7 above! An location of 0.10 to the appropriate suggests that it should have a space of 0.90 to the left, so the answer is aacquire 1.28.

c. Find the Z-score such that P( Z 0 ) = 0.025.

< disclose answer >

Using StatCrunch, we obtain the adhering to result:


So the Z-score is -1.96.

So we"ve talked around how to find a z-score given a room. If you remember, the technology instructions didn"t specify that the distribution essential to be the standard normal - we actually uncover worths in any normal distribution that correspond to a provided area/probability utilizing those same techniques.

Example 9

Referring to IQ scores again, with a expect of 100 and a typical deviation of 15. Find the 90th percentile for IQ scores.


First, we have to analyze the trouble into a space or probability. In Section 3.4, we shelp the kth percentile of a set of data divides the lower k% of a documents collection from the top (100-k)%. So the 90th percentile divides the lower 90% from the upper 10% - definition it has about 90% below and also about 10% above.


Using StatCrunch, we get the complying with result:


Thus, the 90th percentile for IQ scores is around 119.

Example 10

Photo: A Syed

This would be the value through only 5% much less than it. Using StatCrunch, we have the complying with result:


Based on this calculation, the Acme Repaint Company have the right to say that 95% of its cans contain at leastern 1.002 gallons of paint.

Example 11

Using StatCrunch aobtain, we uncover the worth via a space of 0.95 to the left:


So a 1-year-old boy would certainly must weigh around 26.3 lbs. for all yet 5% of all 1-year-old boys to weigh less than he does.

Finding zα

The notation zα ("z-alpha") is the Z-score through a space of α to the ideal.

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The idea of zαis supplied broadly throughout the remainder of the course, so it"s an important one to be comfortable via. The applications won"t be automatically apparent, but the significance is that we"ll be looking for events that are unmost likely - and so have an extremely small probcapability in the "tail".