Normal Curves Introduction
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Transcript Normal Curves Introduction
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Statistics: Chapter 6
Z scores review, Normal Curve Introduction
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Do Now:
Complete the Beijing Olympics worksheet given to you when
you entered class.
This will be an extended do now and I will collect it. You may
work in table groups
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Normal Models
Normal Models are appropriate for distributions whose
shape is uni-modal and symmetric.
“Bell shaped curve”
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Symbols
N(mean, standard deviation)
The normal model with mean 0 and standard deviation of 1 is
called the standard normal model (or standard normal
distribution)
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Symmetric Uni-Model Data
So we said that symmetric, unimodal data can be
standardized into a normal model….
1.
So don’t claim a normal model with skewed data.
2.
Is anything actually ever completely normal?
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Normal Models
The 68 – 95- 99.7 Rule (Empirical Rule)
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+ Sketch Normal Models using the 6895-99.7 Rule
Birth weights of babies N(7.6 lb, 1.3 lb)
+ Sketch Normal Models using the 6895-99.7 Rule
ACT Scores at a certain college, N(21.2, 4.4)
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Do Now
Create a list in your calculator with the following numbers:
4
3
10
12
8
9
3
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Calculating Standard Deviation on
a Calculator
Put all your data into a list
Under the STAT CALC menu, select 1-VAR STATS and hit
ENTER
Specify the location of your data, created a command like 1VAR STATS L1.
Hit ENTER again
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Normal Models
The 68 – 95- 99.7 Rule (Empirical Rule)
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Example: This is a practice
problem in your packet***
A forester measured 27 of the trees in a large wood that is up for
sale. He found a mean diameter of 10.4 inches and a standard
deviation of 4.7 inches. Suppose that these trees provide an
accurate description of the whole forest and the normal model
applies
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N(36, 4)
What percent of data are between 32 and 40?
What percent above the mean are between 36 and 40?
What percent of data are between 28 and 44?
What range contains 99.7% of data?
What range contains 47.5% below the mean?
Where would the top 16% of data be?
What percent of data is outside 24 and 48?
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Helmet Sizes
The
army reports that the distribution of
head circumference among male soldiers is
approximately normal with a mean of 22.8
inches and a standard deviation of 1.1
inches.
What percent of soldiers have a head circumference greater than
23.9in?
A head circumference of 23.9 inches would be what percentile?
What percent of soldiers have a head circumference between
20.6 and 23.9inches?
What interval below the mean would contain 13.5% of soldiers?
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Driving Speed
Suppose it takes you 20 minutes, on average, to drive to
school with a standard deviation of 2 minutes. Suppose a
normal model is appropriate for the distribution of driving
times.
How often will you arrive at school in less than 22 minutes?
How often will it take you more than 24 minutes?
Do you think the distribution of your driving times is unimodal
and symmetric in general?
What does this say about the accuracy of your predictions?
Explain.
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Practice
Try some on your own! As always, call me over if you are
confused!
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Exit Ticket
A company that manufactures rivets believes the shear
strength (in pounds) is modeled by N(500, 50).
Draw and label the normal model (just sketch the curve)
Would it be safe to use these rivets in a situation requiring a shear
strength of 750 pounds? Explain.
About what percent of these rivets would you expect to fall below
900 pounds?
Rivets are used in a variety of applications with varying shear
strength requirements. What is the maximum shear strength for
which you would feel comfortable approving this company rivets?
Explain your reasoning.