Stat 1510: Statistical Thinking and Concepts

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Stat 1510Statistical Thinking and Concepts

• Density Curves and
• Normal Distribution

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Topics

• Density Curves
• Normal Distributions
• The 68-95-99.7 Rule
• The Standard Normal Distribution
• Finding Normal Proportions
• Using the Standard Normal Table
• Finding a Value When Given a Proportion

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Objectives

• Define and describe density curves
• Measure position using percentiles
• Measure position using z-scores
• Describe Normal distributions
• Describe and apply the 68-95-99.7 Rule
• Describe the standard Normal distribution
• Perform Normal calculations

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Density Curves
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In previous classes, we developed a kit of
graphical and numerical tools for describing
distributions. Now, well add one more step to
the strategy.
Exploring Quantitative Data

1. Always plot your data make a graph.
2. Look for the overall pattern (shape, center, and
   spread) and for striking departures such as
   outliers.
3. Calculate a numerical summary to briefly describe
   center and spread.
4. Sometimes the overall pattern of a large number
   of observations is so regular that we can
   describe it by a smooth curve.

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Histogram

• Increase sample size, reduce the class width,
  then we can
• approximate the histogram by a smooth curve

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Density Curves
Example Here is a histogram of vocabulary
scores of 947 seventh graders.
The smooth curve drawn over the histogram is a
mathematical model for the distribution.
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Density Curves
The areas of the shaded bars in this histogram
represent the proportion of scores in the
observed data that are less than or equal to 6.0.
This proportion is equal to 0.303.
Now the area under the smooth curve to the left
of 6.0 is shaded. If the scale is adjusted so the
total area under the curve is exactly 1, then
this curve is called a density curve. The
proportion of the area to the left of 6.0 is now
equal to 0.293.
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Density Curves

• A density curve is a curve that
• is always on or above the horizontal axis
• has an area of exactly 1 underneath it
• A density curve describes the overall pattern of
  a distribution. The area under the curve and
  above any range of values on the horizontal axis
  is the proportion of all observations that fall
  in that range.

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Density Curves
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• Our measures of center and spread apply to
  density curves as well as to actual sets of
  observations.

Distinguishing the Median and Mean of a Density
Curve

• The median of a density curve is the equal-areas
  point, the point that divides the area under the
  curve in half.
• The mean of a density curve is the balance point,
  at which the curve would balance if made of solid
  material.
• The median and the mean are the same for a
  symmetric density curve. They both lie at the
  center of the curve. The mean of a skewed curve
  is pulled away from the median in the direction
  of the long tail.

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Density Curves

• The mean and standard deviation computed from
  actual observations (data) are denoted by and
  s, respectively.
• The mean and standard deviation of the actual
  distribution represented by the density curve are
  denoted by µ (mu) and ? (sigma),
  respectively.

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Normal Distributions

• One particularly important class of density
  curves are the Normal curves, which describe
  Normal distributions.
• All Normal curves are symmetric, single-peaked,
  and bell-shaped
• A Specific Normal curve is described by giving
  its mean µ and standard deviation s.

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Normal Distributions

• A Normal distribution is described by a Normal
  density curve. Any particular Normal
  distribution is completely specified by two
  numbers its mean µ and standard deviation s.
• The mean of a Normal distribution is the center
  of the symmetric Normal curve.
• The standard deviation is the distance from the
  center to the change-of-curvature points on
  either side.
• We abbreviate the Normal distribution with mean µ
  and standard deviation s as N(µ,s).

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Question
Data sets consisting of physical measurements
(heights, weights, lengths of bones, and so on)
for adults of the same species and sex tend to
follow a similar pattern. The pattern is that
most individuals are clumped around the average,
with numbers decreasing the farther values are
from the average in either direction. Describe
what shape a histogram (or density curve) of such
measurements would have?
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The 68-95-99.7 Rule

• The 68-95-99.7 Rule
• In the Normal distribution with mean µ and
  standard deviation s
• Approximately 68 of the observations fall within
  s of µ.
• Approximately 95 of the observations fall within
  2s of µ.
• Approximately 99.7 of the observations fall
  within 3s of µ.

N(0,1)
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68-95-99.7 Rule for any Normal Curve
µ
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Normal Distributions

• The distribution of Iowa Test of Basic Skills
  (ITBS) vocabulary scores for 7th-grade students
  in Gary, Indiana, is close to Normal. Suppose
  the distribution is N(6.84, 1.55).
• Sketch the Normal density curve for this
  distribution.
• What percent of ITBS vocabulary scores are less
  than 3.74?
• What percent of the scores are between 5.29 and
  9.94?

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Health and Nutrition Examination Study of
1976-1980

• Heights of adult men, aged 18-24
• mean 70.0 inches
• standard deviation 2.8 inches
• heights follow a normal distribution, so we have
  that heights of men are N(70, 2.8).

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Health and Nutrition Examination Study of
1976-1980

• 68-95-99.7 Rule for mens heights
• 68 are between 67.2 and 72.8 inches
• µ ? ? 70.0 ? 2.8
• 95 are between 64.4 and 75.6 inches
• µ ? 2? 70.0 ? 2(2.8) 70.0 ? 5.6
• 99.7 are between 61.6 and 78.4 inches
• µ ? 3? 70.0 ? 3(2.8) 70.0 ? 8.4

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Health and Nutrition Examination Study of
1976-1980

• What proportion of men are less than 68 inches
  tall?

How many standard deviations is 68 from 70?
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The Standard Normal Distribution

• All Normal distributions are the same if we
  measure in units of size s from the mean µ as
  center.

Key 203 means203 pounds Stems 10sLeaves
1s
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The Standard Normal Table

• Because all Normal distributions are the same
  when we standardize, we can find areas under any
  Normal curve from a single table.

The Standard Normal Table Table A is a table of
areas under the standard Normal curve. The table
entry for each value z is the area under the
curve to the left of z.
Suppose we want to find the proportion of
observations from the standard Normal
distribution that are less than 0.81. We can
use Table A
P(z lt 0.81)
.7910
Z .00 .01 .02
0.7 .7580 .7611 .7642
0.8 .7881 .7910 .7939
0.9 .8159 .8186 .8212
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Normal Calculations
Find the proportion of observations from the
standard Normal distribution that are between
-1.25 and 0.81.
Can you find the same proportion using a
different approach?
1 (0.10560.2090) 1 0.3146
0.6854
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Revisiting Earlier Example - Health and Nutrition
Examination Study of 1976-1980

• How many standard deviations is 68 from 70?
• standardized score
• (observed value minus mean) / (std dev)
• (68 - 70) / 2.8 -0.71
• The value 68 is 0.71 standard deviations below
  the mean 70.

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Health and Nutrition Examination Study of
1976-1980

• What proportion of men are less than 68 inches
  tall?

-0.71 0 (standardized values)
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Health and Nutrition Examination Study of
1976-1980

• What proportion of men are less than 68 inches
  tall?

.2389
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Normal Calculations
How to Solve Problems Involving Normal
Distributions

• State Express the problem in terms of the
  observed variable x.
• Plan Draw a picture of the distribution and
  shade the area of interest under the curve.
• Do Perform calculations.
• Standardize x to restate the problem in terms of
  a standard Normal variable z.
• Use Table A and the fact that the total area
  under the curve is 1 to find the required area
  under the standard Normal curve.
• Conclude Write your conclusion in the context of
  the problem.

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Normal Calculations

• According to the Health and Nutrition Examination
  Study of 1976-1980, the heights (in inches) of
  adult men aged 18-24 are N(70, 2.8).

• How tall must a man be in the lower 10 for men
  aged 18 to 24?

N(70, 2.8)
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Normal Calculations

• How tall must a man be in the lower 10 for men
  aged 18 to 24?

Look up the closest probability (closest to 0.10)
in the table. Find the corresponding standardized
score. The value you seek is that many standard
deviations from the mean.
z .07 .08 .09
?1.3 .0853 .0838 .0823
-1.2 .1020 .1003 .0985
?1.1 .1210 .1190 .1170
Z -1.28
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Normal Calculations

• How tall must a man be in the lower 10 for men
  aged 18 to 24?

Z -1.28
We need to unstandardize the z-score to find
the observed value (x)
x 70 z(2.8) 70 (-1.28 ) ? (2.8)
70 (?3.58) 66.42
A man would have to be approximately 66.42 inches
tall or less to place in the lower 10 of all men
in the population.
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Question
A company engaged in the manufacturing of special
type of material. Diameter of the this component
is a critical characteristic. From the previous
analysis we know that diameter follows normal
distribution with mean 10.05 and std deviation
0.25. The diameter specification is 10 /- 0.4.
If the company has an order of 10000 items, as a
manger of the company, how many items you will
schedule to produce, so that you may able to sent
10000 good items to the customer?