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```Main Ideas /
Questions:
Algebra 1
Name: ________________________________
Notes
Period: _______Date: _________________
TOPIC
Mean Absolute Deviation and Standard Deviation

Sample 
 Population
Use this
column to
Examples:
write down
a) You want to know the average income of families in Loudoun County. What would be the:
any extra
Sample group? _____________________________________________________________
thoughts you
have while
Population? ________________________________________________________________
watching the
b) You need to know the average height of 5 year olds. What would be the:
video.
Sample group? _____________________________________________________________
Population? ________________________________________________________________

It is many times impossible to know for sure information on the population, but we use the
information on the sample to predict what the same information would be for the entire population
Deviation of a data point 
Subtract data point value minus mean value.

Dispersion of a data set 
We will discuss TWO ways to measure dispersion of a data set:
–
–
Say you have the sample data set: 2, 8, 3, 4, 6, 1
a) What is the mean    ? _______________
b) Recall that deviation is _____________________________________________________
Subtract each data point value minus the mean value.
Distances are always ____________.
*
Remember:
Data point, x
2
8
3
4
6
1
Deviation
x
Absolute Deviation
x
c) What would be a good way to summarize the numbers you got for all the absolute deviations?
Find the mean of all the absolute deviations.






So, the Mean Absolute Deviation for this data set is:
_______________
NOW THINK:
This MAD answer says that on average, the points in that data set should be 2 numbers away from
the mean (i.e., 2 numbers higher or 2 numbers lower than the mean).
What would you think about a data point in that set that is twice the MAD away from the mean?
(i.e., 4 numbers higher or lower than the mean)
That number is a little unusual in this group of points.
What would you think about a data point in that set that is three times the MAD away from the mean?
(i.e., 6 numbers higher or lower than the mean)
That number is definitely very unusual in this group of points. An outlier, maybe?
How can you use the Mean Absolute Deviation of a sample data set to describe the dispersion of that
sample data set?
The smaller the MAD, the less disperse a data set is away from the mean.
The larger the MAD, the more disperse a data set is away from the mean.
You can predict that dispersion of the population will be about the same as the dispersion
you got for the sample group.
Example 1: Calculate the MAD of each data set. Then, describe the dispersion of the data sets.
Height of students in Group ‘A’ (inches)
Height of students in Group ‘B’ (inches)
65, 63, 61, 62, 64, 63
55, 71, 67, 59, 69, 57
Data
point, x
Deviation
x
Absolute
Deviation
x
Data
point, x
Deviation
x
Absolute
Deviation
x
Describe dispersions: Data for Group ‘A” is not very disperse. Data for Group ‘B’ is much more
disperse.
2) STANDARD DEVIATION (  )
This is another way to summarize the deviation from the mean of all the points in a data set.
It’s the same as for MAD, except instead of using absolute value to make all the deviations
positive, this method now squares the deviations to make them all positive!
Let’s find the standard deviation,  , of the same data set we used to learn about MAD:
2, 8, 3, 4, 6, 1
a) What is the mean    ? _______________
b) Now calculate the deviation from the mean of each point but this time square those results in
order to make them positive:
Data point, x
2
8
3
4
6
1
 Why have
and  ?
Square of Deviation
Deviation
x
x  
2
c) Summarize: find the mean of all the square of the deviations






_______________
The mean of all the square of the deviations is called the
___________________.
d) Remember you squared all the deviations? Our last step must be to “undo” all that squaring.
What is the inverse operation of squaring a number?

Find its square root.
=
So, the Standard Deviation,  , for this data set is:
 = _______________
NOW THINK:
This standard deviation,  , says that the points in that data set are expected to be 2.4 numbers away
from the mean (i.e., 2.4 numbers higher or 2.4 numbers lower than the mean).
What would you think about a data point in that set that is twice the  away from the mean?
(i.e., 4.8 numbers higher or lower than the mean)
That number is a little unusual in this group of points.
What would you think about a data point in that set that is three times the  away from the mean?
(i.e., 7.2 numbers higher or lower than the mean)
That number is definitely very unusual in this group of points. An outlier, maybe?
How can you use the standard deviation of a sample data set to describe the dispersion of that
sample data set?
The smaller the, the less disperse a data set is away from the mean.
The larger the , the more disperse a data set is away from the mean.
How can you use the standard deviation,  , of a sample data set to describe the dispersion of the
population? You can predict that the dispersion of the population will be abou
Below are the calculator steps for Finding Standard Deviation. If
you have a graphing calculator, feel free to practice this.
IF not, don’t worry! We will use the calculators in class
to practice!!
Calculator Steps for Standard Deviation
Fortunately, your calculator can find the standard deviation of a data set for you.
Follow the same calculator steps you did last class to find the quartiles for the box-and-whisker plot.
The list of answers you get also includes the standard deviation. REVIEW:
Step 1: Press STAT ,
1
Step 2: Enter your data set as a list under L1. (Data points do not have to be in order)
Step 3: Press STAT , (scroll
to CALC),
1
Step 3: Press ENTER . Your stats results on your screen as shown below.
The mean, is the first number
The standard deviation
is here
10.2222222
2 92
1080
4.17665469
3.93778781
5
9
Example 2: Find the mean and standard deviation of the following set of test grades (use calculator):
95, 76, 85, 92, 78, 84, 71, 80, 65
a) mean,   _____________
  _____________
b) What test grade would be one standard deviation above the mean? ________
c) What test grade would be one standard deviation below the mean? ________
d) What test grades would be two standard deviations away from the mean?
MAD predicts population dispersion better when data has outliers or is skewed data
better for symmetrically distributed data that does not have outliers.
Summary:
What did you
learn??
```
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