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Descriptive Statistics
for one variable
Statistics has two major chapters:
• Descriptive Statistics
• Inferential statistics
Statistics
Descriptive Statistics
• Gives numerical and
graphic procedures to
summarize a collection
of data in a clear and
understandable way
Inferential Statistics
• Provides procedures
to draw inferences
about a population
from a sample
Descriptive Measures
• Central Tendency measures. They are
computed to give a “center” around which the
measurements in the data are distributed.
• Variation or Variability measures. They
describe “data spread” or how far away the
measurements are from the center.
• Relative Standing measures. They describe
the relative position of specific measurements in the
data.
Measures of Central Tendency
• Mean:
Sum of all measurements divided by the number
of measurements.
• Median:
A number such that at most half of the
measurements are below it and at most half of the
measurements are above it.
• Mode:
The most frequent measurement in the data.
Example of Mean
Measurements Deviation
x x - mean
3 -1
5 1
5 1
1 -3
7 3
2 -2
6 2
7 3
0 -4
4 0
40 0
• MEAN = 40/10 = 4
• Notice that the sum of the
“deviations” is 0.
• Notice that every single
observation intervenes in
the computation of the
mean.
Example of Median
• Median: (4+5)/2 =
4.5
• Notice that only the
two central values are
used in the
computation.
• The median is not
sensible to extreme
values
Measurements Measurements
Ranked
x x
3 0
5 1
5 2
1 3
7 4
2 5
6 5
7 6
0 7
4 7
40 40
Example of Mode
Measurements
x
3
5
5
1
7
2
6
7
0
4
• In this case the data have
two modes:
• 5 and 7
• Both measurements are
repeated twice
Example of Mode
Measurements
x
3
5
1
1
4
7
3
8
3
• Mode: 3
• Notice that it is possible for a
data not to have any mode.
Variance (for a sample)
• Steps:
– Compute each deviation
– Square each deviation
– Sum all the squares
– Divide by the data size (sample size) minus
one: n-1
Example of Variance
Measurements Deviations Square of
deviations
x x - mean
3 -1 1
5 1 1
5 1 1
1 -3 9
7 3 9
2 -2 4
6 2 4
7 3 9
0 -4 16
4 0 0
40 0 54
• Variance = 54/9 = 6
• It is a measure of
“spread”.
• Notice that the larger
the deviations (positive
or negative) the larger
the variance
The standard deviation
• It is defines as the square root of the
variance
• In the previous example
• Variance = 6
• Standard deviation = Square root of the
variance = Square root of 6 = 2.45
Percentiles
• The p-the percentile is a number such that at most p%
of the measurements are below it and at most 100 – p
percent of the data are above it.
• Example, if in a certain data the 85th
percentile is 340
means that 15% of the measurements in the data are
above 340. It also means that 85% of the
measurements are below 340
• Notice that the median is the 50th
percentile
For any data
• At least 75% of the measurements differ from the mean
less than twice the standard deviation.
• At least 89% of the measurements differ from the mean
less than three times the standard deviation.
Note: This is a general property and it is called Tchebichev’s Rule: At
least 1-1/k2
of the observation falls within k standard deviations from the
mean. It is true for every dataset.
Example of Tchebichev’s Rule
Suppose that for a certain
data is :
• Mean = 20
• Standard deviation =3
Then:
• A least 75% of the
measurements are
between 14 and 26
• At least 89% of the
measurements are
between 11 and 29
Further Notes
• When the Mean is greater than the Median the
data distribution is skewed to the Right.
• When the Median is greater than the Mean the
data distribution is skewed to the Left.
• When Mean and Median are very close to each
other the data distribution is approximately
symmetric.
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Descriptive statistics -review(2)

  • 2. Statistics has two major chapters: • Descriptive Statistics • Inferential statistics
  • 3. Statistics Descriptive Statistics • Gives numerical and graphic procedures to summarize a collection of data in a clear and understandable way Inferential Statistics • Provides procedures to draw inferences about a population from a sample
  • 4. Descriptive Measures • Central Tendency measures. They are computed to give a “center” around which the measurements in the data are distributed. • Variation or Variability measures. They describe “data spread” or how far away the measurements are from the center. • Relative Standing measures. They describe the relative position of specific measurements in the data.
  • 5. Measures of Central Tendency • Mean: Sum of all measurements divided by the number of measurements. • Median: A number such that at most half of the measurements are below it and at most half of the measurements are above it. • Mode: The most frequent measurement in the data.
  • 6. Example of Mean Measurements Deviation x x - mean 3 -1 5 1 5 1 1 -3 7 3 2 -2 6 2 7 3 0 -4 4 0 40 0 • MEAN = 40/10 = 4 • Notice that the sum of the “deviations” is 0. • Notice that every single observation intervenes in the computation of the mean.
  • 7. Example of Median • Median: (4+5)/2 = 4.5 • Notice that only the two central values are used in the computation. • The median is not sensible to extreme values Measurements Measurements Ranked x x 3 0 5 1 5 2 1 3 7 4 2 5 6 5 7 6 0 7 4 7 40 40
  • 8. Example of Mode Measurements x 3 5 5 1 7 2 6 7 0 4 • In this case the data have two modes: • 5 and 7 • Both measurements are repeated twice
  • 9. Example of Mode Measurements x 3 5 1 1 4 7 3 8 3 • Mode: 3 • Notice that it is possible for a data not to have any mode.
  • 10. Variance (for a sample) • Steps: – Compute each deviation – Square each deviation – Sum all the squares – Divide by the data size (sample size) minus one: n-1
  • 11. Example of Variance Measurements Deviations Square of deviations x x - mean 3 -1 1 5 1 1 5 1 1 1 -3 9 7 3 9 2 -2 4 6 2 4 7 3 9 0 -4 16 4 0 0 40 0 54 • Variance = 54/9 = 6 • It is a measure of “spread”. • Notice that the larger the deviations (positive or negative) the larger the variance
  • 12. The standard deviation • It is defines as the square root of the variance • In the previous example • Variance = 6 • Standard deviation = Square root of the variance = Square root of 6 = 2.45
  • 13. Percentiles • The p-the percentile is a number such that at most p% of the measurements are below it and at most 100 – p percent of the data are above it. • Example, if in a certain data the 85th percentile is 340 means that 15% of the measurements in the data are above 340. It also means that 85% of the measurements are below 340 • Notice that the median is the 50th percentile
  • 14. For any data • At least 75% of the measurements differ from the mean less than twice the standard deviation. • At least 89% of the measurements differ from the mean less than three times the standard deviation. Note: This is a general property and it is called Tchebichev’s Rule: At least 1-1/k2 of the observation falls within k standard deviations from the mean. It is true for every dataset.
  • 15. Example of Tchebichev’s Rule Suppose that for a certain data is : • Mean = 20 • Standard deviation =3 Then: • A least 75% of the measurements are between 14 and 26 • At least 89% of the measurements are between 11 and 29
  • 16. Further Notes • When the Mean is greater than the Median the data distribution is skewed to the Right. • When the Median is greater than the Mean the data distribution is skewed to the Left. • When Mean and Median are very close to each other the data distribution is approximately symmetric.