Download Variability

CHAPTER 3
Descriptive
Statistics
Measures of
Central Tendency
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Descriptive
Statistics
Measures of Central Tendency
 Mean--------Interval or Ratio scale Polygon
– The sum of the values divided by the number of
values--often called the "average." μ=ΣX/N
– Add all of the values together. Divide by the number
of values to obtain the mean.
– Example:
X
7
12
24
20
19
????
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Descriptive
Statistics
The Mean is:
μ=ΣX/N= 82/5=16.4
(7 + 12 + 24 + 20 + 19) / 5 =
16.4.
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The Characteristics of Mean
 1. Changing a score in a distribution will
change the mean
 2. Introducing or removing a score from
the distribution will change the mean
 3. Adding or subtracting a constant from
each score will change the mean
 4. Multiplying or dividing each score by a
constant will change the mean
 5. Adding a score which is same as the
mean will not change the mean
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Descriptive
Statistics
 Measures of Central Tendency
 Median/MiddleOrdinal ScaleBar/Histogram
– Divides the values into two equal halves, with
half of the values being lower than the median
and half higher than the median.
 Sort the values into ascending order.
 If you have an odd number of values, the
median is the middle value.
 If you have an even number of values, the
median is the arithmetic mean (see above) of
the two middle values.
– Example: The median of the same five numbers
(7, 12, 24, 20, 19) is ???.
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Statistics
 The median is 19.
 Mode-Nominal Scale Bar/Histogram
– The most frequently-occurring value (or
values).
 Calculate the frequencies for all of the
values in the data.
 The mode is the value (or values) with
the highest frequency.
– Example: For individuals having the
following ages -- 18, 18, 19, 20, 20, 20, 21,
and 23, the mode is ???? The Mode is 20
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CHARACTERISTICS OF MODE
 Nominal Scale
 Discrete Variable
 Describing Shape
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The Range
 The Range:
The Range is the difference between
the highest number –lowest number +1
2, 4, 7, 8, and 10 -> Discrete Numbers
2, 4.6, 7.3, 8.4, and 10 -> Continues
Numbers
The difference between the upper real
limit of the highest number and the
lower real limit of the lowest number.
CHAPTER 4
Variability
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Variability
 Variability is a measure of
dispersion or spreading of
scores around the mean, and
has 2 purposes:
 1. Describes the distribution
Next slide
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Range, Interquartile Range, Semi-Interquartile
Range, Standard Deviation, and Variance are the
Measures of Variability
 The Range:
The Range is the difference between the
highest number –lowest number +1
2, 4, 7, 8, and 10 -> Discrete Numbers
2, 4.6, 7.3, 8.4, and 10 -> Continues
Numbers
The difference between the upper real
limit of the highest number and the lower
real limit of the lowest number.
Variability
 2. How well an individual score (or
group of scores) represents the
entire distribution. i.e. Z Score
 Ex. In inferential statistics we
collect information from a small
sample then, generalize the results
obtained from the sample to the
entire population.
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Interquartile Range (IQR)
 In descriptive statistics, the
Interquartile Range (IQR),
also called the midspread or
middle fifty, is a measure of
statistical dispersion, being
equal to the difference
between the upper and lower
quartiles. (Q3 − Q1)=IQR
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Interquartile Range (IQR)
IQR is the range covered
by the middle 50% of the
distribution.
IQR is the distance
rd
between the 3 Quartile
st
and 1 Quartile.
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Semi-Interquartile Range (SIQR)
SIQR is ½ or half of
the Interquartile
Range.
SIQR = (Q3-Q1)/2
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Variability
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Variability
Range, SS, Standard Deviations and Variances
 X
1
2
4
5
σ² = ss/N
σ = √ss/N
Pop
s² = ss/n-1 or ss/df Standard deviation
s = √ss/df
Sample
SS=Σx²-(Σx)²/N
 Computation
SS=Σ( x-μ)²
 Definition
Sum of Squared Deviation from Mean
Variance (σ²) is the Mean of Squared Deviations=MS22
Practical Implication for Test
Construction
Variance and Covariance measure the quality of each
item in a test.
Reliability and validity measure the quality of the
entire test.
 σ²=SS/N  used for one set of data
Variance is the degree of variability
of scores from mean.
Correlation is based on a statistic called Covariance (Cov xy
or S xy) ….. r=sp/√ssx.ssy
 COVxy=SP/N-1  used for 2 sets of data
Covariance is a number that reflects the degree to
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which 2 variables vary together.
Variance
 X
1
2
4
5
σ² = ss/N
Pop
s² = ss/n-1 or ss/df Sample
SS=Σx²-(Σx)²/N
SS=Σ( x-μ)²
Sum of Squared Deviation from Mean
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Covariance
 Correlation is based on a statistic called
Covariance (Cov xy or S xy) …..
COVxy=SP/N-1
Correlation-- r=sp/√ssx.ssy
 Covariance is a number that reflects the
degree to which 2 variables vary
together.
 Original Data
X Y
1 3
2 6
4 4
5 7
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Covariance
 Correlation is based on a statistic called
Covariance (Cov xy or S xy) …..
COVxy=SP/N-1
Correlation-- r=sp/√ssx.ssy
 Covariance is a number that reflects the
degree to which 2 variables vary
together.
 Original Data
X Y
8 1
1 0
3 6
0 1
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Covariance

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Descriptive Statistics for
Nondichotomous Variables
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Descriptive Statistics for
Dichotomous Data
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Descriptive Statistics for
Dichotomous Data
Item Variance & Covariance
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FACTORS THAT AFFECT
VARIABILITY
 1. Extreme Scores i.e. 1, 3, 8, 11, 1,000,000.00 . We can’t use
the Range in this situation but we can use the other measures of
variability.
 2. Sample Size If we increase the sample size will change the
Range therefore we can’t use the Range in this situation but we can
use the other measures of variability.
 3. Stability Under Sampling (see next slide) p.130 The
S and S² for all samples should be the same because they come from
same population (all slices of a pizza should taste the same).
 4. Open-Ended Distribution When we don’t have
highest score and lowest score in a distribution
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