frequency graphs
TRANSCRIPT
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“ Frequency Graphs "
By
Dr. Bijaya Bhusan Nanda, Ph. D. (Stat.)
Lecture Series on
Statistics -HSTC
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Histogram
Frequency polygon
Smoothed frequency curve
Cumulative frequency curve or ogives
CONTENT
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Learning Objective
The Trainees will be able to construct and interpret frequency graphs.
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The histogram is a special type of bar graph that represents frequency or relative frequency of continuous distribution.
Histograms are appropriate for continuous, quantitative variables.
A normal curve can be superimposed onto the histogram
What is Histogram?
(Definition)
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It represents the class frequencies in the
form of vertical rectangles erected over
respective class intervals.
Total area of the rectangles is equivalent
to total frequency.
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How to Construct a Histogram?
Class intervals are decided and the class frequencies are obtained.
The class intervals are marked along the X – axis.
A set of adjacent rectangles are erected over the C.Is with area of each rectangle being proportional to the corresponding CI.
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Significance of Histogram?
It helps in the understanding of the frequency distribution.
• Skew ness of the distribution or its deviation from the symmetry
• Peakedness of the distribution
• Comparison of two frequency distribution
LOOK at the Following data
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Age of Respondent
92.582.572.562.552.542.532.522.5
Histogram of Age of Respondents
Source of Data: 1991 General Social Survey
Fre
qu
en
cy
200
180
160
140
120
100
80
60
40
20
0
Std. Dev = 17.29
Mean = 44.5
N = 600.00
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Highest Year School Completed, Mother
20.017.515.012.510.07.55.02.50.0
Highest Year of School: Mother
Fre
qu
en
cy
300
200
100
0
Std. Dev = 3.47
Mean = 11.2
N = 500.00
Highest Year of School Completed
20.018.016.014.012.010.08.06.04.0
Highest Year of School : Respondent
Fre
qu
en
cy
300
200
100
0
Std. Dev = 2.82
Mean = 13.3
N = 598.00
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LINE CHART
No. of Couples vrs. No. of Families
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Frequency polygon is a special type of
line graph representing frequency
distribution
FREQUENCY POLYGON
Frequency polygon can be drawn both for
continuous and discrete data.
Comparison of two frequency
distributions is easier through the
superimposition of two histograms or
their resulting frequency polygons
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How to draw Frequency Polygon?
CIs decided & the CFs obtained.
The CI are marked along the X – axis.
Dot is put above the midpoint of each CI represented on the horizontal axis corresponding to the frequency of the relevant CI.
Connect the dots by straight lines
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The frequency polygon can also be drawn by joining the mid-points of the tops of the rectangles through straight lines in the histogram.
The mid-points of the tops of the first and last rectangles are extended to the mid-points of the classes at the extreme having zero frequencies.
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Significance of Frequency polygon
It helps in the understanding the frequency distribution of data
• Skew ness of the distribution or its deviation from the symmetry
• Peakedness of the distribution
• Comparison of two frequency distribution
• This is useful for Continuous and discrete distribution
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Frequency Polygon of Age
Distribution
0
50
100
150
200
22.5 32.5 42.5 52.5 62.5 72.5 82.5 92.5
Midpoint of the Age Interval
Fre
qu
en
cy
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SMOOTHED FREQUENCY CURVE
For any continuous frequency distribution, if
the class intervals become smaller and
smaller, resulting in the increase of the number
of class intervals, the frequency polygon tends
to a smooth curve called frequency curve.
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The area under the frequency curve represents the
total frequency and approximates the area bounded
by rectangles in the histogram
Advantage in statistical analysis:
Does not put any restriction on the choice of
CI.
Frequency function can be expressed by a
mathematical function.
It is easy to compare two frequency distributions
through their frequency curves.
Histograms and frequency curves may also be
drawn using relative frequency distributions.
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Depending upon the nature of the frequency distributions, the frequency curves may be of different shapes.
• symmetrical frequency curve, and
• Skewed or asymmetrical frequency curve
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Symmetrical Frequency Curve
• The symmetrical frequency curves look like bell-shaped curves where the highest frequency occurs in the central class and other frequencies gradually decrease symmetrically on both sides
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Skewed or asymmetrical
frequency curve Moderately skewed with the highest frequency
leaning either towards left or right (long right tail or
long left tail)
Extreme asymmetrical form like J-shaped or U-
shaped.
A frequency curve with long right tail indicates that
lower values occur more often than the extreme
higher values, e.g., distribution of income of families
in a locality.
Similarly, in a long left tailed frequency distribution,
the extreme higher values occur more often than the
extreme lower values, e.g., educated members
among income groups.
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U Shaped frequency curve
• Mortality according to age group
J Shaped frequency curve
• Distribution of death rate by age group in a population with low IMR
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CUMULATIVE FREQUENCY DISTRUBUTION
A cumulative frequency distribution may construct from the original frequency distribution by cumulating or adding together frequencies successively:
The cumulative frequency distributions so constructed are called upward cumulative frequency distributions because frequencies are cumulated from the lowest class interval to the highest class interval
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OGIVE; The graphical representation of cumulative frequency distrn.
Helpful to find out number or percentage of observations above or below a particular value.
Offer a graphical technique for determining positional measures such as median, quartiles, deciles, percentiles, etc
OGIVE
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0
20
40
60
80
100
120
68 76 84 92 100
108
116
124
132
140
Quantity of glucoza (mg%)
Cu
mu
lati
ve f
req
uen
cy
Figure 2.8 Cumulative frequency distribution for quantity of glucose
(for data in Table 2.1)
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Next Session
Descriptive Statistics – Measures of Central Tendency
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THANK YOU