Chapter 2 Descriptive statistics for pedatric.pptx

2. Descriptive statistics
Presentation of data
• The presentation of data is broadly classified in to the following two
categories:
1. Tabular presentation
2. Diagrammatic and Graphic presentation
• The most convenient way of presenting data is to construct a frequency
distribution.
• Raw data: recorded information in its original collected form, whether it is
counts or measurements.
• Frequency: is the number of values in a specific class of the distribution.
• Frequency distribution: is the organization of raw data in table form using
classes and frequencies.
• The process of arranging data in to classes or categories according to
similarities technically is called classification.

Tabular presentation
• There are three basic types of frequency distributions
– Categorical frequency distribution
– Ungrouped frequency distribution
– Grouped frequency distribution
Categorical FD:-
• A FD in which the data is qualitative i.e. either nominal or ordinal.
• Each category of the variable represents a single class
• the number of times each category repeats represents the
frequency of that class (category).
Example 1. Construct categorical FD the blood type of 25 students is given below
A B B AB O A
O O B AB B A
B B O A O AB
A O O O AB O B
Class
(1)
Tally
(2)
Frequency
(3)
Percent
(4)

Cont……
Ungrouped FD (Frequency Array):-
• A FD of numerical data (quantitative) in which each
value of a variable represents a single class (i.e. the
values of the variable are not grouped) and
• The number of times each value repeats represents the
frequency of that class.
Example 2:- Construct ungrouped frequency distribution
number of children for 21 families.
2 3 5 4 3 3 2
3 1 0 4 3 2 2
1 1 1 4 2 2 2

Cont….
Grouped (Continuous) FD
• A FD of numerical data in which several values of a
variable are grouped into one class.
• The number of observations belonging to the class is
the frequency of the class
• Class limits: Separates one class in a grouped
frequency distribution from another
• Units of measurement (U): the distance between
two possible consecutive measures
• Class boundaries, Class width, Class mark (Mid
points), Cumulative frequency and Relative
frequency (rf)

Cont…
Guidelines for classes
• There should be between 5 and 20 classes.
• The classes must be mutually exclusive. This means
that no data value can fall into two different classes
• The classes must be all inclusive or exhaustive. This
means that all data values must be included.
• The classes must be continuous. There are no gaps in
a frequency distribution.
• The classes must be equal in width. The exception
here is the first or last class. It is possible to have an
"below ..." or "... and above" class. This is often used
with ages.

Steps for constructing Grouped frequency Distribution
1. Find the largest and smallest values then Compute the Range(R)
= Maximum – Minimum
2. Select the number of classes desired, use Sturges rule.
K=1+3.32logn
3. Find the class width by dividing the range by the number of
classes and rounding W=R/K
4. Pick a suitable starting point less than or equal to the minimum
value, This point is called the lower limit of the first class
5. To find the upper limit of the first class, subtract U from the
lower limit of the second class
6. Find the boundaries by subtracting U/2 units from the lower
limits and adding U/2 units from the upper limits.

Cont…
• Example*:
Construct a grouped frequency distribution for
the following data.
11 29 6 33 14 31 22 27 19 20
18 17 22 38 23 21 26 34 39 27

The complete grouped frequency distribution as follows:
Class
limit
Class
boundary
Class
Mark
Tally Freq. Cf (less
than
type)
Cf
(more
than
type)
rf. rcf (less
than type
6 – 11 5.5 – 11.5 8.5 // 2 2 20 0.10 0.10
12 – 17 11.5 – 17.5 14.5 // 2 4 18 0.10 0.20
18 – 23 17.5 – 23.5 20.5 ////// 7 11 16 0.35 0.55
24 – 29 23.5 – 29.5 26.5 //// 4 15 9 0.20 0.75
30 – 35 29.5 – 35.5 32.5 /// 3 18 5 0.15 0.90
36 – 41 35.5 – 41.5 38.5 // 2 20 2 0.10 1.00

Graphic and Diagrammatic presentation of
data
Graphs
Histogram: A graph in which the classes are marked on the X axis
(horizontal axis) and the frequencies are marked along the Y axis
(vertical axis).
• The height of each bar represents the class frequencies and the width
of the bar represents the class width.
• The bars are drawn adjacent to each other.
Frequency Polygon A graph that consists of line segments connecting
the intersection of the class marks and the frequencies.
• Can be constructed from Histogram by joining the mid-points of each
bar.
Cumulative frequency graph : is a smooth free hand curve of frequency
polygon.

Histogram and Frequency Polygon

Less than type and More than cumulative
frequency polygon

Diagrammatic presentation of data
Importance:
• They have greater attraction.
• They facilitate comparison.
• They are easily understandable.
Diagrams are appropriate for presenting discrete data.
The three most commonly used diagrammatic
presentation for discrete as well as qualitative data are:
 Pie charts
Pictogram
 Bar charts

Cont…..
Pie chart
• A pie chart is a circle that is divided in to sections
or wedges according to the percentage of
frequencies in each category of the distribution
Example: Draw a suitable diagram to represent the
following population in a town.
Men Women Girls Boys
2500 2000 4000 1500

Pie chart
Frequency
Men
Women
Girls
Boys

Cont..
Pictogram
• In these diagram, we represent data by means of some picture
symbols.
• We decide abut a suitable picture to represent a definite number of
units
Bar Charts
• A set of bars (thick lines or narrow rectangles) representing some
magnitude over time space.
• They are useful for comparing aggregate over time space.
• Bars can be drawn either vertically or horizontally.
• There are different types of bar charts. The most common being :
– Simple bar chart
– Component or sub divided bar chart.
– Multiple bar charts.

Cont….
• Example: The following data represent sale by product, 1957- 1959
of a given company for three products A, B, C.
Product Sales($) In 1957 Sales($) In 1958 Sales($) In1959
A 12 14 18
B 24 21 18
C 24 35 54

Simple bar chart
A B C
0
5
10
15
20
25
30
Sales($)

Multiple bar chart
A B C
0
10
20
30
40
50
60
Sales($)
Sales($)2
Sales($)3

Component bar chart
A B C
0
20
40
60
80
100
120
Sales($)3
Sales($)2
Sales($)

2.2 Measures of central tendency and dispersion
• Clinicians or physicians usually Perform assessment on
patient and provide information needed to diagnose or
monitor treatment.
• The nature of the data should be exploit in some concise
way that examine and evaluate safety and efficacy of drug
therapies in human subjects.
– Center or middle
– Dispersion
– Shape
• Measures of location
– It is often useful to summarize, in a single number or
statistic, the general location of the data or the point at
which the data tend to cluster.
– Such statistics are called measures of location or
measures of central tendency.
20

Measures of central tendency
• When we want to make comparison between groups of
numbers it is good to have a single value that is
considered to be a good representative of each group.
• This single value is called the average of the group.
• Averages are also called measures of central tendency.
• An average which is representative is called typical
average and
• An average which is not representative and has only a
theoretical value is called a descriptive average

Cont…
The Summation Notation:
• The symbol is a mathematical shorthand for X1+X2+X3+...+XN
Types of measures of central tendency
There are several different measures of central tendency; each has
its advantage and disadvantage.
• The Mean (Arithmetic, Geometric and Harmonic)
• The Mode
• The Median
• The choice of these averages depends up on which best fit the
property under discussion.

Cont…
• The Arithmetic Mean
• Is defined as the sum of the magnitude of the
items divided by the number of items.
• The mean of X1, X2 ,X3 …Xn is denoted by A.M or
and is given by:
• Where

Example
1. Obtain the mean of the following number
2, 7, 8, 2, 7, 3, 7
2. calculate the mean for the following age
distribution.
Class Frequency
6- 10 35
11- 15 23
16- 20 15
21- 25 12
26- 30 9
31- 35 6

Exercises:
Marks of 80 students are summarized in the
following frequency distribution:
Marks No. of students
40-44 7
45-49 10
50-54 22
55-59 f4
60-64 f5
65-69 6
70-74 3
If 25% of the students have marks between 55 and 59
1. Find the missing frequencies f4 and f5.
2. Find the mean.

Properties of the arithmetic mean
• The mean can be used as a summary measure for both
discrete and continuous data, in general however, it is not
appropriate for either nominal or ordinal data.
• For given set of data there is one and only one arithmetic
mean.
• The arithmetic mean is easily understood and easy to
compute.
• Algebraic sum of the deviations of the given values from
their arithmetic mean is always zero.
• The arithmetic mean is greatly affected by the extreme
values.
• In grouped data if any class interval is open, arithmetic
mean can not be calculated.
26

Cont…
• The Geometric Mean
•
– The geometric mean of a set of n observation is
the nth
root of their product.
– The geometric mean of X1, X2 ,X3 …Xn is denoted by
G.M and given by:
• The Harmonic Mean
• The harmonic mean of X1, X2 , X3 …Xn is
denoted by H.M and given by:

Example
1. Find the G.M and HM of the numbers 2, 4, 8.
2. A cyclist pedals from his house to his college
at speed of 10 km/hr and back from the
college to his house at 15 km/hr. Find the
average speed.

The Mode
• Mode is a value which occurs most frequently in a
set of values
• The mode may not exist and even if it does exist, it
may not be unique.
• In case of discrete distribution the value having the
maximum frequency is the modal value.
• Examples: for ungrouped data
– Find the mode of 5, 3, 5, 8, 9
– Find the mode of 8, 9, 9, 7, 8, 2, and 5.
– Find the mode of 4, 12, 3, 6, and 7.

Mode for Grouped data
• If data are given in the shape of continuous
frequency distribution, the mode is defined
as:
• Where:

Example
Example: Following is the distribution of the size of
certain farms selected at random from a district.
Calculate the mode of the distribution.
• Note: The modal class is a class with the highest frequency
Size of farms No. of farms
5-15 8
15-25 12
25-35 17
35-45 29
45-55 31
55-65 5
65-75 3

Properties of mode
• The mode can be used as a summary measure for
nominal, ordinal, discrete and continuous data, in general
however, it is more appropriate for nominal and ordinal
data.
• It is not affected by extreme values
• It can be calculated for distributions with open end classes
• Often its value is not unique
• The main drawback of mode is that often it does not exist
32

The Median
• In a distribution, median is the value of the
variable which divides it in to two equal
halves.
• It is the middle most value in the sense that
the number of values less than the median is
equal to the number of values greater than it.
Median for ungrouped data

Cont..
Example: Find the median of the following numbers.
a) 6, 5, 2, 8, 9, 4.
b) 2, 1, 8, 3, 5, 8.
• Median for grouped data
• If data are given in the shape of continuous
frequency distribution, the median is defined

Example
• Find the median of the following distribution.
Class Frequency
40-44 7
45-49 10
50-54 22
55-59 15
60-64 12
65-69 6
70-74 3

Cont
Class Frequency Cumu.Freq(less than type)
40-44 7 7
45-49 10 17
50-54 22 39
55-59 15 54
60-64 12 66
65-69 6 72
70-74 3 75

Properties of median
• The median can be used as a summary measure for ordinal, discrete
and continuous data, in general however, it is not appropriate for
nominal data.
• There is only one median for a given set of data
• The median is easy to calculate
• Median is a positional average and hence it is not drastically affected
by extreme values
• Median can be calculated even in the case of open end intervals
• It is not a good representative of data if the number of items is small
37

Quantiles
• When a distribution is arranged in order of
magnitude of items, the median is the value of
the middle term.
• Their measures that depend up on their
positions in distribution quartiles, deciles, and
percentiles are collectively called quantiles

Measures of Dispersion (Variation)
• The scatter or spread of items of a distribution is
known as dispersion or variation.
• In other words the degree to which numerical data tend
to spread about an average value is called dispersion or
variation of the data.
• Measures of dispersions are statistical measures which
provide ways of measuring the extent in which data are
dispersed or spread out.
• The measures of dispersion which are expressed in
terms of the original unit of a series are termed as
absolute measures

Cont…
• Relative measures of dispersions are a ratio or
percentage of a measure of dispersion to an appropriate
measure of central tendency and
• It is a pure number and used for making comparisons
between different distributions
Types of Measures of Dispersion
The most commonly used measures of dispersions are:
• Range and relative range
• Quartile deviation and coefficient of Quartile deviation
• Standard deviation and coefficient of variation.

Range(R) and relative range(RR)
Range
• The range is the largest score minus the smallest
score.
R= L-S
• It is a quick and dirty measure of variability,
because the range is greatly affected by extreme
scores.
Relative Range (RR)
• It is also some times called coefficient of range
and given by:

Example
1. If the range and relative range of a series are
4 and 0.25 respectively. Then what is the
value of:
a) Smallest observation
b) Largest observation

Quartile Deviation(QD) and Coefficient of Quartile Deviation
• The QD (inter quartile range) is the difference
between the third and the first quartiles of a set
of items and semi-inter quartile range is half
of the inter quartile range.

The Variance
• If we divide the variation by the number of
values in the population, we get something
called the population variance.
• This variance is the "average squared
deviation from the mean".

Sample Variance
• To counteract this, the sum of the squares of
the deviations is divided by one less than the
sample size

Standard Deviation
• There is a problem with variances. Recall that
the deviations were squared.
• To get the units back the same as the original
data values, the square root must be taken.

Examples
1. Find the variance and standard deviation of the following sample data
i. 5, 17, 12, 10.
ii. The data is given in the form of frequency distribution.
Class Frequency
40-44 7
45-49 10
50-54 22
55-59 15
60-64 12
65-69 6
70-74 3

Special properties of Standard deviations
For normal (symmetric) distribution the following holds.
• Approximately 68.27%, 95.45 % and 99.73% of the data
values fall within one two and three standard deviation of
the mean respectively . i.e. with in
Chebyshev's Theorem
• For any data set, the proportion of the values that fall with in
k standard deviations of the mean or will
be at least
• standard deviations of the mean is at most

Example:
1. Suppose a distribution has mean 50 and
standard deviation 6. What percent of the
numbers are:
a. Between 38 and 62
b. Between 32 and 68
c. Less than 38 or more than 62.
d. Less than 32 or more than 68.

Coefficient of Variation (C.V)
• Is defined as the ratio of standard deviation to
the mean usually expressed as percents.
• The distribution having less C.V is said to be
less variable or more consistent.
Standard Scores (Z-scores)
If X is a measurement from a distribution with mean
and standard deviation S, then its value in standard
units is

Shape of frequency distributions
• Frequency curves arising in practice, take on certain
characteristic shapes
51

Cont…
• For a given set of data often depends on the
way in which the values are distributed and
measured by
Skewness measures departure from symmetry and is
usually characterized as being left or right skewed as
seen previously.
Kurtosis measures “peakedness” of a distribution and
comes in two forms, platykurtosis and leptokurtosis.
52

 Based on the type of skewness, distributions can be:
 Symmetrical distribution: It is neither positively nor
negatively skewed. A curve is symmetrical if one half of the
curve is the mirror image of the other half.
 If the distribution is symmetric and has only one mode, all
three measures are the same, an example being the normal
distribution.
53

Cont..
 Positively skewed distribution: Occurs when the majority of
scores are at the left end of the curve and a few extreme large
scores are scattered at the right end.
 For positively skewed distributions (where the upper, or left, tail
of the distribution is longer (“fatter”) than the lower, or right, tail)
the measures are ordered as follows:
mode < median < mean.
54

 Negatively skewed distribution: occurs when majority of
scores are at the right end of the curve and a few small scores
are scattered at the left end.
 For negatively skewed distributions (where the right tail of the
distribution is longer than the left tail), the reverse ordering
occurs:
mean < median < mode.
55

Median Mode Mean
Fig. 2(a). Symmetric Distribution
Mean = Median = Mode
Mode Median Mean
Fig. 2(b). Distribution skewed to the right
Mean > Median > Mode
Mean Median Mode
Fig. 2(c). Distribution skewed to the left
Mean < Median < Mode 56

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