# 89 - Calculating Mean, Mode and Median from Grouped Data

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This pdf includes the following topics:-
Mean – Grouped Data
Median and Interquartile Range – Grouped Data
Quartiles
Mode – Grouped Data
Variance and Standard Deviation-Grouped Data
1. Lecture 2 – Grouped Data
Calculation
1. Mean, Median and Mode
2. First Quantile, third Quantile and
Interquantile Range.
2. Mean – Grouped Data
Example: The following table gives the frequency distribution of the number
of orders received each day during the past 50 days at the office of a mail-order
company. Calculate the mean. Number f
of order
10 – 12 4
13 – 15 12
16 – 18 20
19 – 21 14
n = 50
X is the midpoint of the
Number f x fx class. It is adding the class
of order limits and divide by 2.
10 – 12 4 11 44
13 – 15 12 14 168
x=
∑ fx = 8 3 2 = 1 6 .6 4
16 – 18 20 17 340 n 50
19 – 21 14 20 280
n = 50 = 832
3. Median and Interquartile Range
– Grouped Data
Step 1: Construct the cumulative frequency distribution.
Step 2: Decide the class that contain the median.
Class Median is the first class with the value of cumulative
frequency equal at least n/2.
Step 3: Find the median by using the following formula:
⎛ n ⎞
⎜ 2 - F ⎟
M e d ia n = L m + ⎜ ⎟ i
⎜ f m ⎟
⎝ ⎠
Where:
n = the total frequency
F = the cumulative frequency before class median
f = the frequency of the class median
m
i = the class width
Lm = the lower boundary of the class median
4. Example: Based on the grouped data below, find the median:
Time to travel to work Frequency
1 – 10 8
11 – 20 14
21 – 30 12
31 – 40 9
41 – 50 7
1st Step: Construct the cumulative frequency distribution
Time to travel Frequency Cumulative
to work Frequency
1 – 10 8 8
11 – 20 14 22
21 – 30 12 34
31 – 40 9 43
41 – 50 7 50
n 50
= = 25 class median is the 3rd class
2 2
So, F = 22, fm = 12, L = 20.5 and i = 10
m
5. ⎛n ⎞
⎜ - F ⎟
Median = Lm + ⎜ 2 ⎟i
f
⎜ m ⎟
⎝ ⎠
⎛ 25 - 22 ⎞
= 21.5 + ⎜ ⎟ 10
⎝ 12 ⎠
= 24
Thus, 25 persons take less than 24 minutes to travel to work and another 25 persons
take more than 24 minutes to travel to work.
6. Using the same method of calculation as in the Median,
we can get Q1 and Q3 equation as follows:
⎛n ⎞ ⎛ 3n ⎞
⎜4-F ⎟ ⎜ 4 -F ⎟
Q1 = LQ1 + ⎜ ⎟i Q3 = LQ3 + ⎜ ⎟i
⎜ f Q1 ⎟ ⎜ f Q3 ⎟
⎝ ⎠ ⎝ ⎠
Example: Based on the grouped data below, find the Interquartile Range
Time to travel to work Frequency
1 – 10 8
11 – 20 14
21 – 30 12
31 – 40 9
41 – 50 7
7. 1st Step: Construct the cumulative frequency distribution
Time to travel Frequency Cumulative
to work Frequency
1 – 10 8 8
11 – 20 14 22
21 – 30 12 34
31 – 40 9 43
41 – 50 7 50
2nd Step: Determine the Q1 and Q3
n 50 ⎛n ⎞
Class Q 1 = = = 12 . 5
4 4 ⎜ 4 -F ⎟
Q1 = LQ1 + ⎜ ⎟i
⎜ fQ1
Class Q1 is the 2nd class

Therefore, ⎝ ⎠
⎛ 12.5 - 8 ⎞
= 10.5 + ⎜ ⎟ 10
⎝ 14 ⎠
= 13.7143
8. ⎛n ⎞
3n 3 ( 50 ) ⎜ 4 -F ⎟
Class Q 3 = = = 37 .5 Q3 = LQ3 + ⎜ ⎟i
⎜ fQ3
4 4 ⎟
⎝ ⎠
⎛ 37.5 - 34 ⎞
Class Q3 is the 4th class = 30.5 + ⎜ ⎟ 10
Therefore, ⎝ 9 ⎠
= 34.3889
Interquartile Range
IQR = Q3 – Q1
IQR = Q3 – Q1
calculate the IQ
IQR = Q3 – Q1 = 34.3889 – 13.7143 = 20.6746
9. Mode – Grouped Data
•Mode is the value that has the highest frequency in a data set.
•For grouped data, class mode (or, modal class) is the class with the highest frequency.
•To find mode for grouped data, use the following formula:
⎛ Δ1 ⎞
M o d e = Lmo + ⎜ ⎟i
Δ
⎝ 1 + Δ 2 ⎠
i is the class width
Δ1 is the difference between the frequency of class mode and the frequency
of the class after the class mode
Δ 2 is the difference between the frequency of class mode
and the frequency of the class before the class mode
Lmo is the lower boundary of class mode
10. Calculation of Grouped Data - Mode
Example: Based on the grouped data below, find the mode
Time to travel to work Frequency
1 – 10 8
11 – 20 14
21 – 30 12
31 – 40 9
41 – 50 7
Based on the table,
Lmo = 10.5, Δ1 = (14 – 8) = 6, Δ 2 = (14 – 12) = 2 and
i = 10
⎛ 6 ⎞
M o d e = 1 0 .5 + ⎜ ⎟ 1 0 = 1 7 .5
⎝ 6 + 2 ⎠
11. Mode can also be obtained from a histogram.
Step 1: Identify the modal class and the bar representing it
Step 2: Draw two cross lines as shown in the diagram.
Step 3: Drop a perpendicular from the intersection of the two lines
until it touch the horizontal axis.
Step 4: Read the mode from the horizontal axis
12. Variance and Standard Deviation
-Grouped Data
( ∑ fx )
2
Population Variance: ∑ fx 2

N
σ2 =
N
( ∑ fx )
2
∑ fx 2

n
Variance for sample data: s =
2
n −1
Standard Deviation:
Population: σ2 = σ2
Sample: s2 = s2
13. Example: Find the variance and standard deviation for the following data:
No. of order f
10 – 12 4
13 – 15 12
16 – 18 20
19 – 21 14
Total n = 50
Solution:
No. of order f x fx fx2
10 – 12 4 11 44 484
13 – 15 12 14 168 2352
16 – 18 20 17 340 5780
19 – 21 14 20 280 5600
Total n = 50 832 14216
14. ( ∑ fx )
2
∑ fx 2

n
Variance, s2 =
n −1
(832 )
2
14216 −
= 50
50 − 1
= 7.5820
Standard Deviation, s = s = 7.5820 = 2.75
2
Thus, the standard deviation of the number of orders received at
the office of this mail-order company during the past 50 days is 2.75.