Cumulative Frequency and Box Plots. Learning Objectives To be able to draw a cumulative frequency...
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Transcript of Cumulative Frequency and Box Plots. Learning Objectives To be able to draw a cumulative frequency...
Learning Objectives
To be able to draw a cumulative frequency curve and use it to estimate the median and interquartile range and draw a box plot
Cumulative Frequency Curves
• Cumulative Frequency = a running total of the data
• Median = Half way up the Cumulative Frequency
• Lower Quartile (LQ) = ¼ way up Cum Freq
• Upper Quartile (UQ) = ¾ way up Cum Freq
• Inter Quartile Range = UQ - LQ
Big Worked ExampleThe table below shows the number of minutes students were late for their fun algebra lesson.
(a) Draw a Cumulative Frequency Diagram of the data
(b) Use it to find the Median, Lower Quartile, Upper Quartile, and Inter Quartile Range
(c) Draw a Box-Plot assuming a minimum time of o minutes and a maximum of 25 minutes
Time t (mins)Number of Students
Cumulative Frequency
0 < t ≤ 5 10
5 < t ≤ 10 16
10 < t ≤ 15 30
15 < t ≤ 20 22
20 < t ≤ 25 2
5 10 15 20 25 t mins
Cum freq
20
40
60
80
x
x
x
x x
Median = Middle Value
QUARTILES•Lower Quartile = ¼ way•Upper Quartile = ¾ way
Interquartile Range
8½ 15½12½
= 15½ - 8½ = 7 mins
Box Plots
• Box Plots (Box and Whisker Diagram) are another way of displaying data for comparison
• To draw a Box Plot we need FIVE pieces of information:– 1. The Minimum Value– 2. Lower Quartile (Q1)– 3. The Median (Q2)– 4. Upper Quartile (Q3)– 5. Maximum Value
• They are drawn in the following way:
• BOX = The location of the middle 50% of the data
• WHISKERS = How the data is spread overall
3 4
12 5
10
20
30
40
50
60
70
0
Cu
mu
lati
ve F
req
uen
cy
10 20 30 40 50 60 70
Minutes Late
Med
ian =
27
LQ =
21 UQ
= 3
8
IQR = 38 – 21 = 17
mins
½
¼
¾
0 10 20 30 40 50 60
Box Plot from Cumulative Frequency Curve