Measures of Position (College Board AP® Statistics)
Study Guide
Written by: Naomi C
Reviewed by: Dan Finlay
Quartiles
What are quartiles?
Recall that the median splits the data set into two parts
At least 50% of the data is less than or equal to the median
and 50% of the data is greater than or equal to it
Quartiles split the data set into four parts
The first quartile (Q1) lies a quarter of the way along the data (when in order)
One quarter (25%) of the data is less than or equal to Q1 (and three quarters is greater than or equal to it)
The third quartile (Q3) lies three quarters of the way along the data (when in order)
Three quarters (75%) of the data is less than or equal to Q3 (and one quarter is greater than or equal to it)
You may come across the median being referred to as the second quartile (Q2)
How do I find the first and third quartiles?
Make sure the data is written in numerical order
Use the median to divide the data set into lower and upper halves
If there are an even number of data values, then
the first half of those values are the lower half,
and the second half are the upper half
All of the data values are included in one or the other of the two halves
If there are an odd number of data values, then
all the values below the median are the lower half
and all the values above the median are the upper half
The median itself is not included as a part of either half
The first quartile is the median of the lower half of the data set
and the third quartile is the median of the upper half of the data set
Find the quartiles in the same way you would find the median for any other data set
just restrict your attention to the lower or upper half of the data accordingly
Worked Example
A naturalist studying crocodiles has recorded the numbers of eggs found in a random selection of 20 crocodile nests
31 32 35 35 36 37 39 40 42 45
46 48 49 50 51 51 53 54 57 60
Find the first and third quartiles for this data set.
Answer:
There are 20 data values (an even number)
So the lower half will be the first 10 values
The first quartile is the median of that lower half of the data
31 32 35 35 36 37 39 40 42 45
So the first quartile is midway between 36 and 37 (i.e. 36.5)
Do the same thing with the upper half of the data to find the third quartile
The third quartile is the median of the upper half of the data
46 48 49 50 51 51 53 54 57 60
So the third quartile is midway between 51 and 51 (i.e. 51)
The first quartile is 36.5
The third quartile is 51
Percentiles
What are percentiles?
A percentile indicates the position of a data value within its distribution
Percentiles divide a data set into 100 equal parts
% of the data values will be less than or equal to the th percentile
e.g. 10% of data values will be less than or equal to the 10th percentile (and 90% will be greater than or equal to it)
99% of data values will be less than or equal to the 99th percentile (and 1% will be greater than or equal to it)
Note that
the 25th percentile is the same as the first quartile
the 50th percentile is the same as the median
the 75th percentile is the same as the third quartile
Also note that percentiles don't need to be whole numbers
e.g. we can talk about the 2.5th percentile
2.5% of the data will be less than or equal to that (and 97.5% will be greater than or equal to it)
Percentiles can be useful for discussing the distribution of data in a data set
e.g. in comparing incomes in the US
you could compare the highest 1% of earners (the ones above the 99th percentile)
with the median income (the 50th percentile)
or the lowest 10% of earners (those below the 10th percentile)
How do I find a percentile?
To find a percentile
Write the percentile as a fraction or a decimal
e.g. 45th percentile is or 0.45
Determine the total number of items in the data set
Multiply the total number of items by the percentile
This will give you the position in the data set (when ordered) of the percentile required
You may be required to find a single value or a class interval in which the percentile is situated
this could apply to data in a table
or in a diagram, e.g. a stem-and-leaf plot or a dotplot
Worked Example
The table shows information about the times, in minutes, taken by 48 students to complete a math pop quiz.
Time (t minutes) | Frequency |
---|---|
8 < t ≤ 10 | 6 |
10 < t ≤ 12 | 24 |
12 < t ≤ 14 | 11 |
14 < t ≤ 16 | 6 |
16 < t ≤ 18 | 1 |
Find the time interval that contains the 65th percentile.
Answer:
65% of data values will be below the 65th percentile
First we need to find what 65% of 48 (the total number of data values) is
So the 65th percentile is the 31.2th data value
That means it lies between the 31st and 32nd data values
The first two time intervals together contain 6+24=30 data values
That means that the 31st and 32nd data values are both in the third time interval
The 65th percentile is in the 12 < t ≤ 14 time interval
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