Measures of Center (College Board AP® Statistics)
Study Guide
Written by: Naomi C
Reviewed by: Dan Finlay
Mode
What is the mode?
The mode is the value that occurs most often in a data set
It is possible for there to be more than one mode
It is possible for there to be no mode
The mode can appear at any point in the distribution of the data set
It is not necessarily in the center of the distribution
The mode is the only measure of center that can be used for categorical data (e.g. favourite colour)
Examiner Tips and Tricks
If there is no mode in a particular data set, state that there is no mode, do not say that the mode is zero!
Median
What is the median?
The median is the middle value when the data is in order of size
If there are an odd number of values in the data set
then the median is the data value in the center of the set
If there are an even number of values in the data set
then there are two values in the middle and the median is the midpoint of these two values
How do I find the median for ungrouped data?
For a data set that contains values
the middle value will be located at the position
The median is a resistant statistic
It is not affected by extreme values
It is a suitable measure of center for distributions that are
either approximately symmetrical
or strongly skewed with outliers
Worked Example
Find the median of the data set given below.
43 29 70 51 64 43
Answer:
First put the values in ascending size order
29 43 43 51 64 70
Find the position of the middle value, using
The middle value lies halfway between the 3rd and the 4th values
Calculate the median
The median value is 47
Mean
What is the mean?
The mean is the sum of all the values divided by the number of values in the data set
It uses all values in a data set
If the distribution of the data is approximately symmetrical
the median and the mean will be close in value
and both values will be representative of the population
If the distribution of the data is strongly skewed or has outliers
The mean will be affected by the extreme values
It is therefore not a resistant statistic
How do I find the mean for ungrouped data?
The formula for calculating the mean is given to you in the exam
Where is the sum of the pieces of data
Worked Example
Find the mean of the data set given below.
27 31 9 65 52 43
Answer:
Use the formula to calculate the mean
The mean is 37.8
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