MATH& 146 Lesson 8. Section 1.6 Averages and Variation

Transcription

1 MATH& 146 Lesson 8 Section 1.6 Averages and Variation 1

2 Summarizing Data The distribution of a variable is the overall pattern of how often the possible values occur. For numerical variables, three summary characteristics of the overall distribution of the data tend to be of the most interest: The Average (Typical Value) The Variability (Spread) The Shape 2

3 Averages Averages provide information about what is considered the "typical" value. If you were to take all of your data and reduce it to a single number, that number would be considered the average. There are many ways to describe the average, but we will focus on just three: the mean, the median, and the mode. 3

4 Averages MEAN or arithmetic mean. It is the sum of all values divided by the count of values. It is the most important of the averages. MEDIAN the middle value in a collection when the values are arranged in order of increasing size. It is the average of choice when outliers are present. MODE the most common value(s) in a dataset. It can be used for any type of data, and it is the only average for regular categorical data. 4

5 Notation for the Sample Mean There are two ways to symbolize the sample mean: 1) with the symbol x-bar, x, or 2) with a capital letter, M. (APA notation) 5

6 The Mean The mean of a collection of values is the sum of all values divided by the count of values. The formula for a mean is x n x where Σ x is the sum of the values and n is the count, or sample size. 6

7 The Mean The mean feels like a typical value because it is the point where the data "balances". 7

8 Example 1 Compute the mean of the following two lists of numbers: a) 13, 24, 25, 34, 37 b) 13, 24, 25, 34, 370 How did changing the last number from 37 to 370 affect the mean? 8

9 Notation for the Median Unlike the mean, there is no standard way to symbolize the median. Some common abbreviations include: 1) with the abbreviation, Med (calculator notation), or 1) with the abbreviation, Mdn (APA notation). 9

10 The Median The median is the middle of your data, with at most half the data values less than it and at most half the data more than it. You can think of it as the value that divides the sorted data into two equal sets of numbers. 10

11 The Median Let a collection of n values be written in order of increasing size. If n is odd, the median is the middle value in the list. Data set 1: 24, 25, 25, 27, 29, 31, 32, 34, 37 (n = 9, odd) Med 29 11

12 The Median If n is even, the median is the average of the two middle values. Data set 2: 42, 42, 43, 44, 44, 46, 47, 47, 47, 49 (n = 10, even) average Med 45 12

13 Example 2 Compute the median of the following list of numbers: 34, 13, 37, 24, 25, 13, 41, 23, 28, 31 13

14 Example 3 Compute the median of the following two lists of numbers: a) 13, 24, 25, 34, 37 b) 13, 24, 25, 34, 370 How did changing the last number from 37 to 370 affect the median? 14

15 The Mode A mode of a collection of values is the value (or values) that occurs the most frequently. For example, the set 1, 2, 2, 3, 6, 6, 6, 6, 7, 8, 10 has a mode at 6. 15

16 The Mode If two or more values occur equally often and more frequently than all other values, then they each would be considered modes. For example, the set 2, 2, 2, 3, 4, 6, 6, 6, 7, 8 has modes at 2 and 6 16

17 The Mode If no number occurs more than once, then no mode exists. For example, 1, 3, 5, 6, 8, 11, 12 has no mode. 17

18 Why Modes? The mean can only be used for numerical data, while the median can be used for numerical and ordinal data. The mode can be found for all data. For instance, for the following sample of colors: red, green, orange, orange, blue, orange the mean and median would be impossible to find. We can still describe the mode as the color orange, however. 18

19 Example 4 Find the mode of the following collection of fruit: Fruit Frequency Apples 11 Oranges 12 Pears 16 Kiwis 10 Bananas 12 19

20 Example 5 Statistics exam scores for 20 students are as follows: 50; 53; 59; 59; 63; 63; 72; 72; 72; 72; 72; 76; 78; 81; 83; 84; 84; 84; 90; 93 Find the mode. 20

21 Variation In addition to describing the average, we should also describe the variation, or spread. Measures of variation tell us how far the numbers are scattered about the center value of the set. The most common ways to measure variation are the range, interquartile range, and standard deviation. 21

22 Measures of Variation RANGE the difference between the maximum and minimum data values. INTERQUARTILE RANGE the difference between the upper and lower quartiles. STANDARD DEVIATION the typical distance the data values are from the mean. 22

23 The Range The simplest way to describe the variation of a data set is to compute the range, defined as the difference between the maximum and minimum values range max min Although the range is easy to compute and can be useful, it occasionally can be misleading. 23

24 Example 6 Consider the following two sets of quiz scores for nine students. Which set has the greater range? Would you also say that the scores in this set are more varied? Quiz 1 Scores: Quiz 2 Scores:

25 Quartiles Quartiles are numbers that separate the data into quarters. To find the quartiles, first find the median and divide the data into two halves: the lower half are the numbers to the left of the median and the upper half are the numbers to the right. The quartiles will then be the medians of each of the halves. 25

26 Quartiles The lower (or first) quartile (denoted Q 1 ) is the median of the lower half of the data. This is the point in which at most 1/4 of the values are smaller than it and at most 3/4 of the values are larger than it. The upper (or third) quartile (denoted Q 3 ) is the median of the upper half of the data. This is the point in which at most 3/4 of the values are smaller than it and at most 1/4 of the values are larger than it. 26

27 Example 7 A group of eight children have the following heights (in inches): 48, 48, 53, 53.5, 54, 60, 62, 71 Find the quartiles for the distribution of the children's heights. 27

28 Interquartile Range When you are using the median to describe the average, an appropriate measure of variation is called the interquartile range. The interquartile range, IQR, tells us how much space the middle 50% of the data roughly occupy. It is given by the formula IQR Q Q

29 Example 8 A group of eight children have the following heights (in inches): 48, 48, 53, 53.5, 54, 60, 62, 71 Q 1 = 50.5 Q 3 = 61 Find the range and the interquartile range for the distribution of the children's heights. 29

30 Example 9 Returning to an Example 6, compute the IQR for each set of quiz scores. Does the IQR appear to be more reliable than the range? Why or why not? Quiz 1 Scores: Quiz 2 Scores: