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Descriptive Statistics (Part 2)

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Presentation on theme: "Descriptive Statistics (Part 2)"— Presentation transcript:

1

2 Descriptive Statistics (Part 2)
4 Chapter Descriptive Statistics (Part 2) Standardized Data Percentiles and Quartiles Box Plots Grouped Data Skewness and Kurtosis (optional)

3 Standardized Data Chebyshev’s Theorem
Developed by mathematicians Jules Bienaymé ( ) and Pafnuty Chebyshev ( ). For any population with mean m and standard deviation s, the percentage of observations that lie within k standard deviations of the mean must be at least 100[1 – 1/k2].

4 Standardized Data Chebyshev’s Theorem
For k = 2 standard deviations, 100[1 – 1/22] = 75% So, at least 75.0% will lie within m + 2s For k = 3 standard deviations, 100[1 – 1/32] = 88.9% So, at least 88.9% will lie within m + 3s Although applicable to any data set, these limits tend to be too wide to be useful.

5 Standardized Data The Empirical Rule
The normal or Gaussian distribution was named for Karl Gauss ( ). The normal distribution is symmetric and is also known as the bell-shaped curve. The Empirical Rule states that for data from a normal distribution, we expect that for k = 1 about 68.26% will lie within m + 1s k = 2 about 95.44% will lie within m + 2s k = 3 about 99.73% will lie within m + 3s

6 Standardized Data The Empirical Rule
Distance from the mean is measured in terms of the number of standard deviations. Note: no upper bound is given. Data values outside m + 3s are rare.

7 Standardized Data Example: Exam Scores
If 80 students take an exam, how many will score within 2 standard deviations of the mean? Assuming exam scores follow a normal distribution, the empirical rule states about 95.44% will lie within m + 2s so 95.44% x 80  76 students will score + 2s from m. How many students will score more than 2 standard deviations from the mean?

8 Standardized Data Unusual Observations
Unusual observations are those that lie beyond m + 2s. Outliers are observations that lie beyond m + 3s.

9 Standardized Data Unusual Observations
For example, the P/E ratio data contains several large data values. Are they unusual or outliers? 7 8 10 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 29 30 31 34 36 37 40 41 45 48 55 68 91

10 Standardized Data The Empirical Rule
If the sample came from a normal distribution, then the Empirical rule states = ± 1(14.08) = (8.9, 38.8) = ± 2(14.08) = (-5.4, 50.9) = ± 3(14.08) = (-19.5, 65.0)

11 Standardized Data The Empirical Rule
Are there any unusual values or outliers? Unusual Outliers 65.0 -19.5 50.9 -5.4 38.8 8.9 22.72

12 Standardized Data Defining a Standardized Variable
A standardized variable (Z) redefines each observation in terms the number of standard deviations from the mean. Standardization formula for a population: Standardization formula for a sample:

13 Standardized Data Defining a Standardized Variable
zi tells how far away the observation is from the mean. For example, for the P/E data, the first value x1 = 7. The associated z value is = 7 – 22.72 14.08 -1.12

14 Standardized Data Defining a Standardized Variable
A negative z value means the observation is below the mean. Positive z means the observation is above the mean. For x68 = 91, = 91 – 22.72 14.08 4.85

15 Standardized Data Defining a Standardized Variable
Here are the standardized z values for the P/E data: What do you conclude for these four values?

16 Standardized Data Defining a Standardized Variable
MegaStat calculates standardized values as well as checks for outliers. In Excel, use =STANDARDIZE(Array, Mean, STDev) to calculate a standardized z value.

17 Standardized Data Outliers What do we do with outliers in a data set?
If due to erroneous data, then discard. An outrageous observation (one completely outside of an expected range) is certainly invalid. Recognize unusual data points and outliers and their potential impact on your study. Research books and articles on how to handle outliers.

18 Standardized Data Estimating Sigma
For a normal distribution, the range of values is 6s (from m – 3s to m + 3s). If you know the range R (high – low), you can estimate the standard deviation as s = R/6. Useful for approximating the standard deviation when only R is known. This estimate depends on the assumption of normality.

19 Percentiles and Quartiles
Percentiles are data that have been divided into 100 groups. For example, you score in the 83rd percentile on a standardized test. That means that 83% of the test-takers scored below you. Deciles are data that have been divided into 10 groups. Quintiles are data that have been divided into 5 groups. Quartiles are data that have been divided into 4 groups.

20 Percentiles and Quartiles
Percentiles are used to establish benchmarks for comparison purposes (e.g., health care, manufacturing and banking industries use 5, 25, 50, 75 and 90 percentiles). Quartiles (25, 50, and 75 percent) are commonly used to assess financial performance and stock portfolios. Percentiles are used in employee merit evaluation and salary benchmarking.

21 Percentiles and Quartiles
Quartiles are scale points that divide the sorted data into four groups of approximately equal size. Q1 Q2 Q3 Lower 25% | Second 25% Third 25% Upper 25% The three values that separate the four groups are called Q1, Q2, and Q3, respectively.

22 Percentiles and Quartiles
The second quartile Q2 is the median, an important indicator of central tendency. Q2  Lower 50%  |  Upper 50%  Q1 and Q3 measure dispersion since the interquartile range Q3 – Q1 measures the degree of spread in the middle 50 percent of data values. Q1 Q3 Lower 25% |  Middle 50%  Upper 25%

23 Percentiles and Quartiles
The first quartile Q1 is the median of the data values below Q2, and the third quartile Q3 is the median of the data values above Q2. Q1 Q2 Q3 Lower 25% | Second 25% Third 25% Upper 25% For first half of data, 50% above, 50% below Q1. For second half of data, 50% above, 50% below Q3.

24 Percentiles and Quartiles
Depending on n, the quartiles Q1,Q2, and Q3 may be members of the data set or may lie between two of the sorted data values.

25 Percentiles and Quartiles
Method of Medians For small data sets, find quartiles using method of medians: Step 1. Sort the observations. Step 2. Find the median Q2. Step 3. Find the median of the data values that lie below Q2. Step 4. Find the median of the data values that lie above Q2.

26 Percentiles and Quartiles
Excel Quartiles Use Excel function =QUARTILE(Array, k) to return the kth quartile. Excel treats quartiles as a special case of percentiles. For example, to calculate Q3 =QUARTILE(Array, 3) =PERCENTILE(Array, 75) Excel calculates the quartile positions as: Position of Q1 0.25n Position of Q2 0.50n Position of Q3 0.75n

27 Percentiles and Quartiles
Example: P/E Ratios and Quartiles Consider the following P/E ratios for 68 stocks in a portfolio. 7 8 10 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 29 30 31 34 36 37 40 41 45 48 55 68 91 Use quartiles to define benchmarks for stocks that are low-priced (bottom quartile) or high-priced (top quartile).

28 Percentiles and Quartiles
Example: P/E Ratios and Quartiles Using Excel’s method of interpolation, the quartile positions are: Quartile Position Formula Interpolate Between Q1 = 0.25(68) = 17.75 X17 + X18 Q2 = 0.50(68) = 34.50 X34 + X35 Q3 = 0.75(68) = 51.25 X51 + X52

29 Percentiles and Quartiles
Example: P/E Ratios and Quartiles The quartiles are: Quartile Formula First (Q1) Q1 = X (X18-X17) = (14-14) = 14 Second (Q2) Q2 = X (X35-X34) = (19-19) = 19 Third (Q3) Q3 = X (X52-X51) = (26-26) = 26

30 Percentiles and Quartiles
Example: P/E Ratios and Quartiles So, to summarize: Q1 Q2 Q3 Lower 25% of P/E Ratios 14 Second 25% of P/E Ratios 19 Third 25% of P/E Ratios 26 Upper 25% of P/E Ratios These quartiles express central tendency and dispersion. What is the interquartile range? Because of clustering of identical data values, these quartiles do not provide clean cut points between groups of observations.

31 Percentiles and Quartiles
Tip Whether you use the method of medians or Excel, your quartiles will be about the same. Small differences in calculation techniques typically do not lead to different conclusions in business applications.

32 Percentiles and Quartiles
Caution Quartiles generally resist outliers. However, quartiles do not provide clean cut points in the sorted data, especially in small samples with repeating data values. Data set A: 1, 2, 4, 4, 8, 8, 8, 8 Q1 = 3, Q2 = 6, Q3 = 8 Data set B: 0, 3, 3, 6, 6, 6, 10, 15 Although they have identical quartiles, these two data sets are not similar. The quartiles do not represent either data set well.

33 Percentiles and Quartiles
Dispersion Using Quartiles Some robust measures of central tendency and dispersion using quartiles are: Statistic Formula Excel Pro Con Midhinge =0.5*(QUARTILE (Data,1)+QUARTILE (Data,3)) Robust to presence of extreme data values. Less familiar to most people.

34 Percentiles and Quartiles
Dispersion Using Quartiles Statistic Formula Excel Pro Con Midspread Q3 – Q1 =QUARTILE(Data,3)-QUARTILE(Data,1) Stable when extreme data values exist. Ignores magnitude of extreme data values. Coefficient of quartile variation (CQV) None Relative variation in percent so we can compare data sets. Less familiar to non-statisticians

35 Percentiles and Quartiles
Midhinge The mean of the first and third quartiles. Midhinge = For the 68 P/E ratios, Midhinge = A robust measure of central tendency since quartiles ignore extreme values.

36 Percentiles and Quartiles
Midspread (Interquartile Range) A robust measure of dispersion Midspread = Q3 – Q1 For the 68 P/E ratios, Midspread = Q3 – Q1 = 26 – 14 = 12

37 Percentiles and Quartiles
Coefficient of Quartile Variation (CQV) Measures relative dispersion, expresses the midspread as a percent of the midhinge. For the 68 P/E ratios, Similar to the CV, CQV can be used to compare data sets measured in different units or with different means.

38 Box Plots A useful tool of exploratory data analysis (EDA).
Also called a box-and-whisker plot. Based on a five-number summary: Xmin, Q1, Q2, Q3, Xmax Consider the five-number summary for the 68 P/E ratios: Xmin, Q1, Q2, Q3, Xmax

39 Center of Box is Midhinge
Box Plots Whiskers Center of Box is Midhinge Box Minimum Right-skewed Maximum Median (Q2) Q1 Q3

40 Box Plots Fences and Unusual Data Values
Use quartiles to detect unusual data points. These points are called fences and can be found using the following formulas: Inner fences Outer fences: Lower fence Q1 – 1.5 (Q3–Q1) Q1 – 3.0 (Q3–Q1) Upper fence Q (Q3–Q1) Q (Q3–Q1) Values outside the inner fences are unusual while those outside the outer fences are outliers.

41 Box Plots Fences and Unusual Data Values
For example, consider the P/E ratio data: Inner fences Outer fences: Lower fence: 14 – 1.5 (26–14) = 4 14 – 3.0 (26–14) = 22 Upper fence: (26–14) = +44 (26–14) = +62 Ignore the lower fence since it is negative and P/E ratios are only positive.

42 Box Plots Fences and Unusual Data Values
Truncate the whisker at the fences and display unusual values and outliers as dots. Inner Fence Outer Fence Unusual Outliers Based on these fences, there are three unusual P/E values and two outliers.

43 Grouped Data Nature of Grouped Data
Although some information is lost, grouped data are easier to display than raw data. When bin limits are given, the mean and standard deviation can be estimated. Accuracy of grouped estimates depend on - the number of bins - distribution of data within bins - bin frequencies

44 Grouped Data Mean and Standard Deviation
Consider the frequency distribution for prices of Lipitor® for three cities: Where mj = class midpoint fj = class frequency k = number of classes n = sample size

45 Grouped Data Nature of Grouped Data
Estimate the mean and standard deviation by Note: don’t round off too soon.

46 Grouped Data Nature of Grouped Data Accuracy Issues
Now estimate the coefficient of variation CV = 100 (s / ) = 100 ( / ) = 9.2% Accuracy Issues How accurate are grouped estimates compared to ungrouped estimates? For the previous example, we can compare the grouped data statistics to the ungrouped data statistics.

47 Grouped Data Accuracy Issues
For this example, very little information was lost due to grouping. However, accuracy could be lost due to the nature of the grouping (i.e., if the groups were not evenly spaced within bins).

48 Grouped Data Accuracy Issues
The dot plot shows a relatively even distribution within the bins. Effects of uneven distributions within bins tend to average out unless there is systematic skewness.

49 Grouped Data Accuracy Issues
Accuracy tends to improve as the number of bins increases. If the first or last class is open-ended, there will be no class midpoint (no mean can be estimated). Assume a lower limit of zero for the first class when the data are nonnegative. You may be able to assume an upper limit for some variables (e.g., age). Median and quartiles may be estimated even with open-ended classes.

50 Skewness and Kurtosis Skewness
Generally, skewness may be indicated by looking at the sample histogram or by comparing the mean and median. This visual indicator is imprecise and does not take into consideration sample size n.

51 Skewness and Kurtosis Skewness Skewness is a unit-free statistic.
The coefficient compares two samples measured in different units or one sample with a known reference distribution (e.g., symmetric normal distribution). Calculate the sample’s skewness coefficient as: Skewness =

52 Skewness and Kurtosis Skewness
In Excel, go to Tools | Data Analysis | Descriptive Statistics or use the function =SKEW(array)

53 Skewness and Kurtosis Skewness
Consider the following table showing the 90% range for the sample skewness coefficient.

54 Skewness and Kurtosis Skewness
Coefficients within the 90% range may be attributed to random variation.

55 Skewness and Kurtosis Skewness
Coefficients outside the range suggest the sample came from a nonnormal population.

56 Skewness and Kurtosis Skewness
As n increases, the range of chance variation narrows.

57 Skewness and Kurtosis Kurtosis
Kurtosis is the relative length of the tails and the degree of concentration in the center. Consider three kurtosis prototype shapes.

58 Skewness and Kurtosis Kurtosis
A histogram is an unreliable guide to kurtosis since scale and axis proportions may differ. Excel and MINITAB calculate kurtosis as: Kurtosis =

59 Skewness and Kurtosis Kurtosis
Consider the following table of expected 90% range for sample kurtosis coefficient.

60 Skewness and Kurtosis Kurtosis
A sample coefficient within the ranges may be attributed to chance variation.

61 Skewness and Kurtosis Kurtosis
Coefficients outside the range would suggest the sample differs from a normal population.

62 Skewness and Kurtosis Kurtosis
As sample size increases, the chance range narrows. Inferences about kurtosis are risky for n < 50.

63 Applied Statistics in Business and Economics
End of Chapter 4


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