##
Summary Statistics
Section* *

The vast amount of numbers on a large number of variables need to be properly organized to extract information from them. Broadly speaking there are two methods to summarize data: visual summarization and numerical summarization. Both have their advantages and disadvantages and applied jointly they will get the maximum information from raw data.

Summary statistics are numbers computed from the sample that present a summary of the attributes.

### Measures of Location

They are single numbers representing a set of observations. Measures of location also include measures of central tendency. Measures of central tendency can also be taken as the most representative values of the set of observations. The most common measures of location are the Mean, the Median, the Mode, and the Quartiles.

- Mean
- the
*arithmetic average*of all the observations. The mean equals the sum of all observations divided by the sample size

- Median
- the
*middle-most value*of the ranked set of observations so that half the observations are greater than the median and the other half is less. Median is a robust measure of central tendency

- Mode
- the
*most frequently*occurring value in the data set. This makes more sense when attributes are not continuous

- Quartiles
- division points which split data into four equal parts after rank-ordering them.
- Division points are called
*Q1*(the first quartile),*Q2*(the second quartile or median), and*Q3*(the third quartile)

Similarly, Deciles and Percentiles are defined as division points that divide the rank-ordered data into 10 and 100 equal segments.

### Measures of Spread

Measures of location are not enough to capture all aspects of the attributes. Measures of dispersion are necessary to understand the variability of the data. The most common measure of dispersion is the Variance, the Standard Deviation, the Interquartile Range and Range.

- Variance
- measures
*how far data values lie from the mean*. It is defined as the average of the squared differences between the mean and the individual data values

- Standard Deviation
- is the square root of the variance. It is defined as the average distance between the mean and the individual data values

- Interquartile range (IQR)
- is the difference between Q3 and Q1. IQR contains the middle 50% of data

- Range
- is the difference between the maximum and minimum values in the sample

### Measures of Skewness

In addition to the measures of location and dispersion, the arrangement of data or the *shape* of the data distribution is also of considerable interest. The most 'well-behaved' distribution is a symmetric distribution where the mean and the median are coincident. The symmetry is lost if there exists a tail in either direction. Skewness measures whether or not a distribution has a single long tail.

Skewness is measured as:

\( \dfrac{\sqrt{n} \left( \Sigma \left(x_{i} - \bar{x} \right)^{3} \right)}{\left(\Sigma \left(x_{i} - \bar{x} \right)^{2}\right)^{\frac{3}{2}}} \)

The figure below gives examples of symmetric and skewed distributions. Note that these diagrams are generated from theoretical distributions and in practice one is likely to see only approximations.

##
Try it!
Section* *

Calculate the answers to these questions then click the icon on the left to reveal the answer.

- Mean
- Median
- Mode
- Q1 and Q3
- Variance and Standard Deviation
- IQR
- Range
- Skewness

- Mean: (3+5+6+9+0+10+1+3+7+4+8)/11= 5.091.
- Median: The ordered data is 0, 1, 3, 3, 4, 5, 6, 7, 8, 9, 10. Thus, 5 is the median.
- Mode: 3.
- Q1 and Q3: Q1 is 3 and Q3 is 8.
- Variance and Standard Deviation: Variance is 10.491 (=((3-5.091)2+...+(8-5.091)2)/10). Thus, the standard deviation is the square root of 10.491, i.e. 3.239.
- IQR: Q3-Q1=8-3=5.
- Range: max-min=10-0=10.
- Skewness: -0.03.

##
Measures of Correlation
Section* *

All the above summary statistics are applicable only for univariate data where information on a single attribute is of interest. Correlation describes the degree of the linear relationship between two attributes, *X* and *Y*.

With *X* taking the values *x*(1), … , *x*(*n*) and *Y* taking the values *y*(1), … , *y*(*n*), the sample correlation coefficient is defined as:

\(\rho (X,Y)=\dfrac{\sum_{i=1}^{n}\left ( x(i)-\bar{x} \right )\left ( y(i)-\bar{y} \right )}{\left( \sum_{i=1}^{n}\left ( x(i)-\bar{x} \right )^2\sum_{i=1}^{n}\left ( y(i)-\bar{y} \right )^2\right)^\frac{1}{2}}\)

The correlation coefficient is always between -1 (perfect negative linear relationship) and +1 (perfect positive linear relationship). If the correlation coefficient is 0, then there is no *linear* relationship between X and Y.

In the figure below a set of representative plots are shown for various values of the population correlation coefficient ρ ranging from - 1 to + 1. At the two extreme values, the relation is a perfectly straight line. As the value of ρ approaches 0, the elliptical shape becomes round and then it moves again towards an elliptical shape with the principal axis in the opposite direction.

##
Try It!
Section* *

Try the applet "CorrelationPicture" and "CorrelationPoints" from the University of Colorado at Boulder.

Try the applet "Guess the Correlation" from the Rossman/Chance Applet Collection.