slides by john loucks st. edward’s university

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Slides by

JohnLoucks

St. Edward’sUniversity



Chapter 3, Part A Descriptive Statistics: Numerical

Measures Measures of Location Measures of Variability



Measures of Location

If the measures are computed for data from a sample,

they are called sample statistics.

If the measures are computed for data from a population,

they are called population parameters.

A sample statistic is referred toas the point estimator of the

corresponding population parameter.

Mean Median Mode Percentiles Quartiles



Mean

The mean of a data set is the average of all the data values.

x The sample mean is the point estimator of the population mean m.

Perhaps the most important measure of location is the mean.

The mean provides a measure of central location.



Sample Mean x

Number ofobservationsin the sample

Number ofobservationsin the sample

Sum of the valuesof the n observations

Sum of the valuesof the n observations

ixx

n



Population Mean m

Number ofobservations inthe population

Number ofobservations inthe population

Sum of the valuesof the N observations

Sum of the valuesof the N observations

ix

N



Seventy efficiency apartments were randomly

sampled in a small college town. The monthly rent

prices for these apartments are listed below.

Sample Mean

Example: Apartment Rents

445 615 430 590 435 600 460 600 440 615440 440 440 525 425 445 575 445 450 450465 450 525 450 450 460 435 460 465 480450 470 490 472 475 475 500 480 570 465600 485 580 470 490 500 549 500 500 480570 515 450 445 525 535 475 550 480 510510 575 490 435 600 435 445 435 430 440



Sample Mean

34,356 490.80

70ix

xn

445 615 430 590 435 600 460 600 440 615440 440 440 525 425 445 575 445 450 450465 450 525 450 450 460 435 460 465 480450 470 490 472 475 475 500 480 570 465600 485 580 470 490 500 549 500 500 480570 515 450 445 525 535 475 550 480 510510 575 490 435 600 435 445 435 430 440




Median

Whenever a data set has extreme values, the median is the preferred measure of central location.

A few extremely large incomes or property values can inflate the mean.

The median is the measure of location most often reported for annual income and property value data.

The median of a data set is the value in the middle when the data items are arranged in ascending order.



Median

12 14 19 26 2718 27

For an odd number of observations:

in ascending order

26 18 27 12 14 27 19 7 observations

the median is the middle value.

Median = 19



12 14 19 26 2718 27

Median

For an even number of observations:

in ascending order

26 18 27 12 14 27 30 8 observations

the median is the average of the middle two values.

Median = (19 + 26)/2 = 22.5

19

30



Median

Averaging the 35th and 36th data values:Median = (475 + 475)/2 = 475

Note: Data is in ascending order.

425 430 430 435 435 435 435 435 440 440440 440 440 445 445 445 445 445 450 450450 450 450 450 450 460 460 460 465 465465 470 470 472 475 475 475 480 480 480480 485 490 490 490 500 500 500 500 510510 515 525 525 525 535 549 550 570 570575 575 580 590 600 600 600 600 615 615




Trimmed Mean

It is obtained by deleting a percentage of the smallest and largest values from a data set and then computing the mean of the remaining values. For example, the 5% trimmed mean is obtained by removing the smallest 5% and the largest 5% of the data values and then computing the mean of the remaining values.

Another measure, sometimes used when extreme values are present, is the trimmed mean.



Mode

The mode of a data set is the value that occurs with greatest frequency. The greatest frequency can occur at two or more different values. If the data have exactly two modes, the data are bimodal.

If the data have more than two modes, the data are multimodal.

Caution: If the data are bimodal or multimodal, Excel’s MODE function will incorrectly identify a single mode.



Mode

450 occurred most frequently (7 times)

Mode = 450


425 430 430 435 435 435 435 435 440 440440 440 440 445 445 445 445 445 450 450450 450 450 450 450 460 460 460 465 465465 470 470 472 475 475 475 480 480 480480 485 490 490 490 500 500 500 500 510510 515 525 525 525 535 549 550 570 570575 575 580 590 600 600 600 600 615 615




Percentiles

A percentile provides information about how the data are spread over the interval from the smallest value to the largest value. Admission test scores for colleges and universities are frequently reported in terms of percentiles. The pth percentile of a data set is a value such

that at least p percent of the items take on this value or less and at least (100 - p) percent of the items take on this value or more.



Percentiles

Arrange the data in ascending order. Arrange the data in ascending order.

Compute index i, the position of the pth percentile. Compute index i, the position of the pth percentile.

i = (p/100)n

If i is not an integer, round up. The p th percentile is the value in the i th position. If i is not an integer, round up. The p th percentile is the value in the i th position.

If i is an integer, the p th percentile is the average of the values in positions i and i +1. If i is an integer, the p th percentile is the average of the values in positions i and i +1.



80th Percentile

i = (p/100)n = (80/100)70 = 56Averaging the 56th and 57th data values:80th Percentile = (535 + 549)/2 = 542


425 430 430 435 435 435 435 435 440 440440 440 440 445 445 445 445 445 450 450450 450 450 450 450 460 460 460 465 465465 470 470 472 475 475 475 480 480 480480 485 490 490 490 500 500 500 500 510510 515 525 525 525 535 549 550 570 570575 575 580 590 600 600 600 600 615 615




80th Percentile

“At least 80% of the items take on a

value of 542 or less.”

“At least 20% of theitems take on a

value of 542 or more.”

56/70 = .8 or 80% 14/70 = .2 or 20%

425 430 430 435 435 435 435 435 440 440440 440 440 445 445 445 445 445 450 450450 450 450 450 450 460 460 460 465 465465 470 470 472 475 475 475 480 480 480480 485 490 490 490 500 500 500 500 510510 515 525 525 525 535 549 550 570 570575 575 580 590 600 600 600 600 615 615




Quartiles

Quartiles are specific percentiles. First Quartile = 25th Percentile

Second Quartile = 50th Percentile = Median Third Quartile = 75th Percentile



Third Quartile

Third quartile = 75th percentilei = (p/100)n = (75/100)70 = 52.5 = 53

Third quartile = 525


425 430 430 435 435 435 435 435 440 440440 440 440 445 445 445 445 445 450 450450 450 450 450 450 460 460 460 465 465465 470 470 472 475 475 475 480 480 480480 485 490 490 490 500 500 500 500 510510 515 525 525 525 535 549 550 570 570575 575 580 590 600 600 600 600 615 615




Measures of Variability

It is often desirable to consider measures of variability (dispersion), as well as measures of location.

For example, in choosing supplier A or supplier B we might consider not only the average delivery time for each, but also the variability in delivery time for each.



Measures of Variability

Range

Interquartile Range

Variance

Standard Deviation

Coefficient of Variation



Range

The range of a data set is the difference between the largest and smallest data values.

It is the simplest measure of variability. It is very sensitive to the smallest and largest data values.



Range

Range = largest value - smallest valueRange = 615 - 425 = 190


425 430 430 435 435 435 435 435 440 440440 440 440 445 445 445 445 445 450 450450 450 450 450 450 460 460 460 465 465465 470 470 472 475 475 475 480 480 480480 485 490 490 490 500 500 500 500 510510 515 525 525 525 535 549 550 570 570575 575 580 590 600 600 600 600 615 615




Interquartile Range

The interquartile range of a data set is the difference between the third quartile and the first quartile. It is the range for the middle 50% of the data.

It overcomes the sensitivity to extreme data values.



425 430 430 435 435 435 435 435 440 440440 440 440 445 445 445 445 445 450 450450 450 450 450 450 460 460 460 465 465465 470 470 472 475 475 475 480 480 480480 485 490 490 490 500 500 500 500 510510 515 525 525 525 535 549 550 570 570575 575 580 590 600 600 600 600 615 615

Interquartile Range

3rd Quartile (Q3) = 5251st Quartile (Q1) = 445

Interquartile Range = Q3 - Q1 = 525 - 445 = 80





The variance is a measure of variability that utilizes all the data.

Variance

It is based on the difference between the value of each observation (xi) and the mean ( for a sample, m for a population).

x

The variance is useful in comparing the variability of two or more variables.



Variance

The variance is computed as follows:

The variance is computed as follows:

The variance is the average of the squared differences between each data value and the mean. The variance is the average of the squared differences between each data value and the mean.

for asample

for apopulation

22

( )xNis

xi x

n2

2

1

( )



Standard Deviation

The standard deviation of a data set is the positive square root of the variance.

It is measured in the same units as the data, making it more easily interpreted than the variance.



The standard deviation is computed as follows:

The standard deviation is computed as follows:

for asample

for apopulation

Standard Deviation

s s 2 2



The coefficient of variation is computed as follows:

The coefficient of variation is computed as follows:

Coefficient of Variation

100 %s

x

The coefficient of variation indicates how large the standard deviation is in relation to the mean. The coefficient of variation indicates how large the standard deviation is in relation to the mean.

for asample

for apopulation

100 %



54.74100 % 100 % 11.15%

490.80sx

22 ( )

2,996.161

ix xs

n

2 2996.16 54.74s s

the standard

deviation isabout 11%

of the mean

• Variance

• Standard Deviation

• Coefficient of Variation

Sample Variance, Standard Deviation,And Coefficient of Variation




End of Chapter 3, Part A

slides by john loucks st. edward’s university

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