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HP 12c Platinum Financial Calculator  Statistics  Average and Standard Deviation
Average and standard deviation concepts
Statistics can be understood as a set of tools involving the study of methods and procedures used for collecting,
classifying, and analyzing data. Statistical tools also offer the means for making scientific inferences from such resulting
summarized data. Two of these tools are the Average and the Standard Deviation.
Given a set of collected data, the average is defined as a measure of central tendency and is the most commonly used.
Its value is computed as the sum of all data points divided by the number of data points included. The standard deviation
is one index of variability used to characterize the dispersion among the data in a given population or a sample. It measures dispersion around the average. The property of the standard deviation is such that when the underlying data is
normally distributed, approximately 68% of all values will lie within one standard deviation on either side of the mean,
and approximately 95% of all values will lie within two standard deviations on either side of the mean. This has
application to many fields, particularly when trying to decide if an observed value is unusual by being significantly
different from the mean.
HP 12c Platinum average and standard deviation
On the HP 12c Platinum, statistics data are stored as a set of summations resulting from the originally collected data.
The collected data set must be typed in before using any statistics features available in the HP 12c Platinum because all
values produced by these statistics tools depend on them. The HP 12c Platinum memory organization allows the study
of statistic data organized as one or twovariable samples. As a general procedure, data is always collected as a pair of
numbers, or (x,y) values, and the HP 12c Platinum computes the following summations:
Figure : Calculating the values for x and y
With these values updated and stored in memory, the HP 12c Platinum computes the average ( ) for each
variable with the following expressions:
Figure : Calculating the average for each variable
The following expressions are used by the HP 12c Platinum to compute the standard deviation of a sample:
Figure : Calculating the standard deviation of a sample
Practice finding average sale prices and standard deviations
Example 1
The sales price of the last 10 homes sold in the Parkdale community were: $198,000; $185,000; $205,200;
$225,300; $206,700; $201,850; $200,000; $189,000; $192,100; $200,400. What is the average of these
sales prices and what is the sample standard deviation? Would a sales price of $240,000 be considered
unusual in the same community?
Solution
Be sure to clear the statistics / summation memories before starting the problem.
Keystroke

Display


Figure : Clearing the statistics/summation memory

Each entered data value causes the display to change and display the number of current entries (n). Now
enter each data value with :
Keystroke

Display


Figure : Entering the first pair of data

The display represented in Figure 2 shows current n value of 1.
Keystroke

Display


Figure : Representing the display after the last entry

Figure 6 represents the display after the last entry. With all data already entered, all summations are ready
and it is possible to compute both the average and the standard deviation. To compute the average press:
Keystroke

Display


Figure : Calculating the average

is the blue function on the front, slanted face of the key, so (the blue prefix key) must be
pressed first.
To compute the standard deviation, press:
Keystroke

Display


Figure : Calculating the standard deviation

is the blue function on the front, slanted face of the key.
Based on these figures, approximately 68% of the prices are in the range $200,355 ± $11,189.04.
Approximately 95% of the prices are in the range $200,355 ± 2×($11,189.04). The following keystroke
sequence gives the lower boundary:
Keystroke (In RPN mode)

Keystroke (In algebraic mode)

Display



Figure : Displaying the lower boundary

The display shows the lower boundary.
Keystroke (In RPN mode)

Keystroke (In algebraic mode)

Display



Figure : Displaying the higher boundary

The display shows the higher boundary.
Answer
$240,000 is an unusual price for a home at the Parkdale community based on the last 10 sales prices.
Practice with average and standard deviation with two variables
Example 2
A land researcher wants to compute the relationship between the constructed area and the land area of
eight homes located in his neighborhood. Initially he needs to know the average and the standard deviation
for both parameters. His measurements allowed him to build the following chart:
Land Area (sq yards)

Construction Area (sq yards)

Land Area (sq yards)

Construction Area (sq yards)

12000

3120

9000

2080

10000

2560

10000

2700

11000

2920

13000

3280

14000

3300

12000

3080

Solution
Be sure to clear the statistics / summation memories before starting the problem.
Keystroke

Display


Figure : Clearing the statistics/summation memory

Each pair must be entered to add it to the statistics summations.
Keystroke

Display


Figure : Entering the first pair of data

The first entered value (construction area) is computed as the y variable and the second value (land area)
is computed as the x variable. The display shows the number of entries. Make sure that all data is entered:
Keystroke

Display


Figure : Entering all the data

To compute the average:
Keystroke

Display


Figure : Calculating the average land area

Average land area: 11,375 sq yards.
Keystroke

Display


Figure : Calculating the average construction area

Average construction area: 2,880 sq yards.
To compute the standard deviation:
Keystroke

Display


Figure : Calculating the standard deviation for land area

Standard deviation for land area: 1,685.02 sq yards.
Keystroke

Display


Figure : Calculating the standard deviation for construction area

Standard deviation for construction area: 415.83 sq yards.
Answer
The average land area for this sample is 11,375 sq yards and the standard deviation is 1,685.02 sq yards.
For the construction area, the average is 2,880 sq yards and the standard deviation is 415.83 sq yards.
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