# Algebra II : Standard Deviation

## Example Questions

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### Example Question #1 : Standard Deviation

Calculate the standard deviation of the following set of values.

Explanation:

To get the standard deviation, we need to calculate the variance, which is the average of the squared differences from the mean, so we will start by getting the mean.

Then, subtract the mean from each value...

...and take the mean of these resulting values, which is equal to the variance.

The square root of this value is the standard deviation. The answer is presented as ,

but you may also calculate it and find it equal to about

### Example Question #2 : Standard Deviation

Determine the standard deviation for the following data set:

12, 15, 30, 5, 27, 19

Explanation:

Formula for the standard deviation:

1. Find the mean

2. Subtract the mean from each number in the data set

3. Sum up the square of the differences and divide by n

4. Take the square root of the variance

### Example Question #3 : Standard Deviation

In the population of high school boys, the variance in height, measured in inches, was found to be 16. Assuming that the height data is normally distributed, 95% of high school boys should have a height within how many inches of the mean?

Explanation:

The 68-95-99.7 rule states that nearly all values lie within 3 standard deviations of the mean in a normal distribution. In this case the question asks for 95% so we want to know what 2 standard deviations from the mean is.

We are given the variance, so to find the standard deviation, take the square root.

So two standard devations is 8 inches. 95% of heights should be within 8 inches of the mean.

### Example Question #4 : Standard Deviation

At the end of the fall semester, a math class of ninth graders had the following grades: 85, 75, 97, 83, 62, 75, 88, 84, 92, and 89.

What is the standard deviation of this class?

Explanation:

The standard deviation of a set of numbers is how much the numbers deviate from the mean. More formally, the standard deviation is

where  is a number in the series,  is the mean, and  is the number of data points. So, to calculate the standard deviation, we must first calculate the mean. The mean of this data set is

Now that we know the mean, we can start calculating the standard deviation. We first need to find the sum of each data point minus the average squared.

Calculating that, we get that the variance from the mean is . Plugging that into our equation for standard deviation, with  being ten data points, we get

### Example Question #1 : How To Find Standard Deviation

Mr. Bell gave out a science test last week to six honors students. The scores were 88, 94, 80, 79, 74, and 83. What is the standard deviation of the scores? (Round to the nearest tenth.)

Explanation:

First, find the mean of the six numbers by adding them all together, and dividing them by six.

88 + 94 + 80 + 79 + 74 + 83 = 498

498/6 = 83

Next, find the variance by subtracting the mean from each of the given numbers and then squaring the answers.

88 – 83 = 5

52 = 25

94 – 83 = 11

112 = 121

80 – 83 = –3

–32 = 9

79 – 83 = –4

–42 = 16

74 – 83 = –9

–92 = 81

83 – 83 = 0

02 = 0

Find the average of the squared answers by adding up all of the squared answers and dividing by six.

25 + 121 + 9 +16 +81 + 0 = 252

252/6 = 42

42 is the variance.

To find the standard deviation, take the square root of the variance.

The square root of 42 is 6.481.

### Example Question #1981 : Algebra 1

On his five tests for the semester, Andrew earned the following scores: 83, 75, 90, 92, and 85. What is the standard deviation of Andrew's scores? Round your answer to the nearest hundredth.

Explanation:

The following is the formula for standard deviation:

Here is a breakdown of what that formula is telling you to do:

1. Solve for the mean (average) of the five test scores
2. Subtract that mean from each of the five original test scores. Square each of the differences.
3. Find the mean (average) of each of these differences you found in Step 2
4. Take the square root of this final mean from #3. This is the standard deviation

Here are those steps:

1. Find the mean of the test scores:

2. Subtract the mean from each of the test scores, then square the differences:

3. Find the mean of the squared values from Step 2:

4. Take the square root of your answer from Step 3:

### Example Question #7 : How To Find Standard Deviation

In her last six basketball games, Jane scored 15, 17, 12, 15, 18, and 22 points per game. What is the standard deviation of these score totals? Round your answer to the nearest tenth.

Explanation:

The following is the formula for standard deviation:

Here is a breakdown of what that formula is telling you to do:

1. Solve for the mean (average) of the five test scores
2. Subtract that mean from each of the five original test scores. Square each of the differences.
3. Find the mean (average) of each of these differences you found in Step 2
4. Take the square root of this final mean from #3. This is the standard deviation

Here are those steps:

1. Find the mean of her score totals:

2. Subtract the mean from each of the test scores, then square the differences:

3. Find the mean of the squared values from Step 2:

4. Take the square root of your answer from Step 3:

### Example Question #5 : Standard Deviation

What is the standard deviation of ?

Explanation:

Standard deviation is  where  represents the data point in the set,  is the mean of the data set and  is number of points in the set.

The mean is  the sum of the data set divided by the number of data points in the set.

Plugging in the values:

### Example Question #6 : Standard Deviation

In a normal distribution, what percentage is covered within one standard deviation?

Explanation:

By drawing a bell curve, the middle line is . One standard deviation left and right of the middle line is  each. That means one standard deviation within is

### Example Question #7 : Standard Deviation

If standard deviation is  and the mean is , what is the range of the number set if it's within one standard deviation?