Understanding Central Tendency & Spread

Understanding how data behaves is a crucial part of any data analysis. Whether you’re working with marketing numbers, medical stats, or machine learning models, being able to describe the average behavior and the spread of your data is essential.

Mean (Average)

Definition:
The mean is the sum of all values divided by the number of values. It represents the “central” or “typical” value in your dataset.

Formula:

\text{Mean} = \frac{\sum x_i}{n}

Example:
If your website had 100, 120, and 110 visitors over three days:

\frac{100 + 120 + 110}{3} = 110

Interpretation:
On average, the site received 110 visitors per day.

Concept 2: Variance

Definition:
Variance measures how far each number in the set is from the mean. It gives a sense of how “spread out” the data is.

Formula:

\sigma^2 = \frac{\sum (x_i - \mu)^2}{n}

Example (using the previous data):

Mean = 110
$(100 − 110)^2 = 100$
$(120 − 110)^2 = 100$
$(110 − 110)^2 = 0$

Sum = 200

\text{Variance} = \frac{200}{3} \approx 66.67

Interpretation:
The average squared distance from the mean is about 66.67.

Standard Deviation

Definition:
Standard deviation is the square root of the variance. It brings the spread back to the original units of the data.

Formula:

\sigma = \sqrt{\sigma^2}

Example:
If variance is 66.67:

\sigma = \sqrt{66.67} \approx 8.16

Interpretation:
On average, each day's visitor count is about 8.16 away from the mean.

Real-World Problem: Website Traffic Analysis

Problem:
A data scientist records the number of website visitors over 5 days:

120, 150, 130, 170, 140

Step 1 — Mean:

\frac{120 + 150 + 130 + 170 + 140}{5} = 142

Step 2 — Variance:

$(120 - 142)^2 = 484$
$(150 - 142)^2 = 64$
$(130 - 142)^2 = 144$
$(170 - 142)^2 = 784$
$(140 - 142)^2 = 4$

\text{Variance} = \frac{484+64+144+784+4}{5} = \frac{1480}{5} = 296

Step 3 — Standard Deviation:

\sigma = \sqrt{296} \approx 17.2

Conclusion:

Mean = 142 visitors per day
Variance = 296
Standard Deviation = 17.2

The website traffic varies by about 17.2 visitors from the average day.

Quiz: Test Your Knowledge

**1.

**2.

**3.

4. Which unit does variance use?
A) Same as data
B) No unit
C) Squared units of data ✅
D) Logarithmic scale

**5.

6. In the dataset [4, 8, 12], what is the mean?
A) 6
B) 8 ✅
C) 12
D) 10

7. Which formula represents variance?
A) —
B) $\sigma^2 = \frac{\sum (x_i - \mu)^2}{n}$ ✅
C) —
D) —

**8.

9. If the variance is 25, what is the standard deviation?
A) 5 ✅
B) 25
C) 2.5
D) 125

10. Why is standard deviation often preferred over variance in interpretation?
A) It’s easier to compute
B) It’s in the original units ✅
C) It gives smaller numbers
D) It avoids using the mean

1. What does the mean represent in a dataset?

2. Which formula correctly represents the mean?

3. What does a high variance indicate?

4. What is the relationship between variance and standard deviation?

5. What is the purpose of squaring the differences in variance?

What does the mean represent in a dataset?

Select the correct answer

The most frequent value

The middle number

The average value

The highest value

Which formula correctly represents the mean?

Select the correct answer

\text{Mean} = \frac{x+y}{n}

\text{Mean} = \frac{\sum x_i}{n}

\text{Mean} = \frac{N}{n}

\text{Mean} = \frac{\sum{x_i y_i}}{n}

What does a high variance indicate?

Select the correct answer

The data points are all the same.

Data points are far from the mean.

The mean is inaccurate.

There's an error in the data.

What is the relationship between variance and standard deviation?

Select the correct answer

Variance is square root of standard deviation.

Standard deviation is variance minus the mean.

Standard deviation is the square root of variance.

They are always equal.

What is the purpose of squaring the differences in variance?

Select the correct answer

To get negative values

To center the data

To eliminate decimals

To ensure all values are positive

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Understanding Central Tendency & Spread

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Mean (Average)

Definition:
The mean is the sum of all values divided by the number of values. It represents the “central” or “typical” value in your dataset.

Formula:

\text{Mean} = \frac{\sum x_i}{n}

Example:
If your website had 100, 120, and 110 visitors over three days:

\frac{100 + 120 + 110}{3} = 110

Interpretation:
On average, the site received 110 visitors per day.

Concept 2: Variance

Definition:
Variance measures how far each number in the set is from the mean. It gives a sense of how “spread out” the data is.

Formula:

\sigma^2 = \frac{\sum (x_i - \mu)^2}{n}

Example (using the previous data):

Mean = 110
$(100 − 110)^2 = 100$
$(120 − 110)^2 = 100$
$(110 − 110)^2 = 0$

Sum = 200

\text{Variance} = \frac{200}{3} \approx 66.67

Interpretation:
The average squared distance from the mean is about 66.67.

Standard Deviation

Definition:
Standard deviation is the square root of the variance. It brings the spread back to the original units of the data.

Formula:

\sigma = \sqrt{\sigma^2}

Example:
If variance is 66.67:

\sigma = \sqrt{66.67} \approx 8.16

Interpretation:
On average, each day's visitor count is about 8.16 away from the mean.

Real-World Problem: Website Traffic Analysis

Problem:
A data scientist records the number of website visitors over 5 days:

120, 150, 130, 170, 140

Step 1 — Mean:

\frac{120 + 150 + 130 + 170 + 140}{5} = 142

Step 2 — Variance:

$(120 - 142)^2 = 484$
$(150 - 142)^2 = 64$
$(130 - 142)^2 = 144$
$(170 - 142)^2 = 784$
$(140 - 142)^2 = 4$

\text{Variance} = \frac{484+64+144+784+4}{5} = \frac{1480}{5} = 296

Step 3 — Standard Deviation:

\sigma = \sqrt{296} \approx 17.2

Conclusion:

Mean = 142 visitors per day
Variance = 296
Standard Deviation = 17.2

The website traffic varies by about 17.2 visitors from the average day.

Quiz: Test Your Knowledge

**1.

**2.

**3.

4. Which unit does variance use?
A) Same as data
B) No unit
C) Squared units of data ✅
D) Logarithmic scale

**5.

6. In the dataset [4, 8, 12], what is the mean?
A) 6
B) 8 ✅
C) 12
D) 10

7. Which formula represents variance?
A) —
B) $\sigma^2 = \frac{\sum (x_i - \mu)^2}{n}$ ✅
C) —
D) —

**8.

9. If the variance is 25, what is the standard deviation?
A) 5 ✅
B) 25
C) 2.5
D) 125

1. What does the mean represent in a dataset?

2. Which formula correctly represents the mean?

3. What does a high variance indicate?

4. What is the relationship between variance and standard deviation?

5. What is the purpose of squaring the differences in variance?

What does the mean represent in a dataset?

Select the correct answer

The most frequent value

The middle number

The average value

The highest value

Which formula correctly represents the mean?

Select the correct answer

\text{Mean} = \frac{x+y}{n}

\text{Mean} = \frac{\sum x_i}{n}

\text{Mean} = \frac{N}{n}

\text{Mean} = \frac{\sum{x_i y_i}}{n}

What does a high variance indicate?

Select the correct answer

The data points are all the same.

Data points are far from the mean.

The mean is inaccurate.

There's an error in the data.

What is the relationship between variance and standard deviation?

Select the correct answer

Variance is square root of standard deviation.

Standard deviation is variance minus the mean.

Standard deviation is the square root of variance.

They are always equal.

What is the purpose of squaring the differences in variance?

Select the correct answer

To get negative values

To center the data

To eliminate decimals

To ensure all values are positive

Oliko kaikki selvää?

Miten voimme parantaa sitä?

Kiitos palautteestasi!

Osio 5. Luku 7