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# Python Data Science Machine Learning Lesson 2: Standard Deviation

What is standard deviation? How to Find/Calculate standard deviation?
Standard deviation is a quantity which expresses by how much the members of a group differ from the mean value for the group.

Standard deviation is a number that describes how spread out the values are.
A low standard deviation means that most of the numbers are close to the mean (average) value.
A high standard deviation means that the values are spread out over a wider range.

Example: Consider the speed of 10 different trains as under.
speed = [92, 90, 95, 97, 85, 99, 88, 97, 84, 101]
In Python programming language you have a module called numpy to calculate the std deviation.
You have to first import mumpy as under

import numpy as np
now the full code in python progamming will be as under:

code

import numpy as np
speed = [92, 90, 95, 97, 85, 99, 88, 97, 84, 101]
print(np.std(speed))

the output will be.
5.617828762075256
So the std deviation is 5.61

Conclusion:
Since the value of standard deviation is high in the above example, it can be concluded that the values are spread out over a wider range.

Note: if you have python installed on your pc you can install numpy as under.
Open Command Prompt from the start menu.
Inside the command prompt, type
pip install numpy
press enter
This command will install numpy on your computer after which you can run on python.