Properties of Normal Distribution
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Transcript of Properties of Normal Distribution
ARUN PRABHAKARME TQEMPEC UNIVERSITY OF TECHNOLOGY, CHANDIGARH
Properties of Normal Distribution
Data can be "distributed" (spread out) in different ways
Normal Distribution
Normal Distribution
• Many things closely follow a Normal Distribution:
• Heights Of People
• Size Of Things Produced By Machines
• Errors In Measurements
• Blood Pressure
• Marks On A Test
Normal Distribution
• MEAN = MEDIAN = MODE
• SYMMETRY ABOUT THE CENTER
• 50% OF VALUES LESS THAN THE MEAN AND 50% GREATER THAN THE MEAN
Standard Deviationsfor 3 sigma level
68% of values are within1 standard deviation of the mean
95% are within 2 standard deviations
99.7% are within 3 standard deviations
Standard Scores
The number of standard deviations from the mean is also called the "Standard Score", "sigma" or "z-score"
Why Standardize ... ?• It can help you make decisions about your data.
EXAMPLE
95% of students at school are between 1.1m and
1.7m tall. Assuming this data is normally distributed
calculate the mean and standard deviation and find Z
Score for one of your friend who is 1.85 m tall ?
Standard Scores
MEAN
The mean is halfway between
1.1m and 1.7m:
Mean = (1.1m + 1.7m ) = 1.4 m
2
• 95% is 2 standard deviations either side of the mean (a total of 4 standard deviations) so:
1 standard deviation = (1.7m-1.1m) / 4
= (0.6m / 4) = 0.15 m
Standard Deviations
• z is the "z-score" (Standard Score)
• x is the value to be standardized
• μ is the mean
• σ is the standard deviation
Z - SCORE
( X- μ ) = ( 1.85 - 1.4 ) = 3
Z - SCORE
σ
1.5
• A Practical Example:
Your company packages sugar in 1 kg bags. When you weigh a sample of bags you get these results:
1007g, 1032g, 1002g, 983g, 1004g, ... (a hundred measurements)
Mean = 1010g
Standard Deviation = 20g
Some values are less than 1000g ... How can you fix that?
APPLICATION
• Adjust the mean amount in each bag
• The standard deviation is 20g, and we need 2.5 of them:
• 2.5 × 20g = 50g
• So the machine should average 1050g, like this:
SOLUTION 1
INCREASE THE AMOUNT OF SUGAR
• Adjust the accuracy of the machine
• Or we can keep the same mean (of 1010g), but then weneed 2.5 standard deviations to be equal to 10g:
• 10g / 2.5 = 4g
• So the standard deviation should be 4g, like this:
Reff: www.mathsisfun.com/data/standard-normal-distribution
SOLUTION 1
INCREASE THE AMOUNT OF SUGAR
“Don't cry because it's over, smile because it happened.”
― Dr. Seuss
THANKYOU