Normal Distribution
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Transcript of Normal Distribution
Evelyn Loekito
2112100032
TUGAS II
NORMAL DISTRIBUTION RESUME
Normal Distribution Definition
The normal distribution refers to a family of continuous probability distributions described by the
normal equation.
The normal distribution is defined by the following equation:
The value of the random variable P is:
Where :
X is a normal random variable
μ is the mean
σ is the standard deviation
π is approximately 3.14159
e is approximately 2.71828
The random variable X in the normal equation is called the normal random variable. The normal
equation is the probability density function for the normal distribution.
Data tends to be around a central value with no bias left or right and it is often called as a Bell Curve
since it’s tendency mostly looks like a bell curve.
A Normal Distribution
The yellow histogram shows some data that follows it closely, but not perfectly (which is usual).
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
The normal distribution is a continuous probability distribution all of which led to several
implications for probability:
The total area under the normal curve is equal to 1.
The probability that a normal random variable X equals any particular value is 0.
The probability that X is greater than a equals the area under the normal curve bounded
by a and plus infinity (as indicated by the non-shaded area in the figure below).
The probability that X is less than a equals the area under the normal curve bounded
by a and minus infinity (as indicated by the shaded area in the figure below).
Parameters and Properties
Normal distribution can be characterized by these two following parameters:
μ = expected value
determines the value of x about which the distribution is centered
with the range -∞ ≤ µ ≤ ∞
σ = standard deviation
determines the width of the distribution about the true mean
with the range σ ˃ 0
The Standard Normal Distribution (SND)
"Standardizing":
Rather than computing each value of µ and σ, any normal distribution can be converted in to The Standard Normal Distribution (SND) with it’s parameter z.
z is the "z-score" (Standard Score)
x is the value to be standardized
μ is the mean
σ is the standard deviation
Thus, the normal distribution equation can be referred as :
P (Z )= 1
√2πe−z
2 /2
We can say that any value is:
likely to be within 1st standard deviation (68%should be)
very likely to be within 2nd standard deviations (95% should be)
almost certainly within 3rd standard deviations (99.7% should be)
Usage of Standard Normal (Z) Table
Values in the table represent areas under the curve to the left of Z (cummulative) quartiles along the
margins.
For Example :
P( 90 < X < 110 ) = P( X < 110 ) - P( X < 90 )
From the Z table :
P( X < 110 ) is 0.84
P( X < 90 ) is 0.16.
We use these findings to compute our final answer as follows:
P( 90 < X < 110 ) = P( X < 110 ) - P( X < 90 )
P( 90 < X < 110 ) = 0.84 - 0.16
P( 90 < X < 110 ) = 0.68
Thus, about 68% of the test scores will fall between 90 and 110.
Source :
http://stattrek.com/probability-distributions/normal.aspx
http://mathworld.wolfram.com/NormalDistribution.html
http://www.mathsisfun.com/data/standard-normal-distribution.html
http://www.oswego.edu/~srp/stats/z.htm
http://www.mathsisfun.com/data/standard-normal-distribution-table.html