STATISTICS: Normal Distribution

Report No. 2
Mr. Roderico Y. Dumaug, Jr.

TOPIC OUTLINE
 The Normal Distribution
1) Introduction
2) Definition of Terms and Statistical Symbols Used
3) How To Find Areas Under the Normal Curve
4) Finding the Unknown Z represented by Zo
5) Examples
 Hypothesis Testing

The Normal Distribution
 Introduction

Before exploring the complicated Standard
Normal Distribution, we must examine how the
concept of Probability Distribution changes when the
Random Variable is Continuous.

 Introduction

A Probability Distribution will give us a Value of P(x) = P(X=x)
to each possible outcome of x. For the values to make a Probability
Distribution, we needed two things to happen:

1. P (x) = P (X = x)
2. 0 ≤ P (x) ≤ 1

For a Continuous Random Variable, a Probability Distribution
must be what is called a Density Curve. This means:

1. The Area under the Curve is 1.
2. 0 ≤ P (x) for all outcomes x.

 Introduction: Example:
Suppose the temperature of a piece of metal is always between 0°F
and 10°F. Furthermore, suppose that it is equally likely to be any
temperature in that range. Then the graph of the probability distribution
for the value of the temperature would look like the one below:
0.10
Uniform Distribution
P (X < 5) ValuesAREA
are spread
uniformly across
Probability 0.05 0.8
the range 0 to 10
0.5 4
P (3 < X ≤ 7) Probability:
Area: 80% P (X > 2)
(4)(0.1) =
0.00 Finding the Area Under the Curve
0.4
7 - 3 = 4
2 3 5
5 7
10
X
Illustration of the fundamental fact about DENSITY CURVE

The Normal Distribution: Definition
of Terms and Symbols Used
 Normal Distribution Definition:

1) A continuous variable X having the symmetrical, bell shaped distribution
is called a Normal Random Variable.
2) The normal probability distribution (Gaussian distribution) is a
continuous distribution which is regarded by many as the most significant
probability distribution in statistics particularly in the field of statistical
inference.
 Symbols Used:
 “z” – z-scores or the standard scores. The table that transforms every normal
distribution to a distribution with mean 0 and standard deviation 1. This
distribution is called the standard normal distribution or simply
standard distribution and the individual values are called standard
scores or the z-scores.
 “µ” – the Greek letter “mu,” which is the Mean, and
 “σ” – the Greek letter “sigma,” which is the Standard Deviation

The Normal Distribution: Definition
of Terms and Symbols Used
 Characteristics of Normal Distribution:
1) It is “Bell-Shaped” and has a single peak at the center of the
distribution,
2) The arithmetic Mean, Median and Mode are equal.
3) The total area under the curve is 1.00; half the area under the
normal curve is to the right of this center point and the other
half to the left of it,
4) It is Symmetrical about the mean,
5) It is Asymptotic: The curve gets closer and closer to the X –
axis but never actually touches it. To put it another way, the
tails of the curve extend indefinitely in both directions.
6) The location of a normal distribution is determined by the
Mean, µ, the Dispersion or spread of the distribution is
determined by the Standard Deviation, σ.

The Normal Distribution: Graphically
Normal Curve is Symmetrical
Two halves identical

Theoretically, curve Theoretically, curve
extends to - ∞ extends to + ∞

Mean, Median
and Mode are
equal.

AREA UNDER THE NORMAL CURVE
P(x1<X<x2)
P(X<x1) P(X>x2)

x1 x2

AREA UNDER THE NORMAL CURVE
Let us consider a variable X which is normally distributed with a mean of 100 and
a standard deviation of 10. We assume that among the values of this variable are
x1= 110 and x2 = 85.
110 100 85 100
z1 1 . 00 z2 1 . 50
10 10

0.7745
0.4332
0.3413

-1.5 1

The Standard Normal Probability
Distribution
 The Standard Normal Distribution is a Normal
Distribution with a Mean of 0 and a Standard Deviation
of 1.
 It is also called the z distribution
 A z –value is the distance between a selected value
, designated X, and the population Mean µ, divided by
the Population Standard Deviation, σ.
 The formula is :

Areas Under the Normal Curve
z 0 1 2 3 4 5 6 7 8 9

0 0 0.004 0.008 0.012 0.016 0.0199 0.0239 0.0279 0.0319 0.0359

0.1 0.0398 0.0438 0.0478 0.0517 0.0557 0.0596 0.0636 0.0675 0.0714 0.0754

1.4 0.4192 0.4207 0.4222 0.4236 0.4251 0.4265 0.4279 0.4292 0.4306 0.4319

1.5 0.4332 0.4345 0.4357 0.437 0.4382 0.4394 0.4406 0.4418 0.4429 0.4441

1.6 0.4452 0.4463 0.4474 0.4484 0.4495 0.4505 0.4515 0.4525 0.4535 0.4545

0.4332

µ = 283 µ = 285.4 Grams
0 1.50 z Values

How to Find Areas Under the
Normal Curve
Let Z be a standardized random variable
P stands for Probability
Φ(z) indicates the area covered
under the Normal Curve.
a.) P (0 ≤ Z≤≤ZZ≤≤2.45) Φ(-0.35) – Φ (0.91)
b.) (-0.35 1.53) = Φ Φ (2.45)
c.) (0.91 0) = (1.53)
= 0.4929 - 0.3186
= 0.1368
0.4370
= 0.1743

This shaded area, from
Z Z = -0.35Z = 1.53, = 2.45,
= 0 and until Z = 0,
Z – 0.91 until Z
represents the probability
value ofof0.1368
value of0.4370.
value 0.1743

Normal Curve

d.) P (-2.0 ≤ Z ≤ 0.95) = Φ (-2.0) + Φ (0.95)
=0.4772 + 0.3289
= 0.8061

Z = -2.0 until Z = 0.95
value of 0.8061.

Normal Curve

e.) P (-1.5 ≤ Z ≤ -0.5) = Φ (-1.5) – Φ (-0.5)
=0.4332 - 0.1915
= 0.2417

Z = -1.5 until Z = -0.5
value of 0.2417.

Normal Curve

f.) P(Z ≥ 2.0) = 0.5 – Φ (2.0) This shaded area, from
= 0.5 – 0.4772 Z = 2.0 until beyond Z = 3
= 0.0228
value of 0.0228.

Normal Curve

g.) P (Z ≤ 1.5) = 0.5 + Φ (1.5)
= 0.5 + 0.4332
= 0.9332

Z = 1.5 until beyond Z = -3
value of 0.9332

Finding the unknown Z represented
by Zo

by Zo

 P(Z ≤ Z0) = 0.8461
0.5 + X = 0.8461
X = 0.8461 – 0.5
X = 0.3461
Z0 = 1.02 ans.
• P(-1.72 ≤ Z ≤ Z0) = 0.9345
• Φ (-1.72) + X = 0.9345
• X = 0.9345 – 0.4573
• X = 0.4772
• Z0 = 2.0

by Zo

CASE MNEMONICS
0 ≤ Z ≤ Zo Φ(Zo )
(-Zo ≤ Z ≤ 0) Φ(Zo )
Z1 ≤ Z ≤ Z2 Φ(Z2 ) – Φ(Z1)
(-Z1 ≤ Z ≤ Z2) Φ(Z1) + Φ(Z2 )
(-Z1 ≤ Z ≤ - Z2) Φ(Z1) - Φ(Z2 )
Z ≥ Zo 0.5 – Φ(Zo )
Z ≤ Zo 0.5 + Φ(Zo )
Z ≤ -Zo 0.5 – Φ(Zo )
Z ≥ -Zo 0.5 + Φ(Zo )

The event X has a normal distribution with mean µ = 10 and
Variance = 9. Find the probability that it will fall:

a.) between 10 and 11
b.) between 12 and 19
c.) above 13
d.) at x = 11
e.) between 8 amd 12

x 10 10
a .) Z 0
3
x 11 10 1
Z 0 . 33
3 3

P ( 10 X 11 ) P(0 Z 0 . 33 )
( 0 . 33 ) 0 . 1293

x 12 10 2
b .) Z 0 . 67
3 3
x 19 10 9
Z 3
3 3

P ( 12 X 19 ) P ( 0 . 67 Z 3)
( 3) ( 0 . 67 ) 0 . 4987 0 . 2486 0 . 251

x 13 10 3
c .) Z 1
3 3

P( X 13 ) P( Z 1) 0 .5 ( 1)

0 .5 0 . 3413 0 . 1587

d .) P ( X 11 ) 0
x 8 10 2
e .) Z 0 . 67
3 3
P( 8 X 12 ) P ( 0 . 67 Z 0 . 67 )
( 0 . 33 ) 0 . 1293

2. A random variable X has a normal distribution
with mean 5 and variance 16.

a.) Find an interval (b,c) so that the probability of X
lying in the interval is 0.95.
b.) Find d so that the probability that X ≥ d is 0.05.

1
Solution: A.
P (b ≤ X ≤ c) = P (Z b ≤ Z ≤ Zc) 2

= P (-1.96 (4) ≤X -5 ≤ 1.96 (4)
= P (-7.84 + 5 ≤ X ≤ 7.84 + 5)
P (b ≤ X ≤ c) = P (-2.84 ≤ X ≤ 12.84 )

thus: b = -2.84 and c = 12.84

2. A random variable X has a normal distribution
with mean 5 and variance 16.

a.) Find an interval (b,c) so that the probability of X
lying in the interval is 0.95.
b.) Find d so that the probability that X ≥ d is 0.05.

1 0.5 – 0.05 = 0.45
Solution B:
P ( X ≥ d ) = P (Z ≥ Zd) = 0.05≥ from the table

0.45 Z = 1.64
= P (X -5 ≥ 6.56)
= P (X ≥ 6.56 + 5)
P ( X ≥ d ) = P (X ≥ 11.56)

thus: d = 11.56

3. A certain type of storage battery last on the
average 3.0 years, with a standard deviation σ of
0.5 year. Assuming that the battery are normally
distributed, find the probability that a given battery
will last less than 2.3 years.

Solution: X 2 .3 3 0 .7
z 1.4
0 .5 0 .5
P (X < 2.3) = P (Z < -1.4)
= 0.5 – Φ (-1.4)
= 0.5 – 0.4192
P (X < 2.3) = 0.0808

STATISTICS: Normal Distribution

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