Probability Distributions

OBJECTIVES

– Identify types of distribution.

– Describe the characteristics of the normal probability distribution.

– Discuss the area beneath the normal distribution curve

– Calculate approximations using the normal curve

– Use the normal distribution tables.

Introduction

• A probability distribution lists, in some form, all the possible outcomes of a probability experiment and the probability associated with each one.

• We have already looked at a few simple examples. E.g. Amount spent on drink.

Examples

• Toss a coin in which the possible outcomes are `head’ or `tail’. We have

P(head) = 0.5; P(tail) = 0.5.• Time the departure of a train to the nearest second on

the platform clock. There will be a certain probability of it departing on time and other probabilities of it departing at later seconds.

• Measure the heights of everyone in this building. In this case it would seem sensible for the outcomes to be `intervals’ rather than actual values.

Types of Distribution

• There are two main types of distribution.• These are discrete and continuous.• Discrete distributions occur when the outcome can be

expressed in terms of variables having discrete values e.g. 0, 1, 2, 3, …

E.g. How many 6’s when we throw a die a number of times.

• Continuous distributions occur when variables are continuous.

E.g. A persons height or weight.

There are two commonly occurring types of discrete distribution

• Binomial – in which there are only two possible outcomes to each experiment and the probability of obtaining one or other outcome does not alter with repetitions (trials) of the experiment. Each is independent.

• E.g. The probability of obtaining a `club’ from a well-shuffled pack of cards. This probability is ¼ = 0.25.

• The probability of a non club is ¾ = 0.75 (See next slide).

• Poisson – similar to the binomial but applicable when the probability of `success’ is very small and there are a large number of trials.

• It is used in problems where events occur over space or time.

• E.g. the number of goals scored in a football match.

The probability distribution for a club being chosen from a deck of cards

CLUBNOT CLUB

0.25

0.5

0.75

Note: 0.25 + 0.75 = 1

Continuous Distributions

• The most important type of continuous distribution is the Normal distribution.

• This will be the subject of the remaining part of this lecture.

• Normal distributions occur commonly in nature.

• E.g. Men’s heights or Women’s heights are both normal distributions.

This is the graph of the standard normal distribution.

The area beneath the curve is always equal to one.

Characteristics of a normal distribution

• Probability is represented by Area under the curve.• The distribution has a single mode.. The modal value

corresponds to the mean µ. For example men’s heights are clustered around the average height;

• It is symmetric about the mean, the left and right halves being mirror images of each other;

• It is bell shaped. The width depends on the standard deviation, σ;

• It extends continuously over all values of x.

The area beneath the normal distribution curve

Total area beneath any normal distribution

curve is always equal to one

0.5

0.5

x

Mean and standard deviation of a Normal distribution

• The normal distribution depends on the mean (µ) and standard deviation (σ) of the population. The mean can be any value, positive or negative.

• In slide 9

µ = 0 and σ = 1.

• This is called the standard Normal distribution.

• The effect of varying σ is to alter the shape of the curve. The smaller the value of σ the narrower the curve.

• 68 % of the curve lies between one standard deviation either side of the mean.

• 95 % lies between 2 standard deviations either side of the mean;

• 99.7 % lies between 3 standard deviations either side of the mean.

Area within one standard deviation of the mean

68%

x

Area within two standard deviations of the mean

95%

x

Sections of the area under the curve represent probabilities of the variable lying within certain ranges.

a bx

In normal distribution below:• Mean () = 15• Standard deviation () = 2• µ

13 17

Area under curve between 13 and 17 is 68% of total areaSo P(13<x<17) = 0.68So P(15<x<17) = 0.34

Q

x

Standardising

• The x value will not always be an exact number of standard deviations away from the mean.

• We can calculate the number of standard deviations which x lies away from the mean.

• Standardised value (z) can be obtained from tables. (See handout.)

We calculate the number of standard deviations between our given ‘x value’ and the mean by the formula:

z = x -

We then use normal distribution tables to find the probability of our variable lying between the given x value and the mean

Example.

A sample of till slips from a particular supermarket show that the weekly amounts spent by the 500 customers were approximately normally distributed, with a mean of £20 and a standard deviation of £4.

Find the expected number of shoppers who:

a) Spend between £20 and £28 per week?

Q

=20

28

=4

X = 20 (mean) is equivalent toZ = 0 x = 28

Z = x – From table, when z = 2.00 Q = 0.4772■ % spending over between £20 and £28 = 0.4772 x 100 =47.72■ Number spending between £20 and £28 = 0.4772 x 500 = 23 shoppers

x

2 d.p.

z 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

0.0 0.0000 0.0040 0.0080 0.0120 0.0160 0.0199 0.0239 0.0279 0.0319 0.03590.1 0.0398 0.0438 0.0478 0.0517 0.0557 0.0596 0.0636 0.0675 0.0714 0.07530.2 0.0793 0.0832 0.0871 0.0910 0.0948 0.0987 0.1026 0.1064 0.1103 0.11410.3 0.1179 0.1217 0.1255 0.1293 0.1331 0.1368 0.1406 0.1443 0.1480 0.15170.4 0.1554 0.1591, 0.1628 0.1664 0.1700 0.1736 0.1772 0.1808 0.1844 0.1879

0.5 0.1915 0.1950 0.1985 0.2019 0.2054 0.2088 0.2123 0.2157 0.2190 0.22240.6 0.2257 0.2291 0.2324 0.2357 0.2389 0.2422 0.2454 0.2486 0.2517 0.25490.7 0.2580 0.2611 0.2642 0.2673 0.2704 0.2734 0.2764 0.2794 0.2823 0.28520.8 0.2881 0.2910 0.2939 0.2967 0.2995 0.3023 0.3051 0.3078 0.3106 0.31330.9 0.3159 0.3186 0.3212 0.3238 0.3264 0.3289 0.3315 0.3340 0.3365 0.3389

1.0 0.3413 0.3438 0.3461 0.3485 0.3508 0.3531 0.3554 0.3577 0.3599 0.36211.1 0.3643 0.3665 0.3686 0.3708 0.3729 0.3749 0.3770 0.3790 0.3810 0.38301.2 0.3849 0.3869 0.3888 0.3907 0.3925 0.3944 0.3962 0.3980 0.3997 0.40151.3 0.4032 0.4049 0.4066 0.4082 0.4099 0.4115 0.4131 0.4147 0.4162 0.41771.4 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 0.4370 0.4382 0.4394 0.4406 0.4418 0.4429 0.44411.6 0.4452 0.4463 0.4474 0.4484 0.4495 0.4505 0.4515 0.4525 0.4535 0.45451.7 0.4554 0.4564 0.4573 0.4582 0.4591 0.4599 0.4608 0.4616 0.4625 0.46331.8 0.4641 0.4649 0.4656 0.4664 0.4671 0.4678 0.4686 0.4693 0.4699 0.47061.92.02.1

0.47130.47720.4821

0.47190.47780.4826

0.47260.47830.4830

0.47320.47880.4834

0.47380.47930.4838

0.47440.47980.4842

0.47500.48030.4846

0.47560.48080.4850

0.47610.48120.4854

0.47670.48170.4857

Normal Distribution Table

b) spend more than £28 per week?

Q

=20

28

=4

x = 28

Z = x –

From table, when z = 2.00 Q = 0.4772 (as before)

■ P(x > 28) = 0.5 – Q = 0.5 - 0.4772 = 0.0228

■ % spending over £28 = 0.0228 x 100 = 2.28%

■ Number spending over £28 = 0.0228 x 500 = 11 shoppers

x

2 d.p.

Distribution Symmetry

• Note that the distribution is only provided for Z positive;

• However symmetry means that we also know areas for Z negative;

The area under the curve between 0 and a

is the same as the area between 0 and –a.

c) spend between £15 and £28?

28

= 4

Q1 Q2

15

x = 28x = 28

QQ2 2 = 0.4772 = 0.4772 ( by part a) ( by part a)

x = 15x = 15

Z =Z = x – x – = = 15 - 2015 - 20 = = -5 -5 = -1.25 = -1.25 4 44 4

From table, when z = -1.25 QFrom table, when z = -1.25 Q11 = 0.3944 = 0.3944

■ ■ P(15 < x < 28) = QP(15 < x < 28) = Q1 1 + Q+ Q2 2 = 0.4772 + 0.3944 = = 0.4772 + 0.3944 = 0.87160.8716

■ ■ Number spending between £15 and £28 is 0.8716 Number spending between £15 and £28 is 0.8716 x 500x 500

= = 435435

x

z 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

0.0 0.0000 0.0040 0.0080 0.0120 0.0160 0.0199 0.0239 0.0279 0.0319 0.03590.1 0.0398 0.0438 0.0478 0.0517 0.0557 0.0596 0.0636 0.0675 0.0714 0.07530.2 0.0793 0.0832 0.0871 0.0910 0.0948 0.0987 0.1026 0.1064 0.1103 0.11410.3 0.1179 0.1217 0.1255 0.1293 0.1331 0.1368 0.1406 0.1443 0.1480 0.15170.4 0.1554 0.1591, 0.1628 0.1664 0.1700 0.1736 0.1772 0.1808 0.1844 0.1879

0.5 0.1915 0.1950 0.1985 0.2019 0.2054 0.2088 0.2123 0.2157 0.2190 0.22240.6 0.2257 0.2291 0.2324 0.2357 0.2389 0.2422 0.2454 0.2486 0.2517 0.25490.7 0.2580 0.2611 0.2642 0.2673 0.2704 0.2734 0.2764 0.2794 0.2823 0.28520.8 0.2881 0.2910 0.2939 0.2967 0.2995 0.3023 0.3051 0.3078 0.3106 0.31330.9 0.3159 0.3186 0.3212 0.3238 0.3264 0.3289 0.3315 0.3340 0.3365 0.3389

1.0 0.3413 0.3438 0.3461 0.3485 0.3508 0.3531 0.3554 0.3577 0.3599 0.36211.1 0.3643 0.3665 0.3686 0.3708 0.3729 0.3749 0.3770 0.3790 0.3810 0.38301.2 0.3849 0.3869 0.3888 0.3907 0.3925 0.3944 0.3962 0.3980 0.3997 0.40151.3 0.4032 0.4049 0.4066 0.4082 0.4099 0.4115 0.4131 0.4147 0.4162 0.41771.4 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 0.4370 0.4382 0.4394 0.4406 0.4418 0.4429 0.44411.6 0.4452 0.4463 0.4474 0.4484 0.4495 0.4505 0.4515 0.4525 0.4535 0.45451.7 0.4554 0.4564 0.4573 0.4582 0.4591 0.4599 0.4608 0.4616 0.4625 0.46331.8 0.4641 0.4649 0.4656 0.4664 0.4671 0.4678 0.4686 0.4693 0.4699 0.47061.92.02.1

0.47130.47720.4821

0.47190.47780.4826

0.47260.47830.4830

0.47320.47880.4834

0.47380.47930.4838

0.47440.47980.4842

0.47500.48030.4846

0.47560.48080.4850

0.47610.48120.4854

0.47670.48170.4857

Normal Distribution Table

d) spend between £22 and £26?

22 26

Q1

Q2

= 4

x = 22x = 22

Z =Z = x – x – = = 22 - 2022 - 20 = = 2 2 = 0.50 = 0.50 4 44 4Q1 = 0.1915

x = 26x = 26

Z =Z = x – x – = = 26 - 2026 - 20 = = 6 6 = 1.50 = 1.50 4 44 4Q2 = 0.4332

■ ■ P(22 < x < 26) = QP(22 < x < 26) = Q22 - Q - Q1 1

= 0.4332 - 0.1915 = = 0.4332 - 0.1915 = 0.2417 0.2417 120 120 peoplepeople

x

z 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

0.0 0.0000 0.0040 0.0080 0.0120 0.0160 0.0199 0.0239 0.0279 0.0319 0.03590.1 0.0398 0.0438 0.0478 0.0517 0.0557 0.0596 0.0636 0.0675 0.0714 0.07530.2 0.0793 0.0832 0.0871 0.0910 0.0948 0.0987 0.1026 0.1064 0.1103 0.11410.3 0.1179 0.1217 0.1255 0.1293 0.1331 0.1368 0.1406 0.1443 0.1480 0.15170.4 0.1554 0.1591, 0.1628 0.1664 0.1700 0.1736 0.1772 0.1808 0.1844 0.1879

0.5 0.1915 0.1950 0.1985 0.2019 0.2054 0.2088 0.2123 0.2157 0.2190 0.22240.6 0.2257 0.2291 0.2324 0.2357 0.2389 0.2422 0.2454 0.2486 0.2517 0.25490.7 0.2580 0.2611 0.2642 0.2673 0.2704 0.2734 0.2764 0.2794 0.2823 0.28520.8 0.2881 0.2910 0.2939 0.2967 0.2995 0.3023 0.3051 0.3078 0.3106 0.31330.9 0.3159 0.3186 0.3212 0.3238 0.3264 0.3289 0.3315 0.3340 0.3365 0.3389

1.0 0.3413 0.3438 0.3461 0.3485 0.3508 0.3531 0.3554 0.3577 0.3599 0.36211.1 0.3643 0.3665 0.3686 0.3708 0.3729 0.3749 0.3770 0.3790 0.3810 0.38301.2 0.3849 0.3869 0.3888 0.3907 0.3925 0.3944 0.3962 0.3980 0.3997 0.40151.3 0.4032 0.4049 0.4066 0.4082 0.4099 0.4115 0.4131 0.4147 0.4162 0.41771.4 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 0.4370 0.4382 0.4394 0.4406 0.4418 0.4429 0.44411.6 0.4452 0.4463 0.4474 0.4484 0.4495 0.4505 0.4515 0.4525 0.4535 0.45451.7 0.4554 0.4564 0.4573 0.4582 0.4591 0.4599 0.4608 0.4616 0.4625 0.46331.8 0.4641 0.4649 0.4656 0.4664 0.4671 0.4678 0.4686 0.4693 0.4699 0.47061.92.02.1

0.47130.47720.4821

0.47190.47780.4826

0.47260.47830.4830

0.47320.47880.4834

0.47380.47930.4838

0.47440.47980.4842

0.47500.48030.4846

0.47560.48080.4850

0.47610.48120.4854

0.47670.48170.4857

Normal Distribution Table

e) what is the value below which 70% of customers spend?

Previous questions

x-value z-value Q value Answer

prob.

%

Here given %

x-value z-value Q value (Answer)

(%)

e) what is the value below which 70% of customers spend?

50%

20%

Q

20% means Q = 0.2000

Using normal table backwards, when Q = 0.2000

Taking nearest value: z = 0.52

Z =Z = x – x – 0.52 = 0.52 = x - 20x - 20 44 4 x 0.52 = x – 204 x 0.52 = x – 20

2.08 = x – 202.08 = x – 20 2.08 + 20 = x2.08 + 20 = x

Value below which 70% spend is £22.08Value below which 70% spend is £22.08

= 4

x? x

4 d.p.

z 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

0.0 0.0000 0.0040 0.0080 0.0120 0.0160 0.0199 0.0239 0.0279 0.0319 0.03590.1 0.0398 0.0438 0.0478 0.0517 0.0557 0.0596 0.0636 0.0675 0.0714 0.07530.2 0.0793 0.0832 0.0871 0.0910 0.0948 0.0987 0.1026 0.1064 0.1103 0.11410.3 0.1179 0.1217 0.1255 0.1293 0.1331 0.1368 0.1406 0.1443 0.1480 0.15170.4 0.1554 0.1591, 0.1628 0.1664 0.1700 0.1736 0.1772 0.1808 0.1844 0.1879

0.5 0.1915 0.1950 0.1985 0.2019 0.2054 0.2088 0.2123 0.2157 0.2190 0.22240.6 0.2257 0.2291 0.2324 0.2357 0.2389 0.2422 0.2454 0.2486 0.2517 0.25490.7 0.2580 0.2611 0.2642 0.2673 0.2704 0.2734 0.2764 0.2794 0.2823 0.28520.8 0.2881 0.2910 0.2939 0.2967 0.2995 0.3023 0.3051 0.3078 0.3106 0.31330.9 0.3159 0.3186 0.3212 0.3238 0.3264 0.3289 0.3315 0.3340 0.3365 0.3389

1.0 0.3413 0.3438 0.3461 0.3485 0.3508 0.3531 0.3554 0.3577 0.3599 0.36211.1 0.3643 0.3665 0.3686 0.3708 0.3729 0.3749 0.3770 0.3790 0.3810 0.38301.2 0.3849 0.3869 0.3888 0.3907 0.3925 0.3944 0.3962 0.3980 0.3997 0.40151.3 0.4032 0.4049 0.4066 0.4082 0.4099 0.4115 0.4131 0.4147 0.4162 0.41771.4 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 0.4370 0.4382 0.4394 0.4406 0.4418 0.4429 0.44411.6 0.4452 0.4463 0.4474 0.4484 0.4495 0.4505 0.4515 0.4525 0.4535 0.45451.7 0.4554 0.4564 0.4573 0.4582 0.4591 0.4599 0.4608 0.4616 0.4625 0.46331.8 0.4641 0.4649 0.4656 0.4664 0.4671 0.4678 0.4686 0.4693 0.4699 0.47061.92.02.1

0.47130.47720.4821

0.47190.47780.4826

0.47260.47830.4830

0.47320.47880.4834

0.47380.47930.4838

0.47440.47980.4842

0.47500.48030.4846

0.47560.48080.4850

0.47610.48120.4854

0.47670.48170.4857

Normal Distribution Table

Summary

• We have looked at different types of distribution – discrete and continuous;

• We have concentrated on one type of continuous distribution to analyse data from Normal distributions.