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PROBABILITY DISTRIBUTIONS
Business Statistics
Probability distribution functions (discrete)
Characteristics of a discrete distribution
Example: uniform (discrete) distribution
Example: Bernoulli distribution
Example: binomial distribution
Probability density functions (continuous)
Characteristics of a continuous distribution
Example: uniform (continuous) distribution
Example: normal (or Gaussian) distribution
Example: standard normal distribution
Back to the normal distribution
Approximations to distributions
Old exam question
Further study
CONTENTS
Today we want to speed up. We will skip some slides or postpone a few. Prepare
well, we want to start the statistical topics as soon as
possible.
โช A sample space is called discrete when its elements can be
counted
โช We will code the elements of a discrete sample space ๐ as
1,2,3, โฆ , ๐ or 0,1,2, โฆ , ๐ โ 1โช Examples
โช die ๐ฅ โ 1,2,3,4,5,6 , so ๐ = 1,2,3,4,5,6โช coin ๐ฅ โ 0,1โช number of broken TV sets ๐ฅ โ 0,1,2,โฆ
PROBABILITY DISTRIBUTION FUNCTIONS (DISCRETE)
Distribution function
๐ ๐ฅ = ๐ ๐ = ๐ฅ
โช the probability that the (discrete) random variable ๐assumes the value ๐ฅ
โช alternative notation: ๐๐ ๐ฅ
PROBABILITY DISTRIBUTION FUNCTIONS (DISCRETE)
Note our convention:
capital letters (๐) for random variables
lowercase letters (๐ฅ) for values
Example
โช die: ๐ ๐ฅ =
1
6if ๐ฅ = 1
1
6if ๐ฅ = 2
1
6if ๐ฅ = 3
1
6if ๐ฅ = 4
1
6if ๐ฅ = 5
1
6if ๐ฅ = 6
0 otherwise
PROBABILITY DISTRIBUTION FUNCTIONS (DISCRETE)
Example: flipping a coin 3 times
โช sample space ๐ = ๐ป๐ป๐ป,๐ป๐ป๐,๐ป๐๐ป, ๐๐ป๐ป,โฆโช define the random variable ๐ = number of heads
โช distribution function ๐ ๐ฅ =
1
8if ๐ฅ = 0
3
8if ๐ฅ = 1
3
8if ๐ฅ = 2
1
8if ๐ฅ = 3
0 otherwise
โช or: ๐๐ 0 =1
8, ๐๐ 1 =
3
8, ๐๐ 2 =
3
8, ๐๐ 3 =
1
8
PROBABILITY DISTRIBUTION FUNCTIONS (DISCRETE)
โช ๐ ๐ฅ is a (discrete) probability distribution function (pdf or
PDF)
โช ๐ ๐ฅ = ๐ ๐ = ๐ฅ expresses the probability that ๐ = ๐ฅโช A random variable ๐ that is distributed with pdf ๐ is written
as
๐~๐
โช Some properties of the pdf:โช 0 โค ๐ ๐ฅ โค 1
โช a probability is always between 0 and 1โช ฯ๐ฅโ๐๐ ๐ฅ = 1
โช the probabilities of all elementary outcomes add up to 1
PROBABILITY DISTRIBUTION FUNCTIONS (DISCRETE)
โช A pdf may have one or more parameters to denote a
collection of different but โsimilarโpdfs
โช Example: a regular die with ๐ faces
โช ๐ ๐ = ๐ฅ;๐ = ๐๐ ๐ฅ;๐ = ๐ ๐ฅ;๐ =1
๐(for ๐ฅ = 1,โฆ ,๐)
โช ๐~๐ ๐
PROBABILITY DISTRIBUTION FUNCTIONS (DISCRETE)
๐ = 4 ๐ = 6 ๐ = 8 ๐ = 12 ๐ = 20
In addition to the (discrete) probability distribution function
(pdf)
โช ๐ ๐ = ๐ฅ = ๐๐ ๐ฅ = ๐ ๐ฅwe define the (discrete) cumulative distribution function (cdf or
CDF)
๐น ๐ฅ = ๐น๐ ๐ฅ = ๐ ๐ โค ๐ฅ
and therefore
๐น ๐ฅ =
๐=โโ
๐ฅ
๐ ๐ = ๐ =
๐=โโ
๐ฅ
๐ ๐
PROBABILITY DISTRIBUTION FUNCTIONS (DISCRETE)
Depending on how we
count, you may also start
at ๐ = 0 or ๐ = 1
Example
โช die: ๐ ๐ = 2 =1
6, but ๐ ๐ โค 2 = ๐ ๐ = 1 +
๐ ๐ = 2 =1
3
โช Some properties of the cdf:โช ๐น โโ = 0 and ๐น โ = 1โช monotonously increasing
PROBABILITY DISTRIBUTION FUNCTIONS (DISCRETE)
โช pdf
โช cdf
PROBABILITY DISTRIBUTION FUNCTIONS (DISCRETE)
Expected value of ๐
๐ธ ๐ =
๐=1
๐
๐ฅ๐๐ ๐ = ๐ฅ๐ =
๐=1
๐
๐ฅ๐๐ ๐ฅ๐
โช Exampleโช die with ๐ 1 = ๐ 2 = โฏ = ๐ 6 =
1
6โช ๐ธ ๐ = 1 ร
1
6+ 2 ร
1
6+ 3 ร
1
6+ 4 ร
1
6+ 5 ร
1
6+ 6 ร
1
6=
7
2= 3
1
2โช Interpretation: mean (average)
โช alternative notation: ๐ or ๐๐โช so ๐ธ ๐ = ๐๐
โช Note difference between ๐ and the sample mean าง๐ฅโช e.g., rolling a specific die ๐ = 100 times may return a mean าง๐ฅ = 3.72 or
3.43โช while ๐ = 7/2, always (property of die, property of โpopulationโ)
CHARACTERISTICS OF A DISCRETE DISTRIBUTION
Variance
var ๐ =
๐=1
๐
๐ฅ๐ โ ๐ธ ๐2๐ ๐ฅ๐
โช Interpretation: dispersionโช alternative notation: ๐2 or ๐๐
2 or ๐ ๐
โช so var ๐ = ๐๐2
โช Note difference between ๐2 and the sample variance ๐ 2
โช e.g., rolling a specific die 100 times may return a variance ๐ 2 = 2.86 or 3.04
โช while ๐2 =35
12, always (property of die, property of โpopulationโ)
โช And of course: standard deviation ๐๐ = var ๐
CHARACTERISTICS OF A DISCRETE DISTRIBUTION
Transformation rules of random variable ๐ and ๐โช For means:
โช ๐ธ ๐ + ๐ = ๐ + ๐ธ ๐โช ๐ธ ๐๐ = ๐๐ธ ๐โช ๐ธ ๐ + ๐ = ๐ธ ๐ + ๐ธ ๐
โช For variances:โช var ๐ + ๐ = var ๐โช var ๐๐ = ๐2var ๐โช if ๐ and ๐ independent (so if cov ๐, `๐ ):
โช var ๐ + ๐ = var ๐ + var ๐โช if ๐ and ๐ dependent:
โช var ๐ + ๐ = var ๐ + 2cov ๐, ๐ + var ๐
CHARACTERISTICS OF A DISCRETE DISTRIBUTION
โช Generalization of fair die:โช equal probability of integer outcomes from ๐ through ๐
โช conditions: ๐, ๐ โ โค, ๐ < ๐โช zero probability elsewhere
โช uniform discrete distribution
โช pdf: ๐ ๐ฅ; ๐, ๐ = เต1
๐โ๐+1๐ฅ โ โค and ๐ฅ โ ๐, ๐
0 otherwiseโช Examples:
โช coin: ๐ = 0, ๐ = 1โช die: ๐ = 1, ๐ = 6
โช Random variable:โช ๐~๐ ๐, ๐
EXAMPLE: UNIFORM DISTRIBUTION
EXAMPLE: UNIFORM DISTRIBUTION
No need to memorize or even
discuss this sheet. Most
information is either on the
formula sheet or unimportant.
โช Example: choose a random number from 1 through 100with equal probability and denote it by ๐โช random variable: ๐~๐ 1,100
โช pdf: ๐ ๐ฅ = ๐ ๐ = ๐ฅ =1
100(๐ฅ โ 1,2,โฆ , 100 )
โช cdf: ๐น ๐ฅ = ๐ ๐ โค ๐ฅ =๐ฅ
100(๐ฅ โ 1,2,โฆ , 100 )
โช expected value: ๐ธ ๐ = 501
2
โช variance: var ๐ =9999
12โ 833.25
โช Sample (๐ = 1000): โช values (e.g.): 45, 96, 33, 7, 44, 96, 20, โฆโช mean: าง๐ฅ = 50.92 (e.g.)
โช variance: ๐ ๐ฅ2 = 823.25 (e.g.)
EXAMPLE: UNIFORM DISTRIBUTION
Given are two dice, with outcomes ๐ and ๐.
a. Find ๐ธ ๐ + ๐b. Find var ๐ + ๐
EXERCISE 1
โช Bernoulli experimentโช random experiment with 2 discrete outcomes (coin type)
โช head, true, โsuccessโ, female: ๐ = 1โช tail, false, โfailโ, male: ๐ = 0โช Bernoulli distribution
โช Examples:โช winning a price in a lottery (buying one ticket)
โช your luggage arrives in time at a destination
โช Probability of success is parameter ๐ (with 0 โค ๐ โค 1)โช ๐ 1 = ๐ ๐ = 1 = ๐โช ๐ 0 = ๐ ๐ = 0 = 1 โ ๐
โช Random variableโช ๐~๐ต๐๐๐๐๐ข๐๐๐ ๐ or ๐~๐๐๐ก ๐
EXAMPLE: BERNOULLI DISTRIBUTION
โช Expected valueโช ๐ธ ๐ = ๐ (obviously!)
โช Varianceโช var ๐ = ๐ 1 โ ๐โช variance zero when ๐ = 0 or ๐ = 1 (obviously!)
โช variance maximal when ๐ = 1 โ ๐ =1
2(obviously!)
โช pdf: ๐ ๐ฅ; ๐ = แ๐ if ๐ฅ = 1
1 โ ๐ if ๐ฅ = 00 otherwise
โช cdf: (not so interesting)
EXAMPLE: BERNOULLI DISTRIBUTION
โช Repeating a Bernoulli experiment ๐ timesโช ๐ is total number of โsuccessesโ
โช ๐ ๐ = ๐ฅ is probality of ๐ฅ โsuccessesโ in sample
โช ๐ = ๐1 + ๐2 +โฏ+ ๐๐โช where ๐๐ is the outcome of Bernoulli experiment number ๐ =1,2,โฆ , ๐
โช ๐ has a binomial distribution
EXAMPLE: BINOMIAL DISTRIBUTION
โช Exampleโช flip a coin 10 times:๐ is number of โheads upโ
โช roll 100 dice: ๐ is number of โsixesโ
โช produce 1000 TV sets: ๐ is number of broken sets
โช What is important?โช the number of repitions (๐)
โช the probability of success (๐) per item
โช the constancy of ๐โช the independence of the โexperimentsโ
EXAMPLE: BINOMIAL DISTRIBUTION
โช Expected valueโช ๐ธ ๐ = ๐๐ (obviously!)
โช Varianceโช var ๐ = ๐๐ 1 โ ๐โช minimum (0) when ๐ = 0 or ๐ = 1 (obviously!)
โช maximum for given ๐ when ๐ = 1 โ ๐ =1
2(obviously!)
โช pdf:
โช ๐ ๐ฅ; ๐, ๐ =๐!
๐ฅ! ๐โ๐ฅ !๐๐ฅ 1 โ ๐ ๐โ๐ฅ (๐ฅ โ 0,1,2,โฆ , ๐ )
โช cdf:โช ๐น ๐ฅ; ๐, ๐ = ฯ๐=0
๐ฅ ๐ ๐ฅ; ๐, ๐
โช Random variable:โช ๐~๐๐๐ ๐, ๐ or ๐~๐๐๐๐๐ ๐, ๐
EXAMPLE: BINOMIAL DISTRIBUTION
Recall the factorial function:
5! = 5 ร 4 ร 3 ร 2 ร 1
โช Example:โช roll 10 dice: what is the distribution of ๐ = number of โsixesโ?
โช What is the probability model?โช you repeat an experiment 10 times (๐ = 10)
โช with a probability ๐ =1
6of success and a probability 1 โ ๐ =
5
6of failure per
experiment
โช What is the probability distribution?
โช ๐~๐๐๐ 10,1
6
โช where the random variable ๐ represents the total number of sixes
โช so ๐ is not the outcome of a roll of the die!
โช ๐ธ ๐ = 10 ร1
6= 1
2
3
โช so we expect on average 12
3sixes in 10 rolls
โช var ๐ = 10 ร1
6ร
5
6=
25
18
EXAMPLE: BINOMIAL DISTRIBUTION
EXAMPLE: BINOMIAL DISTRIBUTION
No need to memorize or even discuss this
sheet. Most information is either on the
formula sheet or unimportant.
โช Calculating pdf and cdf values
โช Example: binomial distrbution with ๐ = 8, ๐ = 0.5โช what is ๐ 3 = ๐ ๐ = 3 (pdf)?
โช what is ๐น 3 = ๐ ๐ โค 3 (cdf)?
โช Different methods:โช using a graphical calculator (not at the exam)
โช using the formula (see next slides)
โช using a table (see next slides)
โช using Excel (see the computer tutorials)
โช using online calculators (figure out for yourself)
EXAMPLE: BINOMIAL DISTRIBUTION
โช pdf using the formula
โช ๐ 3; 8,0.5 =8!
3! 8โ3 !0.53 1 โ 0.5 8โ3 = 0.2188
โช or
โช ๐ 3; 8,0.5 = 830.53 1 โ 0.5 8โ3 = 0.2188
โช using the binomial coefficient ๐๐
= ๐๐ถ๐ =๐!
๐! ๐โ๐ !
EXAMPLE: BINOMIAL DISTRIBUTION
At the exam, you can just use the tables.
Much easier!
โช pdf using the table in Appendix Aโช ๐ 3; 8,0.50 = 0.2188
EXAMPLE: BINOMIAL DISTRIBUTION
โช At the exam: non-cumulative table only
โช Problem: how to do the cdf?
โช Use the definition:
๐น ๐ฅ = ๐ ๐ โค ๐ฅ =
๐=0
๐ฅ
๐ ๐ = ๐
โช ๐ ๐ โค 3 = ๐ ๐ = 0 + ๐ ๐ = 1 + ๐ ๐ = 2 +๐ ๐ = 3
โช use table, four times
EXAMPLE: BINOMIAL DISTRIBUTION
โช Exampleโช ๐น 3; 8,0.50 = 0.0039 + 0.0313 + 0.1094 + 0.2188
EXAMPLE: BINOMIAL DISTRIBUTION
Note that this table gives a
pdf, not a cdf
โช Note that cdf is ๐น ๐ฅ = ๐ ๐ โค ๐ฅโช How to find ๐ ๐ < ๐ฅ ?
โช use ๐ ๐ โค ๐ฅ = ๐ ๐ โค ๐ฅ โ 1โช How to find ๐ ๐ > ๐ฅ ?
โช use ๐ X > x = 1 โ ๐ ๐ โค ๐ฅโช How to find ๐ ๐ฅ1 < ๐ < ๐ฅ2 ?
โช use ๐ ๐ฅ1 < ๐ < ๐ฅ2 = ๐ ๐ < ๐ฅ2 โ ๐ ๐ โค ๐ฅ1โช Etc.
EXAMPLE: BINOMIAL DISTRIBUTION
โช Use such rules to efficiently use the (pdf) table (๐ = 8)โช ๐ ๐ โค 7 = ๐ 0 + ๐ 1 +โฏ+ ๐ 7
โช Much easier:โช ๐ ๐ โค 7 = 1 โ ๐ 8
EXAMPLE: BINOMIAL DISTRIBUTION
Example:
โช Context:โช on average, 20% of the emergency room patients at Greenwood
General Hospital lack health insurance
โช In a random sample of 4 patients, what is the probability
that at least 2 will be uninsured?
EXAMPLE: BINOMIAL DISTRIBUTION
โช Binomial model (patient is uninsured or not, ๐uninsured =0.20)โช ๐ is number of uninsured patients in sample
โช ๐ ๐ โฅ 2 = ๐ ๐ = 2 + ๐ ๐ = 3 + ๐ ๐ = 4 =0.1536 + 0.0256 + 0.0016 = 0.1808
EXAMPLE: BINOMIAL DISTRIBUTION
Note that this table gives a
pdf, not a cdf
Discrete distributionsโช probability distribution function (pdf): ๐ ๐ฅ = ๐ ๐ = ๐ฅโช probability of obtaining the value ๐ฅ
Continuous distributionsโช the probability of obtaining the value ๐ฅ is 0โช define probability density function (pdf): ๐ ๐ฅ
โช ๐ ๐ โค ๐ โค ๐ = ๐๐๐ ๐ฅ ๐๐ฅ
โช probability of obtaining a value between ๐ and ๐
PROBABILITY DENSITY FUNCTION (CONTINUOUS)
Compare with the
probability distribution
function (pdf) ๐ ๐ = ๐ฅfor the discrete case
The red curve is the pdf, ๐ ๐ฅThe integral is the grey area
under the pdf
So pdf refers to two distinct but related things:โช probability distribution function ๐ ๐ฅ (discrete case)
โช probability density function ๐ ๐ฅ (continuous case)
Note also that the dimensions are differentโช ๐ is a dimensionless probability
โช example:
โช if ๐ is in kg, the discrete pdf ๐ ๐ is dimensionless
โช while the continuous pdf ๐ ๐ฅ is in 1/kg
PROBABILITY DENSITY FUNCTION (CONTINUOUS)
Because ๐ ๐ฅ ๐๐ฅ should be
dimensionless, and ๐๐ฅ is in in kg
In addition to the probability density function ...โช ๐ ๐ฅ = ๐๐ ๐ฅ
... we define the cumulative distribution function (cdf or CDF)
๐น ๐ฅ = ๐ ๐ โค ๐ฅ = เถฑ
โโ
๐ฅ
๐ ๐ฆ ๐๐ฆ
Some properties of the cdf:โช ๐น โโ = 0 and ๐น โ = 1โช monotonously increasing
PROBABILITY DENSITY FUNCTION (CONTINUOUS)
Compare with
๐น ๐ฅ = ๐ ๐ โค ๐ฅ =
๐=โโ
๐ฅ
๐ ๐ = ๐
for the discrete case
๐ฅ
๐น ๐ฅ
โช pdf
โช cdf
PROBABILITY DENSITY FUNCTION (CONTINUOUS)
๐ 70 โค ๐ โค 75
= เถฑ
70
75
๐ ๐ฅ ๐๐ฅ
๐ 70 โค ๐ โค 75= ๐น 75 โ ๐น 70
โช Expected value
๐ธ ๐ = เถฑ
โโ
โ
๐ฅ๐ ๐ฅ ๐๐ฅ
โช Example: let ๐ ๐ฅ = 1 for ๐ฅ โ 0,1
โช ๐ธ ๐ = 01๐ฅ๐๐ฅ = แ
1
2๐ฅ2
0
1=
1
2
โช Interpretation: mean (average)โช alternative notation for ๐ธ ๐ : ๐ or ๐๐
CHARACTERISTICS OF A CONTINUOUS DISTRIBUTION
Compare with
๐ธ ๐ =
๐=1
๐
๐ฅ๐๐ ๐ฅ
for the discrete case
โช Variance
var ๐ = เถฑ
โโ
โ
๐ฅ โ ๐ธ ๐2๐ ๐ฅ ๐๐ฅ
โช Interpretation: dispersionโช alternative notation for var ๐ : ๐2 or ๐๐
2 or V(๐)
CHARACTERISTICS OF A CONTINUOUS DISTRIBUTION
Compare with
var ๐ =
๐=1
๐
๐ฅ๐ โ ๐ธ ๐2๐ ๐ฅ๐
for the discrete case
โช Analogy with uniform discrete distributionโช equal density for all outcomes between ๐ and ๐
โช condition: ๐ < ๐โช zero probability elsewhere
โช uniform continuous distribution
โช pdf: ๐ ๐ฅ; ๐, ๐ = เต1
๐โ๐๐ฅ โ ๐, ๐
0 otherwise
โช or easier: ๐ ๐ฅ; ๐, ๐ =1
๐โ๐(๐ฅ โ ๐, ๐ )
โช Examples:โช โstandardโ uniform deviate: ๐ = 0, ๐ = 1
EXAMPLE: UNIFORM (CONTINUOUS) DISTRIBUTION
Example: let ๐ be exam grade of randomly selected studentโช assume uniform distribution: ๐~๐ 1,10โช what is ๐ ๐ โฅ 6.5 ?
Solutionโช use ๐ ๐ โฅ 6.5 = 1 โ ๐ ๐ < 6.5 = 1 โ ๐ ๐ โค 6.5
โช cdf: ๐ ๐ โค ๐ฅ = ๐น ๐ฅ = โโ๐ฅ๐ ๐ฆ ๐๐ฆ
โช uniform continuous with ๐ = 1 and ๐ = 10
โช pdf: ๐ ๐ฅ =1
9(๐ฅ โ 1,10 )
โช cdf: ๐ ๐ โค ๐ฅ = 1๐ฅ 1
9๐๐ฆ =
1
9๐ฅ โ 1
โช answer: ๐ ๐ โฅ 6.5 = 1 โ1
96.5 โ 1
โช or: area of black rectangle
EXAMPLE: UNIFORM (CONTINUOUS) DISTRIBUTION
For a continuous distribution
๐ ๐ < ๐ฅ = ๐ ๐ โค ๐ฅbecause ๐ ๐ = ๐ฅ = 0
1 6.5 10
1
9
๐ ๐ โฅ 6.5 is the black area
โช Expected value
โช ๐ธ ๐ =๐+๐
2
โช Variance
โช var ๐ =๐โ๐ 2
12๐)๐๐ฅ โ
๐+๐
2
2ร
1
๐โ๐๐๐ฅ =
๐โ๐ 2
12)
โช pdf
โช ๐ ๐ฅ =1
๐โ๐
โช cdfโช ๐น ๐ฅ =
๐ฅโ๐
๐โ๐
โช Random variableโช ๐~๐ ๐, ๐ or ๐~โ๐๐ 0, ๐ or ๐~โ๐๐ ๐ etc.
EXAMPLE: UNIFORM (CONTINUOUS) DISTRIBUTION
โช pdf
โช ๐ ๐ฅ; ๐, ๐ =1
๐ 2๐๐โ1
2
๐ฅโ๐
๐
2
โช cdf
โช ๐น ๐ฅ = โโ๐ฅ๐ ๐ฆ; ๐, ๐ ๐๐ฆ =? ? ?
โช Expected valueโช ๐ธ ๐ = ๐
โช Varianceโช var ๐ = ๐2
โช Random variableโช ๐~๐ ๐, ๐ or ๐~๐ ๐, ๐2
EXAMPLE: NORMAL (OR GAUSSIAN) DISTRIBUTION
In a concrete case indicate the
parameterโs symbol:
๐ 12, ๐ = 2 or ๐ 12, ๐2 = 4
Remember notation ๐๐ for expected
value and ๐๐2 for variance.
So here ๐๐ = ๐ and ๐๐2 = ๐2.
This is no coincedence!
Now, ๐ = 3.1415 ...
โช Some characteristicsโช range: ๐ฅ โ โโ,โโช pdf has maximum at ๐ฅ = ๐โช pdf is symmetric around ๐ฅ = ๐โช not too interesting for ๐ฅ < ๐ โ 3๐ and for ๐ฅ > ๐ + 3๐
EXAMPLE: NORMAL (OR GAUSSIAN) DISTRIBUTION
โช Normal distribution with ๐ = 0 and ๐ = 1โช so a 0-parameter distribution: standard normal
โช pdf
โช ๐ ๐ฅ =1
2๐๐โ
1
2๐ฅ2
โช cdfโช ๐น ๐ฅ = โโ
๐ฅ๐ ๐ฆ ๐๐ฆ =? ? ?= ฮฆ ๐ฅ
โช with ฮฆ โโ = 0, ฮฆ โ = 1, ฮฆ 0 = 0.5, ๐ฮฆ
๐๐ฅ= ๐ ๐ฅ
โช Expected valueโช ๐ธ ๐ = 0
โช Varianceโช var ๐ = 1
โช Random variableโช ๐~๐ 0,1 , we often write ๐~๐ 0,1
EXAMPLE: STANDARD NORMAL DISTRIBUTION
Remember the trick:
if you donโt know
something, just give it
a name
โช Important because any normally distributed variable can be
โstandardizedโ to standard normal distribution
โช Methods for determing the values of ฮฆ ๐ฅ :โช using a graphical calculator (not at the exam)
โช not using a formula (no formula available for ฮฆ ๐ฅ )
โช using a table (see next slides)
โช using Excel (see the computer tutorials)
โช using online calculators (figure out for yourself)
EXAMPLE: STANDARD NORMAL DISTRIBUTION
โช Calculating the value of the cdf with a tableโช ๐ ๐ โค 1.36 = ฮฆ 1.36โช table C-2 (p.768): ๐ ๐ โค 1.36 = 0.9131
EXAMPLE: STANDARD NORMAL DISTRIBUTION
Note that cdf is ๐ ๐ โค ๐ฅโช How to find ๐ ๐ < ๐ฅ ?
โช use ๐ ๐ โค ๐ฅ (why?)
โช How to find ๐ ๐ > ๐ฅ ?โช use 1 โ ๐ ๐ โค ๐ฅ (why?)
โช or use ๐ ๐ > ๐ฅ = ๐ ๐ < โ๐ฅ (why?)
โช How to find ๐ ๐ โฅ ๐ฅ ?โช is easy now ...
โช How to find ๐ ๐ฅ โค ๐ โค ๐ฆ ?โช use ๐ ๐ โค ๐ฆ โ ๐ ๐ โค ๐ฅ
โช Etc.
EXAMPLE: STANDARD NORMAL DISTRIBUTION
= โ
Scale for standard normal,
but this applies to any
continuous distribution
โช Inverse lookupโช ๐ ๐ โค ๐ฅ = ฮฆ ๐ฅ = 0.90โช table C-2 (p.768): ๐ฅ โ 1.28
EXAMPLE: STANDARD NORMAL DISTRIBUTION
No need to know this table by heart...
but two values can be convenient to know
โช ๐ ๐ โค 1.96 = 0.95, a ๐ง-value as large as 1.96 or
larger occurs only with 5% probability
โช ๐ โ1.645 โค ๐ โค 1.645 = 0.95, a ๐ง-value as large as
1.96 or larger or as small as โ1.645 or smaller occurs
only with 5% probability
โช so remember 1.96 and 1.645โช (you can always look them up if you forgot or are unsure)
EXAMPLE: STANDARD NORMAL DISTRIBUTION
Note: ๐~๐ ๐, ๐2 โ ๐ โ ๐~๐ 0, ๐2 โ๐โ๐
๐~๐ 0,1
โช Standardization
โช ๐ฅ โ ๐ง =๐ฅโ๐
๐and ๐ โ ๐ =
๐โ๐
๐
โช If ๐~๐ ๐, ๐2 , how to determine ๐ ๐ โค ๐ฅ ?
โช ๐ ๐ โค ๐ฅ = ๐ ๐ โ ๐ โค ๐ฅ โ ๐ = ๐๐โ๐
๐โค
๐ฅโ๐
๐= ๐ ๐ โค
๐ฅโ๐
๐
โช Exampleโช suppose ๐~๐ 180, ๐2 = 25
โช ๐ ๐ โค 190 = ๐ ๐ โค190โ180
5= ๐ ๐ โค 2 = 0.9772
โช ๐ ๐ โค ๐ฅ = 0.90 = ๐ ๐ โค๐ฅโ180
5โ
๐ฅโ180
5= 1.28 โ ๐ฅ = 186.4
BACK TO THE NORMAL DISTRIBUTION
This is our way of doing
normalcdf and invnorm if you
donโt have a graphical calculator!
โช What is โnormalโ about the normal distribution?โช it has quite a weird pdf formula
โช and an even weirder cdf formula
โช Butโช it is unimodal
โช it is symmetric
โช very often empirical distributions โlookโ normal
โช a quantity is approximately normal if it is influenced by many
additive factors, none of which is dominating
โช several statistics (mean, sum, ...) are normally distributed
โช Youโll learn that soonโช when we discuss the Central Limit Theorem (CLT)
BACK TO THE NORMAL DISTRIBUTION
โช Scalingโช If ๐~๐ ๐๐, ๐๐
2 then ๐๐ + ๐~๐ ๐๐๐ + ๐, ๐2๐๐2
โช Additivityโช If ๐~๐ ๐๐, ๐๐
2 and ๐~๐ ๐๐, ๐๐2 and ๐, ๐ independent, then
๐ + ๐~๐ ๐๐ + ๐๐, ๐๐2 + ๐๐
2
PROPERTIES OF THE NORMAL DISTRIBUTION
pdf of 0.825๐ + 11
pdf of ๐
Sometimes, we can approximate a โdifficultโ distribution by a
โsimplerโ one
โช Important case: binomial normalโช example 1: flipping a coin (๐ = 0.50, ๐ = #heads) very often
APPROXIMATIONS TO DISTRIBUTIONS
โช But also when ๐ โ 0.50โช example 2: flipping a biased coin (๐ = 0.30, ๐ = #heads) very
often
APPROXIMATIONS TO DISTRIBUTIONS
๐ = 10; ๐ = .30 ๐ = 20; ๐ = .30 ๐ = 40; ๐ = .30
โช binomial normalโช ๐๐๐ ๐, ๐ ๐ ๐, ๐2
โช using ๐ =? ? ? and ๐2 =? ? ?
We know that when ๐~๐๐๐ ๐, ๐โช ๐ธ ๐ = ๐๐โช var ๐ = ๐๐ 1 โ ๐
So, replaceโช ๐ = ๐๐โช ๐2 = ๐๐ 1 โ ๐
So,โช ๐๐๐ ๐, ๐ ๐ ๐๐, ๐๐ 1 โ ๐
โช rule: allowed when ๐๐ โฅ 5 and ๐ 1 โ ๐ โฅ 5
APPROXIMATIONS TO DISTRIBUTIONS
The book says โฅ 10instead of โฅ 5
โช Example binomial normalโช roll a die ๐ = 900 times
โช study the occurrence of โsixesโ (so ๐ =1
6)
โช what is the probability of no more then 170 โsixesโ?
โช Exact: ๐๐๐๐ ๐=900;๐=1/6 X โค 170 =?
โช Two problems:โช need to add 171 pdf-terms (๐ ๐ = 0 until ๐ ๐ = 170 )
โช 900! gives an ERROR
โช Approximation: ๐๐ ๐=150;๐2=125 ๐ โค 170 =
๐๐ ๐ โค170โ150
125= ฮฆ๐ 1.7888 โ 0.9631
APPROXIMATIONS TO DISTRIBUTIONS
900 ร1
6= 150
900 ร1
6ร 1 โ
1
6= 125
โช Now take ๐~๐๐๐ 18,0.5โช In a โbinomialโ context ๐ ๐ โค 11 = ๐ ๐ < 12โช But in a โnormalโ context ๐ ๐ โค 11 = ๐ ๐ < 11
โช So, take care about using integers
โช Safest: go half-way: ๐ ๐ โค 11.5 = ๐ ๐ < 11.5โช This is the continuity correction
APPROXIMATIONS TO DISTRIBUTIONS
The intuitive notion of the continuity correctionโช when approximating a discrete distribution by a continuous
distribution
APPROXIMATIONS TO DISTRIBUTIONS
๐๐๐๐ ๐ โค 7 โ ๐๐ ๐ โค 71
2๐๐๐๐ ๐ โฅ 7 โ ๐๐ ๐ โฅ 6
1
2
Improving previous result
โช without continuity correctionโช ๐๐๐๐ ๐=900;๐=1/6 X โค 170 = ๐๐ ๐=150;๐2=125 (
)
๐ โค
170 = ๐๐ ๐ โค170โ150
125= ฮฆ๐ 1.788 โ 0.9631
โช with continuity correctionโช ๐๐๐๐ ๐=900;๐=1/6 X โค 170 = ๐๐ ๐=150;๐2=125 (
)
๐ โค
170.5 = ๐๐ ๐ โค170.5โ150
125= ฮฆ๐ 1.833 โ 0.9664
APPROXIMATIONS TO DISTRIBUTIONS
30 June 2014, Q1d
OLD EXAM QUESTION
30 June 2014, Q1f
OLD EXAM QUESTION
Doane & Seward 5/E 6.1-6.4, 6.8, 7.1-7.5
Tutorial exercises week 1
discrete probability distributions
continuous probability distributions
expectation and variance
FURTHER STUDY
Recommended