<< Chapter < Page Chapter >> Page >

Definition. If C is an event having positive probability, the conditional probability of A , given C is

P ( A | C ) = P ( A C ) P ( C )

For a fixed conditioning event C , we have a new likelihood assignment to the event A . Now

P ( A | C ) 0 , P ( Ω | C ) = 1 , and P ( j A j | C ) = P j A j C P ( C ) = j P ( A j C ) / P ( C ) = j P ( A j | C )

Thus, the new function P ( | C ) satisfies the three defining properties (P1), (P2), and (P3) for probability, so that for fixed C , we have a new probability measure , with all the properties of an ordinary probability measure.

Remark . When we write P ( A | C ) we are evaluating the likelihood of event A when it is known that event C has occurred. This is not the probability of a conditional event A | C . Conditional events have no meaning in the model we are developing.

Conditional probabilities from joint frequency data

A survey of student opinion on a proposed national health care program included 250 students, of whom 150 were undergraduates and 100 were graduate students.Their responses were categorized Y (affirmative), N (negative), and D (uncertain or no opinion). Results are tabulated below.

U 60 40 50
G 70 20 10

Suppose the sample is representative, so the results can be taken as typical of the student body. A student is picked at random. Let Y be the event he or she is favorable to the plan, N be the event he or she is unfavorable, and D is the event of no opinion (or uncertain). Let U be the event the student is an undergraduate and G be the event he or she is a graduate student. The data may reasonably be interpreted

P ( G ) = 100 / 250 , P ( U ) = 150 / 250 , P ( Y ) = ( 60 + 70 ) / 250 , P ( Y U ) = 60 / 250 , etc.


P ( Y | U ) = P ( Y U ) P ( U ) = 60 / 250 150 / 250 = 60 150

Similarly, we can calculate

P ( N | U ) = 40 / 150 , P ( D | U ) = 50 / 150 , P ( Y | G ) = 70 / 100 , P ( N | G ) = 20 / 100 , P ( D | G ) = 10 / 100

We may also calculate directly

P ( U | Y ) = 60 / 130 , P ( G | N ) = 20 / 60 , etc.
Got questions? Get instant answers now!

Conditional probability often provides a natural way to deal with compound trials carried out in several steps.

Jet aircraft with two engines

An aircraft has two jet engines. It will fly with only one engine operating. Let F 1 be the event one engine fails on a long distance flight, and F 2 the event the second fails. Experience indicates that P ( F 1 ) = 0 . 0003 . Once the first engine fails, added load is placed on the second, so that P ( F 2 | F 1 ) = 0 . 001 . Now the second engine can fail only if the other has already failed. Thus F 2 F 1 so that

P ( F 2 ) = P ( F 1 F 2 ) = P ( F 1 ) P ( F 2 | F 1 ) = 3 × 10 - 7

Thus reliability of any one engine may be less than satisfactory, yet the overall reliability may be quite high.

Got questions? Get instant answers now!

The following example is taken from the UMAP Module 576, by Paul Mullenix, reprinted in UMAP Journal, vol 2, no. 4. More extensivetreatment of the problem is given there.

Responses to a sensitive question on a survey

In a survey, if answering “yes” to a question may tend to incriminate or otherwise embarrass the subject, the response given may beincorrect or misleading. Nonetheless, it may be desirable to obtain correct responses for purposes of social analysis. Thefollowing device for dealing with this problem is attributed to B. G. Greenberg.By a chance process, each subject is instructed to do one of three things:

  1. Respond with an honest answer to the question.
  2. Respond “yes” to the question, regardless of the truth in the matter.
  3. Respond “no” regardless of the true answer.

Let A be the event the subject is told to reply honestly, B be the event the subject is instructed to reply “yes,” and C be the event the answer is to be “no.” The probabilities P ( A ) , P ( B ) , and P ( C ) are determined by a chance mechanism (i.e., a fraction P ( A ) selected randomly are told to answer honestly, etc.). Let E be the event the reply is “yes.” We wish to calculate P ( E | A ) , the probability the answer is “yes” given the response is honest.


Since E = E A B , we have

P ( E ) = P ( E A ) + P ( B ) = P ( E | A ) P ( A ) + P ( B )

which may be solved algebraically to give

P ( E | A ) = P ( E ) - P ( B ) P ( A )

Suppose there are 250 subjects. The chance mechanism is such that P ( A ) = 0 . 7 , P ( B ) = 0 . 14 and P ( C ) = 0 . 16 . There are 62 responses “yes,” which we take to mean P ( E ) = 62 / 250 . According to the pattern above

P ( E | A ) = 62 / 250 - 14 / 100 70 / 100 = 27 175 0 . 154
Got questions? Get instant answers now!

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
A fair die is tossed 180 times. Find the probability P that the face 6 will appear between 29 and 32 times inclusive
Samson Reply

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play

Source:  OpenStax, Applied probability. OpenStax CNX. Aug 31, 2009 Download for free at http://cnx.org/content/col10708/1.6
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Applied probability' conversation and receive update notifications?