Results are out! Find what you need...fast. Get quick advice or join the chat
x

Unlock these great extras with your FREE membership

  • One-on-one advice about results day and Clearing
  • Free access to our personal statement wizard
  • Customise TSR to suit how you want to use it

Stats Qustion - can get rep and lots of thanks from me!! :D

Announcements Posted on
Rate your uni — help us build a league table based on real student views 19-08-2015
  1. Offline

    ReputationRep:
    Its this June 2009 paper question from OCR.. it uses logs but i swear i have never come across this topic through out the year!


    Q9. repeated independent trials of a certain experiment are carried out. On each trial the probability of success is 0.12.
    i) Find the smallest value of n such that the probability of at least one success in n trials is more than 0.95.

    ii) Find the probability that the 3rd success occurs on the 7th trial

    the answer for part i) says log0.05/log0.88

    i know that 1-0.95 is where they get 0.05 from and 1-0.12 is 0.88

    but i don't understand why/how they used logs and whether logs is always applied with these sorts of questions
  2. Offline

    ReputationRep:
    :bump:
  3. Offline

    ReputationRep:
    (Original post by HEY_101)
    Its this June 2009 paper question from OCR.. it uses logs but i swear i have never come across this topic through out the year!


    Q9. repeated independent trials of a certain experiment are carried out. On each trial the probability of success is 0.12.
    i) Find the smallest value of n such that the probability of at least one success in n trials is more than 0.95.

    ii) Find the probability that the 3rd success occurs on the 7th trial

    the answer for part i) says log0.05/log0.88

    i know that 1-0.95 is where they get 0.05 from and 1-0.12 is 0.88

    but i don't understand why/how they used logs and whether logs is always applied with these sorts of questions
    you can use trial and improvement... just keep evaluating (0.88)n until it is less than 0.05.... because you want 1 - 0.88n to be more than 0.95
  4. Offline

    ReputationRep:
    why do you do 0.88n and not use any other numbers

    and how do you use logs for this question, the mark scheme used logs and im confused how they used it
  5. Offline

    ReputationRep:
    oh no wait i get ur method but how do you use logs though??
  6. Offline

    ReputationRep:
    So hard to explain this on here.

    Do you know about Binomial Distribution?

    Let X be the prob of success

    therefore X- B(n, .12)

    We want the P(X>=1) > .95

    P(X>=1) = 1-P(X=0)

    so we want the P(X=0) <= .05

    which is nC0 x .88^n <= .05

    we are left with .88^n <=.05

    you understand it so far? try logging both sides yourself to get the minimum value for n
  7. Offline

    ReputationRep:
    For part two, you know that the P(success) = .12 and P(failure) = .88

    just try working it out by using S for sucess and F for fail and work out how many variations there can be when the 3rd success is on the 7th trial


    e.g SSFFFFS, SFSFFFS, SFFSFFS, etc
  8. Offline

    ReputationRep:
    Quote me for anymore help, feels so great to help people on here after all the help I have got (and hopefully will continue to get) from others on TSR
  9. Offline

    ReputationRep:
    OHHHHHHH yeah i did the working out for log and got the answer finally!!! ill give all u guys reps!!

    and like the title, MANY THANKS!!

Reply

Submit reply

Register

Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?
  2. this can't be left blank
    this email is already registered. Forgotten your password?
  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. By joining you agree to our Ts and Cs, privacy policy and site rules

  2. Slide to join now Processing…

Updated: April 1, 2012
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

New on TSR

Rate your uni

Help build a new league table

Poll
How do you read?
Study resources
Quick reply
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.