• We are pleased to announce that the winner of our Feedback Prize Draw for the Winter 2024-25 session and winning £150 of gift vouchers is Zhao Liang Tay. Congratulations to Zhao Liang. If you fancy winning £150 worth of gift vouchers (from a major UK store) for the Summer 2025 exam sitting for just a few minutes of your time throughout the session, please see our website at https://www.acted.co.uk/further-info.html?pat=feedback#feedback-prize for more information on how you can make sure your name is included in the draw at the end of the session.
  • Please be advised that the SP1, SP5 and SP7 X1 deadline is the 14th July and not the 17th June as first stated. Please accept out apologies for any confusion caused.

Assignment X4 Qu 4.2(i)

S

Study Break

Member
The question asks you to calculate the probability that 2 deviations lie outside the range -3 to 3.

I am just unsure about the probability given in the solution, i.e. the probability of an individual standardised deviation falling outside the range -3 to 3 should be approximately 2*0.00135.

Why is it 0.135%?

Does this following from comparings ISD's with standard normal, e.g. 34% should lie in (-1,0), 14% in (-2,-1) etc...
Also is it possible to get these percentages from the tables?

Thanks in advance.
 
Hi
Yes, you are right. The value of 0.00135 is the probabilty (from the Normal tables) of observing a unit-normal variable that is greater than 3. If you look on page 161 of the orange tables, and look up under x=3, you will find the value 0.99865. This is the probability of a unit-normal variable being less than 3. So take this from 1, and you have the required answer of 0.00135.
Everything else you say makes sense.
Good luck!
Robert
 
Back
Top