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
