STATISTICAL INFERENCE PART V CONFIDENCE INTERVALS 1

2 INTERVAL ESTIMATION Point estimation of  : The inference is a guess of a single value as the value of . No accuracy associated with it. Interval estimation for  : Specify an interval in which the unknown parameter, , is likely to lie. It contains measure of accuracy through variance.

3 INTERVAL ESTIMATION An interval with random end points is called a random interval. E.g., is a random interval that contains the true value of  with probability 0.95.

Interpretation ( ) μ (unknown, but true value) 4 90% CI  Expect 9 out of 10 intervals to cover the true μ

5 INTERVAL ESTIMATION An interval (l(x 1,x 2,…,x n ), u(x 1,x 2,…,x n )) is called a 100  % confidence interval (CI) for  if where 0<  <1. The observed values l(x 1,x 2,…,x n ) is a lower confidence limit and u(x 1,x 2,…,x n ) is an upper confidence limit. The probability  is called the confidence coefficient or the confidence level.

6 INTERVAL ESTIMATION If Pr (l(x 1,x 2,…,x n )  )= , then l(x 1,x 2,…,x n ) is called a one-sided lower 100  % confidence limit for . If Pr(  u(x 1,x 2,…,x n ))= , then u(x 1,x 2,…,x n ) is called a one-sided upper 100  % confidence limit for .

7 METHODS OF FINDING PIVOTAL QUANTITIES PIVOTAL QUANTITY METHOD: If Q=q(x 1,x 2,…,x n ) is a r.v. that is a function of only X 1,…,X n and , and if its distribution does not depend on  or any other unknown parameter (nuisance parameters), then Q is called a pivotal quantity. nuisance parameters: parameters that are not of direct interest

8 PIVOTAL QUANTITY METHOD Theorem: Let X 1,X 2,…,X n be a r.s. from a distribution with pdf f(x;  ) for  and assume that an MLE (or ss) of  exists: If  is a location parameter, then Q=  is a pivotal quantity. If  is a scale parameter, then Q= /  is a pivotal quantity. If  1 and  2 are location and scale parameters respectively, then are PQs for  1 and  2.

Note Example: If  1 and  2 are location and scale parameters respectively, then is NOT a pivotal quantity for  1 because it is a function of  2 A pivotal quantity for  1 should be a function of only  1 and X’s, and its distribution should be free of  1 and  2. 9

Example X 1,…,X n be a r.s. from Exp(θ). Then, is SS for θ, and θ is a scale parameter.  S/θ is a pivotal quantity.  So is 2S/θ, and using this might be more convenient since this has a distribution of χ²(2n) which has tabulated percentiles. 10

11 CONSTRUCTION OF CI USING PIVOTAL QUANTITIES If Q is a PQ for a parameter  and if percentiles of Q say q 1 and q 2 are available such that Pr{q 1  Q  q 2 }= , Then for an observed sample x 1,x 2,…,x n ; a 100  % confidence region for  is the set of  that satisfy q 1  q(x 1,x 2,…,x n ;  )  q 2.

12 EXAMPLE Let X 1,X 2,…,X n be a r.s. of Exp(  ),  >0. Find a 100  % CI for . Interpret the result.

13 EXAMPLE Let X 1,X 2,…,X n be a r.s. of N( ,  2 ). Find a 100  % CI for  and  2. Interpret the results.

14 APPROXIMATE CI USING CLT Let X 1,X 2,…,X n be a r.s. By CLT, The approximate 100(1−  )% random interval for μ: The approximate 100(1 −  )% CI for μ: Non-normal random sample

15 APPROXIMATE CI USING CLT Usually,  is unknown. So, the approximate 100(1  )% CI for  : When the sample size n ≥ 30, t  /2,n-1 ~N(0,1). Non-normal random sample

16 Lower confidence limit Upper confidence limit 1 -  Confidence level Graphical Demonstration of the Confidence Interval for 

17 Inference About the Population Mean when  is Unknown The Student t Distribution Standard Normal Student t 0

18 Effect of the Degrees of Freedom on the t Density Function Student t with 10 DF 0 Student t with 2 DF Student t with 30 DF The “degrees of freedom” (a function of the sample size) determines how spread the distribution is compared to the normal distribution.

19 Finding t-scores Under a t- Distribution (t-tables) t.100 t.05 t.025 t.01 t.005 Degrees of Freedom t 0.05, 10 = t 0

20 EXAMPLE A new breakfast cereal is test-marked for 1 month at stores of a large supermarket chain. The result for a sample of 16 stores indicate average sales of $1200 with a sample standard deviation of $180. Set up 99% confidence interval estimate of the true average sales of this new breakfast cereal. Assume normality.

21 ANSWER 99% CI for  : ( , ) With 99% confidence, the limits and cover the true average sales of the new breakfast cereal.

22 Checking the required conditions We need to check that the population is normally distributed, or at least not extremely nonnormal. Look at the sample histograms, Q-Q plots … There are statistical methods to test for normality