Summary: the confidence interval for the mean (σ 2 known) with gaussian assumption

Transcription

1 Summary: the confidence interval for the mean (σ known) with gaussian assumption on X Let X be a Gaussian r.v. with mean µ and variance σ. If X 1, X,..., X n is a random sample drawn from X then the confidence interval for µ of level 1 can be written as follows µ ( X n ± z 1 ) σ n The same result holds if X is any other random variable provided that the sample size is not so small. 1

2 Confidence intervals Confidence interval for the mean (σ unknown) under gaussian assumption If the value of the variance σ is not known we estimate it using S n. If X 1, X,..., X n is random sample i.i.d as X then the confidence interval for µ with confidence level 1 can be rewritten in the form µ X n ± t (n 1) 1 S n n

3 Student-t random variable The value t (n 1) in the previous formula of the confidence interval for the mean 1 / iwth unknown variance is the analogue of z 1 but calculated for the Student-t random variable. That value can be found in a opportune statistical table. The Student-t distribution is quite similar to the gaussian in shape but it has higher tails and it is more concentrated around the mean. If we denote by T n 1 the Student-t random variable with n 1 degrees of freedom we have that ( P T n 1 < t (n 1) ) = 1 1 / Example: Calculate P ( T 8 < t ) = 0.90; P ( T 6 > t ) = 0.99; P ( T 11 >.5 ) ; P ( T 3 < t ) =

4 Student-t random variable g p = P(t t pg ) = tp f(x)dx p 0 t p g The shaded area correspond to p = P freedom and p is the probability ( T g < t (g) p ) where g are the degrees of 4

5 Example: weight, in grams, of some grains of dust on silicon circuits is supposed to be normally distributed with parameters µ and σ. Data are riportati di seguito: After getting an estimate for µ build confidence interval with confidence level 95% eand 99% assuming σ unknown Let us calculate x n x n = 1 n n i=1 x i = =

6 As σ ìn unknown we use the statistics s n s n = 1 n 1 n i=1 (x i x n ) = 0.85 The confidence interval can be obtained from the formula s µ x n n ± n t(n 1) 1 6

7 In the two cases the values of Student-t are and Substituting the values o t (11) t (11) = t (11) =.01 = t (11) = µ (1.10,.7) level 95%[(1.16,.1) σ known] µ (0.86,.51) level 99%[(1.00,.37) σ known] Warning 1: for fixed n the length of the interval grows with the confidence level Warning : for given n and 1, the length of the interval is bigger if the variance is to be estimated Warning 3: the length of a confidence interval depends on the following two quantities: the sample size n and the confidence level 1. 7

8 The right choice of the sample size n The length of the confidence interval when σ is known, is L(n, ) = ( X n + z 1 = z 1 σ n σ n ( X n z 1 The length of the interval L(n, ) when σ is not known is L(n, ) = X n + t n 1 s 1 n n = t n 1 1 s n n X n t n 1 1 )) σ n which also depends on s n. The length of first interval (σ known) is not affected by the value of σ from one sample to another s n n 8

9 We want to find n such that L(n, ) < C and then L(n, ) = z 1 n > σ n < C ( ) σ z 1 C z 1 σ C < n 9

10 Suppose now that we are interested in a confidence interval for the proportion p in a Bernoulli scheme. If X i are i.i.d. Bernoulli random variable with unknown parameter p we have seen that n i=1 X i Bin(n, p). We know that we can use the gaussian approximation for the binomial random variable in the framework of large samples. The natural estimator for p is ˆp n which is essentially a Binomial random variable rescaled by the factor 1/n. For large n we have then Z = ˆp n p p(1 p) n N(0, 1) Making the same steps as in previous cases we get the following interval p(1 p) p ˆp n ± z 1 n but we cannot calculate it as p is unknown. 10

11 We can then bypass this problem by plugging in ˆp n in place of p in the previous interval and we obtain an asymptotic and approximated confidence interval for p Confidence interval for the proportion Let X be a Bernoulli random variable with mean p. If X 1, X,..., X n is i.i.d. random sample drawn from X then the asymptotic (and approximated) confidence interval for p of level 1 can be written in this form p ˆp n ± z 1 ˆp n (1 ˆp n ) n 11

12 Hypotheses testing If we have some idea of a possible value of the unknown parameter we can test this hypothesis to a test which, after empirical evidence, allows us to accept or reject our thesis. Hypotheses testing for the mean Assume a gaussian model X with unknown mean µ and known variance σ. Vi would like to verify the hypothesis that the true value of the mean is µ 0. We denote this hypothesis by which is also called the null hypotheses H 0 : µ = µ 0 A test is a decision rule that leads to two alternatives only: either we reject the null hypothesis H 0 or we accept it (better: there is no evidence for rejecting it) The decision is based on the observation of an i.i.d. sample 1

13 Given that the decision is based on a random sample, there is always the possibility of taking the wrong decision. Such errors are reported below We have then Reject H 0 Non Rejection of H 0 H 0 is true I st type error no errors 1 H 0 is false no errors II nd type error 1 β β = P (rejection H 0 H 0 is true) β = P (non rejection H 0 H 0 is false) In general, we will observe value of x n different from µ 0. A decision rule of a test will then consider the distance of x n to µ 0 to figure if it is low or high. Switching to random variables: the test will consider the distance between X n and µ 0 to verify that this is not too high (in probability). 13

14 To decide when to reject H 0 we always need to specify an alternative hypothesis H 1 which can be of any kind but different from H 0. We start considering a bilateral alternative hypothesis. This hypothesis is denoted by H 1 : µ µ 0 We can deduce a decision rule of the following kind: if X n µ 0 is greater than some fixed value k we reject the null hypothesis H 0 : µ = µ 0 in favor of the alternative hypothesis H 1 : µ µ 0 14

15 How to choose k? Let us fix in such a way to guarantee that, with that particular choice of k we will make, at most, a first type error at most equal to The value of k must satisfy The value k = k is said test threshold P ( X n µ 0 > k H 0 true) = How to determine it? 15

16 If H 0 is true, then X n N(µ 0, σ /n) e Form which follows X n µ 0 σ n N(0, 1) = P ( X n µ 0 > k H 0 ) = P = P Z > k n σ = P By the symmetry of Z the value k is such that X n µ 0 σ n Z < k σ n e Z > k σ = z 1 cioè k = σ z n n 1 > k σ n H 0 k n σ 16

17 Summary: Let us denote by Z the test statistic Z = X n µ 0 σ n Assume that for a particular sample we obtain z as realization of Z z = x n µ 0 σ n The test tells us to reject the null hypothesis H 0 : µ = µ 0 in favor of H 1 : µ µ 0 if z lies outside the (rejection region) of the interval ( z 1, z 1 ) called acceptation region of the test. 17

18 Regione di rifiuto Regione di accettazione Regione di rifiuto z 0 z 1 18

19 There could be other type of alternative hypotheses. If we assume a null hypothesis against the alternative H 0 : µ = µ 0 H 1 : µ > µ 0 = P ( X n µ 0 > k H 0 ) = P The test will reject the null hypothesis if Z > k n σ z = x n µ 0 σ n > z 1 19

20 Regione di rifiuto Regione di accettazione 0 z 1 0

21 Of course, if the hypothesis is H 1 : µ < µ 0 the test will reject for values of z too small and in particular if z < z 1

22 Regione di rifiuto Regione di accettazione z 0

23 Let us summarize in a global scheme Test on the mean (σ known) If X is a gaussian random variable with unknown mean µ and variance σ known. If X 1, X,..., X n is an i.i.d. sample drawn from X then, at level, the test that verifies hypothesis of the type H 0 : µ = µ 0, has the following rejection region on the bases of the following alternatives: if H 1 : µ µ 0, Reject H 0 se z > z 1 when H 1 : µ > µ 0, Reject H 0 se z > z 1 when H 1 : µ < µ 0, Reject H 0 se z < z where z = x n µ 0 σ n 3

24 If the variance is unknown, we estimate it using s n. Test for the mean (σ nota) If X is a gaussian random variable with unknown parameters µ and σ. If X 1, X,..., X n is random sample i.i.d. drawn from X the, at level, the test that verifies H 0 : µ = µ 0, has the following rejection test when H 1 : µ µ 0, when H 1 : µ > µ 0, when H 1 : µ < µ 0, where Reject H 0 se t > t n 1 1 Reject H 0 se t > t n 1 1 Reject H 0 se t < t n 1 t = x n µ 0 e s n s n n = s n 4

25 Hypothesis testing for the proportion: we build a test to verify the distance between p 0 (our null hypothesis) and the observed sample proportion ˆp n. The decision rule has always the same form but it is based on the following test statistics Z Z = ˆp n p 0 p 0 (1 p 0 ) n Please notice that, contrary to what we did for the confidence interval case, we use the true variance under H 0, p 0 (1 p 0 )/n, to normalize the distance ˆp n p 0 and not the approximated variance ˆp n (1 ˆp n )/n 5

26 Test on the proportion Let X be a Bernoulli random variable with parameter p and X 1, X,..., X n a i.i.d. random sample drawn from X. The level test to verify the hypothesis H 0 : p = p 0, has the following form for different alternative hypotheses if H 1 : p p 0, Reject H 0 se z > z 1 if H 1 : p > p 0, Reject H 0 se z > z 1 where if H 1 : p < p 0, Reject H 0 se z < z z = ˆp n p 0 p 0 (1 p 0 ) n Be careful: this is an asymptotic test which can be used if n >> 30. 6