Inferential Statistics Hypothesis tests Confidence intervals

Transcription

1 Inferential Statistics Hypothesis tests Confidence intervals Eva Riccomagno, Maria Piera Rogantin DIMA Università di Genova

2 Part G. Multiple tests Part H. Confidence intervals 1. Introduction 2. Confidence interval for the mean (a of a Normal variable known variance (b of a Normal variable unknown variance (c of a variable with unknown distribution (approximate 3. Different levels 1 α 4. Confidence intervals and tests 1

3 Part G. Multiple tests We may need to conduct many hypothesis tests concurrently Suppose each test is conducted at level α. For any one test, the chance of a false rejection of the null is α. But the chance of at least one false rejection is much higher Examples: Measuring the state of anxiety by questionnaire in two groups of subjects. Various questions help define the level of anxiety. As more questions are compared, it becomes more likely that the two groups will appear to differ on at least one topic by random chance alone. Efficacy of a drug in terms of the reduction of any one of a number of disease symptoms. It becomes more likely that the drug will appear to be an improvement over existing drugs in terms of at least one symptom. 2

4 Consider m hypothesis tests: H i 0 and Hi 1 for i = 1,..., m Example For α = 0.05 and m = 2 Probability to retain both H 1 0 and H2 0 when true: (1 α2 = = 0.90 Probability to reject at least one true hypothesis: 1 (1 α 2 = = 0.10 For α = 0.05 and m = 20 Probability to reject at least one true hypothesis: 1 (1 α 20 = = 0.64 α There are many ways to deal with this problem. Here we discuss two methods 3

5 Bonferroni (B Method Let p 1,..., p m denote the m p-values for these tests. Reject null hypothesis H i 0 if p i α m The probability of falsely rejecting any null hypotheses is less than or equal to α Example (continue m = 2: 1 (1 (0.05/2 2 = m = 20: 1 (1 (0.05/20 20 = Benjamini-Hochberg (BH Method 1. Let p (1 < < p (m denote the ordered p-values 2. Reject all null hypotheses H (i 0 for which p (i < i m α If the tests are not independent the value to compare p (i appropriately adjusted is 4

6 Example Consider the following 10 (sorted p-values. Fix α = 0.05 p=c( , , , , , , , , , alpha=0.05; m=length(p; i=seq(1,m; b=i*alpha/m; BH=(p<b; B=(p<alpha/m; cbind(p,b,bh,b p b BH B [1,] [2,] [3,] [4,] [5,] [6,] [7,] [8,] [9,] [10,] Reject H i 0 for - i = 1, 2 with Bonferroni method - i = 1, 2, 3, 4, 5 with Benjamini-Hochberg method 5

7 Abuse of test Warning! There is a tendency to use hypothesis testing methods even when they are not appropriate. Often, estimation and confidence intervals are better tools. Use hypothesis testing only when you want to test a well-defined hypothesis (from Wassermann A summary of the paper by Regina Nuzzo. (2014 Statistical Errors P values, the gold standard of statistical validity, are not as reliable as many scientists assume. Nature, vol. 506, p Ronald Fisher 1920s intended p-values as an informal way to judge whether evidence was significant for a second look one part of a fluid, non-numerical process that blended data and background knowledge to lead to scientific conclusions Interpretation the p-value summarizes the data assuming a specific null hypothesis 6

8 Caveats tendency to deflect attention from the actual size of an effect P-hacking or significance-chasing, including making assumptions, monitor data while it is being collected, excluding data points,... Measures that can help look for replicability do not ignore exploratory studies nor prior knowledge report effect sizes and confidence intervals take advantage of Bayes rule (not part of this course unfortunately try multiple methods on the same data set adopt a two-stage analysis, or preregistered replication 7

9 Part H Confidence intervals 1. Introduction 2. Confidence interval for the mean (a of a Normal variable known variance (b of a Normal variable unknown variance (c of a variable with unknown distribution (approximate 3. Different levels 1 α 4. Confidence intervals and tests 8

10 1. Introduction Let θ be a real valued parameter and L and U two real valued functions of the random sample X = (X 1,..., X n such that L(x U(x for all instances x of the random sample. Then (L, U is called an interval estimation for θ If P (θ (L, U 1 α then (L, U is a 1 α confidence interval 1 α is called the coverage of the confidence interval Usually 1 α = 0.95 We have over a 1 α chance of covering the unknown parameter with the estimator interval (from Casella Berger 9

11 From Wassermann Warning! (L, U is random and θ is fixed Warning! There is much confusion about how to interpret a confidence interval. A confidence interval is not a probability statement about θ since θ is a fixed quantity [... ] Warning! Some texts interpret confidence intervals as follows: if I repeat the experiment over and over, the interval will contain the parameter 1 α percent of the time, e.g. 95% of the time. This is correct but useless since we rarely repeat the same experiment over and over. [... ] Rather - day 1: θ 1 collect data construct a 95% IC for θ 1 - day 2: θ 2 collect data construct a 95% IC for θ 2 - day 3: θ 3 collect data construct a 95% IC for θ Then 95 percent of your intervals will trap the true parameter value. There is no need to introduce the idea of repeating the same experiment over and over 10

12 2. Confidence interval for the mean of a random variable X 1,..., X n i.i.d. random sample. Parameter of interest µ Point estimator : X; point estimate x (sample value of X at the observed data points Confidence interval or Interval estimator with coverage 1 α: ( X δ, X + δ with δ such that P ( X δ < µ < X + δ = 1 α The limit of the interval X δ and X +δ are random variables The sample confidence interval is: (x δ, x + δ How to compute δ? Using the (exact or approximate distribution of the point estimator X 11

13 2. (a Confidence interval for the mean of a Normal variable known variance For X 1,..., X n i.i.d. sample random variables with X 1 N (µ, σ 2 X N ( µ, σ2 n or Z = X µ σ/ n N (0, 1 1 α = P ( X δ < µ < X + δ = P ( µ δ < X < µ + δ Example X 1 N (µ, 4 n = 9 1 α = 0.95 X N ( µ, µ δ µ µ + δ 12

14 Computation of δ 1 α = P ( µ δ < X < µ + δ ( µ δ µ = P σ < X µ σ < µ + δ µ σ ( n n n = P δ σ n < Z < δ σ n δ σ n σ = z 1 α/2 δ = z 1 α/2 n Density functions of - Z N (0, 1 - X N (µ, 4/9 z /2 = 1.96 δ = µ δ µ µ + δ Confidence interval for µ: ( X z 1 α/2 σ n, X + z 1 α/2 σ n 13

15 Sample confidence interval for µ: ( x z 1 α/2 σ n, x + z 1 α/2 σ n - we do not know if µ belongs or not to this sample interval whose limits are computed using the sample value x - another x the interval would be different Among all possible confidence intervals constructed as before, 95% contains µ and 5% does not Simulation for 100 samples: n = 80 σ 2 = 4 1 α = 95% ( x / 80, x / 80 6 intervals do not contain µ 14

16 2. (b Confidence interval for the mean of a Normal variable unknown variance For X 1,..., X n random sample and X 1 N (µ, σ 2 as point estimator of µ and σ 2 take X and S 2 respectively Consider the random variable T = X µ S/ n t [n 1] The computation of the confidence interval for µ is similar to the normal case (X t 1 α/2 S n, X + t 1 α/2 S n 2. (c Confidence interval for the mean of a random variable with unknown distribution If the sample size is large we can use the approximate distribution of X via CLT: (X z 1 α/2 S n, X + z 1 α/2 S n 15

17 3. Different coverage coefficients 1 α A 95%-confidence interval or a 99%-confidence interval? Values of the 1 α/2 quantile of a standard normal random variable N (0, 1: z = 1.64 z = 1.96 z = What is gained in precision is lost in range Example X N (µ, 4/80 and assume x = 2.5: - at 90%: δ = 0.37 sample confidence interval (1.92, at 95%: δ = 0.44 sample confidence interval (2.06, at 99%: δ = 0.58 sample confidence interval (2.13,

18 5. Confidence intervals and tests Parameter of interest µ of a N (µ, σ with σ known Two-sided 1 α confidence interval for µ and two-sided test at level α (H 0 : µ = µ 0 and H 1 : µ µ 0 H 0 is retained for x (µ 0 δ, µ 0 + δ The sample confidence interval is (x δ, x + δ µ ( 0 ( x ( x A B where δ = z 1 α/2 σ n in both cases The interval where H 0 is retained is centered in µ 0 while the confidence interval is centered in x If the sample confidence interval contains µ 0 then H 0 is retained, and viceversa 17

19 Compare tests and confidence intervals in R output Example. Chicago Tribune (continue > np=750*0.2347; prop.test(np,750, sample proportions test with continuity correction data: np out of 750, null probability 0.25 X-squared = , df = 1, p-value = alternative hypothesis: true p is not equal to percent confidence interval: sample estimates: p > prop.test(np,750,0.25,"less" 1-sample proportions test with continuity correction data: np out of 750, null probability 0.25 X-squared = , df = 1, p-value = alternative hypothesis: true p is less than percent confidence interval: sample estimates: p Remark. If the parameter of interest is a proportion p then δ is different for confidence intervals and tests, because it depends on the standard deviation. In the first case it is calculated using the sample value ˆp, in the second one using p 0 18