Sampling Distributions: Central Limit Theorem

Transcription

1 Review for Exam 2

2 Sampling Distributions: Central Limit Theorem Conceptually, we can break up the theorem into three parts: 1. The mean (µ M ) of a population of sample means (M) is equal to the mean (µ) of the underlying population of scores 2. The standard deviation (σ M ) of a population of sample means (called the standard error) becomes smaller as the size of the sample (n, the number of scores used to compute M) increases 3. Whatever the distribution of the underlying scores, the sampling distribution of the mean will approach the normal distribution as the sample size (n) increases

3 p(m) Exam 2 Review Sample Size & Standard Error n 1 M 7 n 4 M n 16 M (M)

4 Sampling Error & Sampling Distributions Exam 2 Review Sampling error prevents us from directly comparing sample statistics to population parameters to evaluate hypotheses The sampling distribution tells us how much variability (error) to expect in our sample statistics (e.g., M) The standard error of a statistic (the standard deviation of its sampling distribution) depends both on the variability of the underlying population scores (σ) and on the size of the sample (n).

5 Hypothesis Testing The purpose of a hypothesis test is to decide between two explanations: 1. H 0 : The difference between the sample and the population can be explained by sampling error (there does not appear to be an effect of treatment or condition) 2. H 1 : The difference between the sample and the population is too large to be explained by sampling error (there does appear to be an effect of treatment or condition).

6 Hypothesis Testing The null hypothesis is the default hypothesis that there is NO treatment effect We compute the sampling distribution of the statistic of interest (e.g., the mean exam score of the sample) under the assumption that the null hypothesis (H 0 ) is true We determine the critical values & rejection regions What values of the statistic, were they observed, would be extreme enough to make us reject the null hypothesis? Generally, we will compute the sampling distribution of a standardized test statistic (e.g., z, t, or F statistics).

7 0 1 Population distributions Raw scores (x) n 4 M n 2 Sampling distributions M crit Sample means (M)

8 Errors in Hypothesis Testing P = α P = 1-β P = 1-α P = β

9 Step 3: Make a Decision critical region (extreme 5%) reject H 0 middle 95% retain H 0 α= 0.05; 2-tailed test M µ from H 0 z Z= 1.60 Retain H 0 : no difference

10 critical region (extreme 5%) reject H 0 middle 95% retain H 0 α= 0.05; 2-tailed test n = 16 M µ from H 0 z z = 2.86 reject H 0 : significant difference

11 Hypothesis testing: Steps 1. Determine appropriate test and hypotheses 2. Use distribution table to find critical statistic value(s) representing rejection region Remember: the rejection region represents values that you would be unlikely to obtain if the null hypothesis were true 3. Compute appropriate test statistic from data 4. Make a decision: does the statistic for your sample fall into the critical (rejection) region or not?

12 Computing Mean-Difference Statistics 2 M 0 ˆM ˆ Test Sample Data Hypothesized Population Parameter Estimated Standard Error Estimated Variance Exam 2 Review Degrees of Freedom z-test M µ σ 2 n σ 2 n Singlesample t-test M µ s 2 n s 2 = SS df df = n 1 Relatedsamples t-test M D, where D = x 2 x 1 μ D =0 s D 2 n D s D 2 = SS D df D df = n D 1 Independentsamples t-test M 1 M 2 μ 1 μ 2 =0 s p s p s 2 p = SS 1 + SS 2 df = n n 1 n 2 df 1 + df 1 +n Post-hoc tests M A M B μ A μ B =0 MS error n A + MS error n B MS error = SS error df error df error

13 yes z-test one Is σ provided? no One-sample t-test Number of Samples two Are scores matched across samples? yes no Related samples t-test Independent samples t-test >2 ANOVA

14 Example Scenario A professor thinks that this year s freshman class seems to be smarter than previous classes. To test this, she administers an IQ test to a sample of 36 freshman and computes the mean (M=114.5) and standard deviation (s = 18) of their scores. College records indicate that the mean IQ across previous years was What is the appropriate statistical test for this problem? a) A z-test b) A one-sample t-test c) An independent-samples t-test d) A related-samples t-test

15 Example Scenario A psychologist examined the effect of exercise on a standardized memory test. Scores on this test for the general population form a normal distribution with a mean of 50 and a standard deviation of 8. A sample of 62 people who exercise at least 3 hours per week has a mean score of 57. What is the appropriate statistical test for this problem? a) A z-test b) A one-sample t-test c) An independent-samples t-test d) A related-samples t-test

16 Example Scenario A researcher studies the effect of a drug on nightmares in veterans with PTSD. A sample of clients with PTSD kept count of their nightmares for 1 month before treatment. They were then given the medication and asked to record counts of their nightmares again for a month following treatment. What is the appropriate statistical test for this problem? a) A z-test b) A one-sample t-test c) An independent-samples t-test d) A related-samples t-test

17 Example Scenario A neurologist had two groups of patients with different types of aphasia (a brain disorder) name objects presented to them as line drawings. He wanted to determine whether the number of objects correctly named differed across the two groups. What is the appropriate statistical test for this problem? a) A z-test b) A one-sample t-test c) An independent-samples t-test d) A related-samples t-test

18 Computational Example 1 A psychologist is investigating the effect of being an only child on personality. A sample of 30 only children is obtained and each child is given a standardized personality test. Population scores on the test form a normal distribution with µ = 50 and σ =15. a) If the mean for the sample is 58, what can the researcher conclude about his hypothesis? Use a two-tailed test with α = b) Compute a 90% confidence interval for the mean of the only child population

19 Clicker Question What is the appropriate statistical test for part (a) of this problem? a) A z-test b) A one-sample t-test c) An independent-samples t-test d) A related-samples t-test

20 Example 1: Research Hypothesis H 1 : Only children have different personality scores than the general population I.e., µ only children µ general population Null Hypothesis H 0 : Only children have personality scores that are not significantly different than those of the general population I.e., µ only children = µ general population We have,, n, and M

21 Find Critical z value z Upper-Tail Probabilities

22 Example1: Compute z-statistic n 30 M 58.0 z crit 1.96 z M n ; reject H 0 The personalities of only children differ significantly from those of the general population z = 2.92, p =

23 Example 1: Compute Confidence Interval b) Compute a 90% confidence interval for the mean of the only child population Recall that to compute a confidence interval, you: Exam 2 Review 1. Select a level of confidence and look up the corresponding t α (or z α ) values in the t (or z) distribution table. 2. Use two-tailed probabilities (e.g., for z α, look up p(z>z) = α/2) 3. The confidence interval is computed by inverting the t (or z) transformation CI 0.1 M z M M z n

24 z Upper-Tail Probabilities

25 Example 1: Compute Confidence Interval Exam 2 Review n 30 M 58.0 z CI 0.90 M z 0. 1 n [53.49, 62.51]

26 Top Incorrect Problems (Exam 2) If the population from which we sample is normal, the sampling distribution of the mean a) will approach normal for large sample sizes. b) will be normal. c) will be normal only for small samples. d) will be slightly positively skewed.

27 Top Incorrect Problems (Exam 2) If the population from which we sample is normal, the sampling distribution of the mean a) will approach normal for large sample sizes. b) will be normal. c) will be normal only for small samples. d) will be slightly positively skewed. Why? If the population is normal, then the distribution of the mean for any sample size (including a sample size of 1) will be normal.

28 Top Incorrect Problems (Exam 2) Sampling distributions help us test hypotheses about means by a) telling us what kinds of means to expect if the null hypothesis is false. b) telling us what kinds of means to expect if the null hypothesis is true. c) telling us how variable the population is. d) telling us exactly what the population mean is.

29 Top Incorrect Problems (Exam 2) Sampling distributions help us test hypotheses about means by a) telling us what kinds of means to expect if the null hypothesis is false. b) telling us what kinds of means to expect if the null hypothesis is true. c) telling us how variable the population is. d) telling us exactly what the population mean is. Why? Sampling distributions are the distributions of a sample statistic. In many hypothesis tests (e.g., t-tests and ANOVAs), we use these distributions to predict the distribution of means or mean differences under the null hypothesis.

30 Example 2 An educational psychologist studies the effect of frequent testing on retention of class material. In one section of an introductory course, students are given quizzes each week. A second section of the same course receives only two tests during the semester. At the end of the semester, a sample from each of the sections receives the same final exam, and the number of errors made are recorded. X 1 (quiz) X 2 (no quiz) M SS Does frequent testing significantly affect retention of class material? Use a two-tailed test, with α = 0.05.

31 Clicker Question What is the appropriate statistical test for this problem? a) A z-test b) A one-sample t-test c) An independent-samples t-test d) A related-samples t-test

32 Example 2: Research Hypothesis H 1 : Testing frequency affects retention of class material I.e., µ quiz µ no quiz Null Hypothesis H 0 : Testing frequency does not significantly affect retention of class material I.e., µ quiz = µ no quiz We have no population data, and sample data from two independent samples, so we must use the independentsamples t-test df = n 1 + n 2 2 = = 11

33 t-distribution Table t One-tailed test α/2 -t t Two-tailed test α α/2 Level of significance for one-tailed test Level of significance for two-tailed test df

34 Compute t Statistic M SS n 1 M SS n df 11 t crit s M Compute Pooled Variance: s SS SS p df1 df Estimate Standard Error: M 1 2 s n 2 p s n 2 p Compute t-statistic: t df t 11 M s 1 2 M M M ; retain H 0 Frequent testing does not significantly affect the amount of information retained by students t(11) = 1.64, p>0.05.

35 Top Incorrect Problems (Exam 2) The reason why we need to solve for t instead of z in some situations is due to a) the sampling distribution of the mean. b) the size of our sample mean. c) the sampling distribution of the sample size. d) the sampling distribution of the variance.

36 Top Incorrect Problems (Exam 2) The reason why we need to solve for t instead of z in some situations is due to a) the sampling distribution of the mean. b) the size of our sample mean. c) the sampling distribution of the sample size. d) the sampling distribution of the variance. Why? We solve for the z-statistic using σ, which is a constant, but we solve for the t-statistic using the sample standard deviation s which varies from sample to sample.