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Sampling Distributions, Hypothesis Testing and One-sample Tests Media Violence • Does violent content in a video affect later behavior? Bushman (1998) • Two groups of 100 subjects saw a video Violent video versus nonviolent video • Then free associated to 26 homonyms with aggressive & nonaggressive forms. e.g. cuff, mug, plaster, pound, sock Cont. Media Violence--cont. • Results Mean number of aggressive free associates = 7.10 • Assume we know that without aggressive video the mean would be 5.65, and the standard deviation = 4.5 These are parameters (m and s) • Is 7.10 enough larger than 5.65 to conclude that video affected results? Sampling Distribution of the Mean • We need to know what kinds of sample means to expect if video has no effect. i. e. What kinds of means if m = 5.65 and s = 4.5? This is the sampling distribution of the mean. Cont. Sampling Distribution Number of Aggressive Associates 1400 1200 Fr equency 1000 800 600 400 Std. Dev = .45 200 Mean = 5.65 0 N = 10000.00 7. 7. 6. 6. 6. 6. 5. 5. 5. 5. 4. 4. 4. 4. 3. 25 00 75 50 25 00 75 50 25 00 75 50 25 00 75 Mean Number A ggr essive Ass ociates Cont. Sampling Distribution of the Mean--cont. • The sampling distribution of the mean depends on Mean of sampled population • Why? St. dev. of sampled population • Why? Size of sample • Why? Cont. Sampling Distribution of the mean--cont. • Shape of the sampling distribution Approaches normal • Why? Rate of approach depends on sample size • Why? • Basic theorem Central limit theorem Central Limit Theorem • Given a population with mean = m and standard deviation = s, the sampling distribution of the mean (the distribution of sample means) has a mean = m, and a standard deviation = s /n. The distribution approaches normal as n, the sample size, increases. Demonstration • Let population be very skewed • Draw samples of 3 and calculate means • Draw samples of 10 and calculate means • Plot means • Note changes in means, standard deviations, and shapes Cont. Parent Population Skewed Population 3000 Frequency 2000 1000 Std. Dev = 2.43 Mean = 3.0 N = 10000.00 0 .0 20 .0 18 .0 16 .0 14 .0 12 .0 10 0 8. 0 6. 0 4. 0 2. 0 0. X Cont. Sampling Distribution n = 3 Sampling Distribution Sample size = n = 3 Frequency 2000 1000 Std. Dev = 1.40 Mean = 2.99 N = 10000.00 0 0 .0 13 0 .0 12 0 .0 11 0 .0 10 00 9. 00 8. 00 7. 00 6. 00 5. 00 4. 00 3. 00 2. 00 1. 00 0. Sample Mean Cont. Sampling Distribution n = 10 Sampling Distribution Sample size = n = 10 1600 1400 Frequency 1200 1000 800 600 400 Std. Dev = .77 200 Mean = 2.99 N = 10000.00 0 50 6. 00 6. 50 5. 00 5. 50 4. 00 4. 50 3. 00 3. 50 2. 00 2. 50 1. 00 1. Sample Mean Cont. Demonstration--cont. • Means have stayed at 3.00 throughout-except for minor sampling error • Standard deviations have decreased appropriately • Shapes have become more normal--see superimposed normal distribution for reference Steps in Hypothesis Testing • Define the null hypothesis. • Decide what you would expect to find if the null hypothesis were true. • Look at what you actually found. • Reject the null if what you found is not what you expected. The Null Hypothesis • The hypothesis that our subjects came from a population of normal responders. • The hypothesis that watching a violent video does not change mean number of aggressive interpretations. • The hypothesis we usually want to reject. Important Concepts • Concepts critical to hypothesis testing Decision Type I error Type II error Critical values One- and two-tailed tests Decisions • When we test a hypothesis we draw a conclusion; either correct or incorrect. Type I error • Reject the null hypothesis when it is actually correct. Type II error • Retain the null hypothesis when it is actually false. Type I Errors • Assume violent videos really have no effect on associations • Assume we conclude that they do. • This is a Type I error Probability set at alpha () • usually at .05 Therefore, probability of Type I error = .05 Type II Errors • Assume violent videos make a difference • Assume that we conclude they don’t • This is also an error (Type II) Probability denoted beta () • We can’t set beta easily. • We’ll talk about this issue later. • Power = (1 - ) = probability of correctly rejecting false null hypothesis. Critical Values • These represent the point at which we decide to reject null hypothesis. • e.g. We might decide to reject null when (p|null) < .05. Our test statistic has some value with p = .05 We reject when we exceed that value. That value is the critical value. One- and Two-Tailed Tests • Two-tailed test rejects null when obtained value too extreme in either direction Decide on this before collecting data. • One-tailed test rejects null if obtained value is too low (or too high) We only set aside one direction for rejection. Cont. One- & Two-Tailed Example • One-tailed test Reject null if violent video group had too many aggressive associates • Probably wouldn’t expect “too few,” and therefore no point guarding against it. • Two-tailed test Reject null if violent video group had an extreme number of aggressive associates; either too many or too few. Testing Hypotheses: s known • H0: m = 5.65 • H1: m 5.65 (Two-tailed) • Calculate p (sample mean) = 7.10 if m = 5.65 • Use z from normal distribution • Sampling distribution would be normal Using z To Test H0 • Calculate z X m 7.1 5.65 1.45 z 3.22 s 4.5 .45 n 1.96, reject 100 H0 • If z > + • 3.22 > 1.96 The difference is significant. Cont. z--cont. • Compare computed z to histogram of sampling distribution • The results should look consistent. • Logic of test Calculate probability of getting this mean if null true. Reject if that probability is too small. Testing When s Not Known • Assume same example, but s not known • Can’t substitute s for s because s more likely to be too small See next slide. • Do it anyway, but call answer t • Compare t to tabled values of t. Sampling Distribution of the Variance 1400 1200 138.89 Population variance = 138.89 Frequency 1000 n=5 800 10,000 samples 600 58.94% < 138.89 400 200 0 0 0. 80 0 0. 75 0 0. 70 0 0. 65 0 0. 60 0 0. 55 0 0. 50 0 0. 45 0 0. 40 0 0. 35 0 0. 30 0 0. 25 0 0. 20 0 0. 15 0 0. 10 .0 50 0 0. Sample variance t Test for One Mean • Same as z except for s in place of s. • For Bushman, s = 4.40 X m 7.1 5.65 1.45 t 3.30 s 4.40 .44 n 100 Degrees of Freedom • Skewness of sampling distribution of variance decreases as n increases • t will differ from z less as sample size increases • Therefore need to adjust t accordingly • df = n - 1 • t based on df t Distribution Two-Tailed Significance Level df 10 15 20 25 30 100 .10 1.812 1.753 1.725 1.708 1.697 1.660 .05 2.228 2.131 2.086 2.060 2.042 1.984 .02 2.764 2.602 2.528 2.485 2.457 2.364 .01 3.169 2.947 2.845 2.787 2.750 2.626 Conclusions • With n = 100, t.0599 = 1.98 • Because t = 3.30 > 1.98, reject H0 • Conclude that viewing violent video leads to more aggressive free associates than normal. Factors Affecting t • Difference between sample and population means • Magnitude of sample variance • Sample size Factors Affecting Decision • Significance level • One-tailed versus two-tailed test Size of the Effect • We know that the difference is significant. That doesn’t mean that it is important. • Population mean = 5.65, Sample mean = 7.10 • Difference is nearly 1.5 words, or 25% more violent words than normal. Cont. Effect Size (cont.) • Later we will express this in terms of standard deviations. 1.45 units is 1.45/4.40 = 1/3 of a standard deviation. Confidence Limits on Mean • Sample mean is a point estimate • We want interval estimate Probability that interval computed this way includes m = 0.95 CI .95 X t.025 s X For Our Data CI X t sX .95 .025 7.1 1.98 0.44 7.1 0.87 6.23 m 7.97 Confidence Interval • The interval does not include 5.65--the population mean without a violent video • Consistent with result of t test. • Confidence interval and effect size tell us about the magnitude of the effect. • What can we conclude from confidence interval?