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Review You run a t-test and get a result of t = 0.5. What is your conclusion? A.Reject the null hypothesis because t is bigger than expected by chance.

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Presentation on theme: "Review You run a t-test and get a result of t = 0.5. What is your conclusion? A.Reject the null hypothesis because t is bigger than expected by chance."— Presentation transcript:

1 Review You run a t-test and get a result of t = 0.5. What is your conclusion? A.Reject the null hypothesis because t is bigger than expected by chance B.Reject the null hypothesis because t is smaller than expected by chance C.Keep the null hypothesis because t is bigger than expected by chance D.Keep the null hypothesis because t is smaller than expected by chance

2 Review Your null hypothesis states µ = 50, and your sample has a mean of M = 58. If your t statistic equals 4, what is the standard error of the mean (s M )? A.Depends on the sample size B.0.5 C.4 D.2

3 Review Your null hypothesis predicts the population mean should be µ 0 = 100. You measure a sample of 25 people and calculate statistics of M = 94 and s = 10. What is the value of your t statistic? A.5 B.-0.6 C.-3 D.2.4

4 Hypothesis Testing 10/9

5 Where Am I? Wake up after a rough night in unfamiliar surroundings Still in Boulder? Expected if in Boulder (large likelihood) Surprising but not impossible (moderate likelihood) Couldn’t happen IF in Boulder (likelihood near zero)  Can’t be in Boulder

6 Steps of Hypothesis Testing 1.State clearly the two hypotheses 2.Determine which is the null hypothesis (H 0 ) and which is the alternative hypothesis (H 1 ) 3.Compute a relevant test statistic from the sample 4.Find the likelihood function of the test statistic according to the null hypothesis 5.Choose alpha level (  ): how willing you are to abandon null (usually.05) 6.Find the critical value: cutoff with probability  of being exceeded under H 0 7.Compare the actual result to the critical value Less than critical value  retain null hypothesis Greater than critical value  reject null hypothesis; accept alternative hypothesis

7 Specifying Hypotheses Both hypotheses are statements about population parameters Null Hypothesis (H 0 ) – Always more specific, e.g. 50% chance, mean of 100 – Usually the less interesting, "default" explanation Alternative Hypothesis (H 1 ) – More interesting – researcher’s goal is usually to support the alternative hypothesis – Less precise, e.g. > 50% chance,  > 100

8 Test Statistic Statistic computed from sample to decide between hypotheses Relevant to hypotheses being tested – Based on mean if hypotheses are about means – Based on number correct (frequency) if hypotheses are about probability correct Sampling distribution according to null hypothesis must be fully determined – Can only depend on data and on values assumed by H 0 Often a complex formula with little intuitive meaning – Inferential statistic: Only used in testing reliability

9 Likelihood Function Probability distribution of a statistic according to a hypothesis – Gives probability of obtaining any possible result Usually interested in distribution of test statistic according to null hypothesis Same as sampling distribution, assuming the population is accurately described by the hypothesis Test statistic chosen because we know its likelihood function – Binomial test: Binomial distribution – t-test: t distribution

10 Critical Value Cutoff for test statistic between retaining and rejecting null hypothesis – If test statistic is beyond critical value, null will be rejected – Otherwise, null will be retained Before collecting data: What strength of evidence will you require to reject null? – How many correct outcomes? – How big a difference between M and  0, relative to  M ? Critical region – Range of values that will lead to rejecting null hypothesis – All values beyond critical value Frequency Probability t

11 Types of Errors Goal: Reject null hypothesis when it’s false; retain it when it’s true Two ways to be wrong – Type I Error: Null is correct but you reject it – Type II Error: Null is false but you retain it Type I Error rate – IF H 0 is true, probability of mistakenly rejecting H 0 – Proportion of false theories we conclude are true E.g., proportion of useless treatments that are deemed effective Logic of hypothesis testing is founded on controlling Type I Error rate – Set critical value to give desired Type I Error rate

12 Alpha Level Choice of acceptable Type I Error rate – Usually.05 in psychology – Higher  more willing to abandon null hypothesis – Lower  require stronger evidence before abandoning null hypothesis Determines critical value – Under the sampling distribution of the test statistic according to the null hypothesis, the probability of a result beyond the critical value is  Test Statistic Sampling Distribution from H 0 Critical Value 

13 Doping Analogy Measure athletes' blood for signs of doping – Cheaters have high RBCs, but even honest people vary What rule to use? – Must set some cutoff, and punish anyone above it – Will inevitably punish some innocent people H 0 likelihood function is like distribution of innocent athletes’ RBCs Cutoff determines fraction of innocent people that get unfairly punished – This fraction is alpha Distribution of Innocent Athletes RBC Don’t PunishPunish

14 Power Type II Error rate – IF H 0 is false, probability of failing to reject it – E.g., fraction of cheaters that don’t get caught Power – IF H 0 is false, probability of correctly rejecting it – Equal to one minus Type II Error rate – E.g., fraction of cheaters that get caught Power depends on sample size – Choose sample size to give adequate power – Researchers must make a guess at effect size to compute power H1H1 H0H0 Type I error rate (  ) Type II error rate H0H0 H1H1 Power

15 Two-Tailed Tests Sometimes want to detect effects in either direction – Drugs that help or drugs that hurt Formalized in alternative hypothesis –   0 Two critical values, one in each tail Type I error rate is sum from both critical regions – Need to divide errors between both tails – Each gets  /2 (2.5%) t 0t crit -t crit  /2 M 00 Reject H 0

16 One-Tailed vs. Two-Tailed Tests t 0t crit -t crit  /2 t 0t crit  One-tailed Two-tailed

17 An Alternative View: p-values Reversed approach to hypothesis testing – After you collect sample and compute test statistic – How big must  be to reject H 0 p-value – Measure of how consistent data are with H 0 – Probability of a value equal to or more extreme than what you actually got – Large p-value  H 0 is a good explanation of the data – Small p-value  H 0 is a poor explanation of the data p >  : Retain null hypothesis p <  : Reject null hypothesis; accept alternative hypothesis Researchers generally report p-values, because then reader can choose own alpha level – E.g. “p =.03” – If willing to allow 5% error rate, then accept result as reliable – If more stringent, say 1% (  =.01), then remain skeptical t = 2.15 t crit for  =.05 t crit for  =.01 t crit for  =.03  p =.03 t t

18 Review Later this semester, we’ll learn about hypothesis tests for distributions of nominal variables. For example, we’ll poll everyone on their favorite colors and count the frequency for each color. What would be a reasonable null hypothesis? A.Each color is chosen by the same number of people in the class B.Each color would be chosen by the same number of people in the population C.Some colors are more popular than others among people in this class D.Some colors are more popular than others among the population

19 Review If you run a 1-tailed t-test with a sample size of n = 10 and  =.05, the critical value is t crit = Now imagine you ran the same test, but 2-tailed. Which of the following are the new critical values? (You should be able to rule out all wrong answers.) A. 1.21, 2.41 B.-2.01, 2.45 C.-2.23, 2.23 D.-1.67, 1.67

20 Review You run a t-test and get a result of t = 0.56 and p =.32. If your chosen alpha level was 5%, what do you conclude? A.Retain the null hypothesis, because p >  B.Reject the null hypothesis, because p >  C.Retain the null hypothesis, because p < t D.Reject the null hypothesis, because p < t


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