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Hypothesis Tests IEF 217a: Lecture 2.b Fall 2002

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Hypothesis Testing Correct models? Data similar? –Use one series to predict another Has something changed in the data? –Quality control, portfolio strategies

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Outline Introduction (Basketball) Proportion changes (Political polls) Difference in means (Airline arrivals, Firestone) Testing a distribution (die) Causality Multiple comparisons and data snooping Statistical power

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Outline Introduction (Basketball) Proportion changes (Political polls) Difference in means (Airline arrivals, Firestone) Testing a distribution (die) Causality Multiple comparisons and data snooping Statistical power

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Hypothesis Testing Null hypothesis –Assumption about how the world works –Assume this is true –Could data have come from this machine/theory/conjecture??? –Do you need more/other data?

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Basketball and Larry Bird Facts –Bird normally makes 48 percent of his shots –Bird has just finished a series of games where he made only 20 of 57 shots –Question: Is this the usual Larry Bird, or has something changed? –Is he in a slump? –On to matlab (bird1.m)

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Hypothesis Testing Terms Null hypothesis –Assumption about the world Test statistic –Observed statistic (Random variable) p-value (probability null is true) –Prob( shots <= 20 )

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Outline Introduction (Basketball) Proportion changes (Political polls) Difference in means (Airline arrivals, Firestone) Testing a distribution (die) Causality Multiple comparisons and data snooping Statistical power

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Political Poll Gore/Bush 0/1 Two polls (100 people) –First 50/50 –Second 55/45 What is the probability that something has changed in the population? Matlab: pollchange.m

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Outline Introduction (Basketball) Proportion changes (Political polls) Difference in means (Airline arrivals, Firestone) Testing a distribution (die) Causality Multiple comparisons and data snooping Statistical power

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Differences in Means Two samples Different means Could they be drawn from the same population? Examples –Has something changed? Flights (time) Tires (Firestone)

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Flight Delays Two series (minutes late) –Before mechanics threat of delays –After mechanics threat of delays More delays after threat Compare to pooled data Null = two series are the same Could the mean difference between the two come from the pooled series?

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Flight Delays Matlab code: airline.m Note: Fancy histogram code

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Firestone Overall tires have a failure rate of 5 in 1000 You have observed in a sample of 10,000 tires a failure rate of 60 Is something wrong with Firestone tires? Matlab: firestone.m

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Outline Introduction (Basketball) Proportion changes (Political polls) Difference in means (Airline arrivals, Firestone) Testing a distribution (die) Causality Multiple comparisons and data snooping Statistical power

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Testing a Die Problem: –You’ve observed the following rolls of a die out of 6000 rolls 1: 1014, 2: 958, 3: 986, 4: 995, 5: 1055, 6:992 –Could this have come from a fair die with probs of 1/6 for each side?

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Dietest.m Method: –Think up a test statistic –Roll 6000 dies with sample –Check how the value of the test statistic from the original data compares with the distribution from the simulations dietest.m

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Outline Introduction (Basketball) Proportion changes (Political polls) Difference in means (Airline arrivals, Firestone) Testing a distribution (die) Causality Multiple comparisons and data snooping Statistical power

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Causality Stock returns and weather Are returns higher when it is sunny? Given some data on weather and returns test this hypothesis on to matlab: sunny.m

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Outline Introduction (Basketball) Proportion changes (Political polls) Difference in means (Airline arrivals, Firestone) Testing a distribution (die) Causality Multiple comparisons and data snooping Statistical power

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Multiple Tests and Data Snooping In the search for patterns you often look at many different things –Different trading rules –Different regression runs –Different drugs Each is often tested alone Then get excited when 1 is significant

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Data Snooping and Trading Strategies Efficient markets world (no predictability) Someone claims to have a buy/sell (short/long) strategy which generates significantly large returns They pretested 10 strategies and chose the best out of the 10 Return sample is independent and normal

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Questions What is the likelihood that some “best” strategy beats a buy and hold benchmark? What if this strategy were tested to see if it was “significant” using traditional statistical tests, ignoring that it had been snooped? Matlab: snooptest.m

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Other Applications Many other trading strategies –More later Multiple regressions –Run 20 regressions of y = a + bx for different x –Report only those with significant b –Common economist sin

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Outline Introduction (Basketball) Proportion changes (Political polls) Difference in means (Airline arrivals, Firestone) Testing a distribution (die) Causality Multiple comparisons and data snooping Statistical power

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Hypothesis Tests Again P-value or significance level –Probability of rejecting null hypothesis given that it is true

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P-Value, Size, and Type I error Observe 2 Prob(x>2) Null: Normal(0,1)

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Hypothesis Tests Again Type II error –Probability of accepting null hypothesis given that it is false

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Hypothesis Tests Again Power –Probability of rejecting null hypothesis when it is false –Probability of catching a deviation

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Type I and Type II errors Which do you prefer? Mushroom/Toadstool(poison) test –Null = Mushroom –Type I: Reject mushroom given mushroom –Type II: Accept mushroom given toadstool Makes a difference

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Hypothesis Tests: Final Word Traditional Goals –Correct Size –Maximum Power Specific situations –Costs of Type II error (mushrooms) –Finance: Using incorrect model Missing risks (LTCM)

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Problems for Monte-Carlo Tests of Power Test a null hypothesis under some alternative Need to commit to which alternative Power(alternative)

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Outline Introduction (Basketball) Proportion changes (Political polls) Difference in means (Airline arrivals, Firestone) Testing a distribution (die) Causality (stocks and weather) Multiple comparisons and data snooping Statistical power

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