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Transportation Planning CE 573 Lecture 5
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Topics Data collection issues Sample size estimation Statistical inference
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Data Collection Location –Cordon –Screen line –Isolated point Time –Peak –Off-peak –Days of week –Days of year Survey instrument –Questions –Device Personnel –Work shifts –Individual capabilities –Communications
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Sample Size Terminology Mean ( ) Sample standard deviation (s x ) Sample size (n) Confidence level ( α ) Confidence interval limits ( +/- Δ) Test statistic (t) Population size (N) More help: http://edis.ifas.ufl.edu/PD006http://edis.ifas.ufl.edu/PD006
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Sample Size Test statistic –Critical value (see table given α/2 and degrees of freedom) n-1 –Calculated value –Confidence interval limit as proportion of the mean n
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Sample Size Adjustment Adjusting for small population size (N) –Redefine n as n’, with n being the adjusted sample size n’ unadjusted population size calculated n adjusted population size
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Survey Scenario What kind of sampling should you do? –Random or –Stratified random Depends on –Cost of sending survey –Cost of processing survey Population Proportions N = 20,000 0 autos1 autos $10,0000.10.3 $40,0000.050.55
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Survey Scenario Say n must be at least 50 for each cell or population category Random sample –Smallest proportion 0.05 –Total sample size is 50/0.05 = 1,000 observations Stratified sample –Preliminary survey of 1,000 –Get addresses of 50 households in each cell –Total sample size is 4 * 50 = 200 (50 for each cell) –Do you need to adjust the sample size by cell?
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