1. Transportation Planning CE 573 Lecture 5

2. Topics Data collection issues Sample size estimation Statistical inference

3. 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

4. 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

5. 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  

6. 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

7. 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

8. 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?