Transportation Planning

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 (sx)
• Sample size (n)
• Confidence level (a)
• Confidence interval limits (/- ?)
• Test statistic (t)
• Population size (N)
• More help http//edis.ifas.ufl.edu/PD006

5
Sample Size

• Test statistic
• Critical value (see table given a/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 Population Proportions Population Proportions
N 20,000 0 autos 1 autos
10,000 0.1 0.3
40,000 0.05 0.55
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?