torrance transit system sampling method analysis presented by paul m. yun
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Torrance Transit System Sampling Method Analysis
presented by Paul M. Yun
Contents
• Background
• Understanding Question
• Approach
• Writing
Background
• Letter from Torrance Transit System to Dr. Drew.
Their Question
If we only do two round-trips per survey day (362 round-trips, or 724 one-way trips), would our data meet the 95% confidence , 10% accuracy FTA criteria? If not, would there be a way of adding a few extra surveys to meet the standard?
What????
Sampling Schemes
• FTA(Federal Transportation Administration) Sampling scheme: Every 2nd day/ 3 one-way trips/ 549 one-way trips per year
• Torrance Transit Sampling scheme: Every 2nd day/ 2 round trips/ 362 round trips (724 one-way trips)
Finding what to do
Things to do
• Read US Department Transportation Booklet for Sampling Procedure.
• Contact with David Banks, statistician of US Department of Transportation.
• Read relevant documents from Torrance Transit System.
Close examination
Idea!!
Initial thought
• Correlation between the average travel mileage in Westbound & the average travel mileage in Eastbound
Confirmation
I got you!!
Meeting with folks in Torrance Transit System
Homework Assignment to Torrance Transit System folks
• Collect a sample of 13 round trips.
Magic number
.3188
• Var[ m ] =
• n-2 ( Σi Var[ Xi ] + Σ i≠j Cov[ Xi, Xj ])
• = n-2 ( n σ2 + nc )
• = (σ2 + c )/n = σ2 (1 + ρ )/n
• Let’s find the correlation coefficient for which the proposed sampling scheme provides information equivalent to the FTA sampling scheme.
• σ2/549 = σ2 (1 + ρ )/ 724
• ρ=.3188.
0.536
• Estoy mucha triste!!
Find the sample size (n) required to meet the federally defined criteria.
• ρ = 0.536
• σ2/549 = σ2 (1 + ρ )/ n
• σ2/549 = σ2 (1 + 0.536 )/ n
• n = 1.536 X 549
• n = 844 one-way trips or 422 round trips
Conclusion
• Torrance Transit System needs to make 60 more round-trips (= 422 – 362) or 120 more one-way trips (= 60 X 2).
Torrance Transit System is HAPPY!!
• We saved over $7,000!!
Me??
Lots of work!!
Lots of money!!
Lots of Learning!!
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