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Stat 100
• Read Chapter 14
Time Series
• A collection of measurements of a variable made at a sequence of time points
Albuquerque BATmobile• Albuquerque, NM – began aggressive
program of Breathalyzer Testing for DUI
• Police had several vans (BATmobiles) equipped to do testing
• Analyzed quarterly counts of fatal and injury accidents
• 29 quarters before BATmobiles and 23 after
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QTR
AC
CQuarterly Data ; Red = BAT periodAlberquerque Fatal and Injury Accidents
Feature of Data
• Seasonal - always more accidents in the third quarter
• Upward Trend prior to BAT program
• Perhaps reduction after the BAT program
Seasonally Adjusted Data
• Seasonally adjusted =data adjusted to take away seasonal effects
• Use seasonally adjusted data to see trend more clearly
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Time
Mov
. Avg
Trend Based on Four Quarter Moving Averages
Red = BAT Period
Observational or Experiment?
• Is the comparison of the BATmobile time to the pre BATmobile time an observational study or an experiment?
It’s an experiment (but not a good one)
• “Researchers” created two different situations.
• But, there easily could be confounding in a time series experiment.
• Additional safety factors could have changed during the BATmobile period
Important Components of Time Series
• Trend - overall up or down pattern
• Seasonality - pattern related to seasons of the year
• Random Fluctuation - completely random effects
Note about detecting seasonal patterns
• It’s necessary to collect data on a seasonal basis
• We could not detect seasonal effects in amount of auto travel in Pennsylvania from a time series of annual amounts of travel