Stat 100 Read Chapter 14. Time Series A collection of measurements of a variable made at a sequence...

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