Stat 551, Fall 2009
Homework 5
Due date: Thursday, November 5, 2009
Impact of seatbelt legislation: The file SBL-UK.txt contains the numbers of monthly
deaths and serious injuries on UK roads for 10 years, starting in January 1975. Seatbelt
legislation was introduced in February 1983 in the hope of reducing the mean number of
monthly ”deaths and serious injuries”. The purpose of this analysis is to model the effect of
this legislature.
(a) Plot the series of deaths and serious injuries over time, and comment on the main features
of the data.
(b) Propose and justify one or two possible forms for the change function mt . What is the
intervention time T?
(c) Find a suitable model for the pre-intervention series, i.e. a model for Nt .
(d) Use R to fit the proposed intervention model(s). Report estimated model parameters
and their standard errors, check diagnostics, present a plot of the original data and the
fitted values and comment on the quality of your model fit.
(e) Bonus question: Present a plot of the estimated effect of the intervention and its behavior
over time. Comment on the effect of the legislature on reducing the number of deaths
and serious injuries and its extent in time.