 Solution

Under the more plausible Quasi-Poisson regression, the year Wald statistic is $-1.686$, with reported p-value $0.0922$. An approximate 95% confidence interval isThus the estimated fall is about 9.35%, but the data do not establish a reduction at the 5% level after allowing for overdispersion. The interval allows both a sizeable reduction and a small increase. The before–after comparison also lacks a contemporaneous randomized control, so causal inference about the campaign would require addressing other changes between years. The small Poisson regression p-value is not sufficient evidence of a campaign effect when its variance assumption is unsuitable.