Effectiveness is about equal in injury and noninjury crashes.
Here, with the data pooled across seven calendar years, sample size is less of an issue, and it is worthwhile to take several steps that improve data quality or reduce the potential for biases in the analysis, even if those steps reduce available sample size. Limited to crashes involving two or more vehicles, and where the first harmful event is a collision between vehicles. Comparing only MY 1985 and 1986 cars simplifies the analysis and avoids biases that may enter when longer series of model years are included. It is conceivable that the vehicle age effect could vary among subsets. We are not asking here if the proportion of daytime to nighttime crashes changes as cars get older - nor if the proportion of crash involvements that are rear impacts is different in the daytime and the nighttime.