70PR05B-ES HESG332204R0001 ABB programmable processor
Technical data of product parameters
A naïve before-after study is a simple before-after comparison where the safety effect of a treatment is
assessed by directly comparing the crash frequency in the after period with the crash frequency in the
before period (Hauer, 1997). The typical naïve before-after study design does not account for possible
bias due to 70PR05B-ES HESG332204R0001and does not account for temporal effects or trends. A variation of this study design
may consider the comparison of crash rates (per vehicle miles traveled70PR05B-ES HESG332204R0001 between the before and
after periods rather than crash counts. Crash rates attempt to consider the change in traffic volume
between the before and after periods, but the use of rates implicitly assumes that crash frequency and
traffic volume are linearly related. Many studies have shown that the relationship between crash
frequency and traffic volume is often non-linear.
Before-after study with comparison group
In this study design, an untreated comparison group that is similar to the treatment group is identified
to account for temporal effects and changes in traffic volume. One way to apply this method is to use
the comparison group to first calculate a comparison ratio which is the ratio of observed crash
frequency in the after period to that in the before period 70PR05B-ES HESG332204R0001in the before period at the treatment sites is multiplied by this comparison ratio to estimate
the expected number of crashes in the after period had the treatment not occurred. The expected
number of crashes in the after period (had the treatment not occurred) is then compared with the70PR05B-ES HESG332204R0001
actual number of crashes in the after period to determine the safety impact of the treatment. In order
for this approach to work, we have to assume that the trends in the crash counts in the treatment and
comparison group are similar. Hauer (1997) proposes a test that makes use of a sequence of sample70PR05B-ES HESG332204R0001
odds ratios to determine if the trends in the two groups are indeed similar. By selecting the comparison
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