Use the percentage column for the comparison. Repeat the steps above for all crash types under consideration. As an example of Step 3, comparing severity of pedestrian accidents shows almost ten times the percentage of fatal accidents as in the general population of accidents. This is clearly statistically significant, but other smaller differences should be subjected to the appropriate statistical tests if significance is questionable. The process given above has traditionally been performed manually by placing the hard-copy global frequency distributions next to that of the subset and comparing the results for each variable.
Large discrepancies become readily apparent and warrant further investigation.
This was the typical method for performing mainframe-based PI in the past. These steps provide the initial look at the data. This is a very simple and straightforward assessment, but it has been the author's experience that it produces more usable information than any other phase of the analysis. It is also critical to being able to know what information to seek next, the process of which is described below. For example, alcohol crashes will be over-represented in the early morning and on weekend days.