98991114_rime_in_the_United_States_2006__Reason_for_Estimation
[Year] 1997 [State(s)] Illinois [Estimation Method] Valid Crime Index (Part I) counts were available for most of the largest cities (100,000 and over in population). For other agencies, the only available counts generated by the Illinois State Program were state totals based upon an incident-level system without indication of multiple offenses recorded within single incidents. Therefore, the UCR Hierarchy Rule could not be applied in order to convert the state's data to Summary format. (The Hierarchy Rule requires that only the most serious offense in a multiple-offense criminal incident is counted.) To arrive at a comparable state estimate to be included in national compilations, the Illinois State Program's state totals (which were inflated because of the nonapplication of the Hierarchy Rule) were reduced by the proportion of multiple offenses reported within single incidents in the NIBRS database. Valid totals for the large cities were excluded from the reduction process. [Reason for Estimation] 
The state UCR Program was unable to provide complete offense figures in accordance with UCR guidelines.
Table 01, Data Declaration - Crime in the United States 2006
Reason for Estimation