I could not find a breakdown of the rape statistics for Chicago but the following should be helpful in understanding the numbers-
Reporting Statistics
Only 16% of rapes and sexual assaults are reported to the police (Rape in America: A Report to the Nation. 1992). In 1995 there were 97,460 rapes reported to law enforcement officials. At a 16% reporting rate, this means that there were actually closer to 649,733 rapes in the United States. Along the same lines, the number of rapes reported in New York state in 1996 was 20,911. At a 16% reporting rate, this means the actual number of rapes was closer to 139,406. (Computerized Criminal History, Feb. 1998)
The rate of false reports of rape is approximately 2 - 3% which is no different than that for other crimes. This is different than the 8% of reports which are unfounded. This means that in 8% of the rape cases reported the investigators or prosecutors deemed that the case was not prosecutable for any number of reasons. Only 2 - 3% of the reports however were fabricated stories.
Victim Characteristics
1 in 3 sexual assault victims are under the age of 12 (Snyder & Sickmund, 1999) and convicted rape and sexual assault offenders report that 2/3 of their victims were under the age of 18. Among victims age 18 - 29, two thirds had a prior relationship with the rapist. (National Crime Victimization Survey, Criminal Victimization, 1996)
18% of women who reported being raped before age 18 said they were also raped after age 18. (Violence Against Women Survey, 1998)
Perpetrator Characteristics
In 1997, 68.3% were perpetrated by someone who knew the victim. (Bureau of Justice's National Crime Victimization Survey, 1997) 78% of women raped or physically assaulted since they turned 18 were assaulted by a current or former husband, live-in partner or date. 17% were victimized by an acquaintance, 9% by a relative other than a husband and only 14% were assaulted by a stranger. (National Violence Against Women Survey, 1998)
Assault Characteristics
Rape and sexual assault are not crimes that usually occur in dark alleys or in deserted areas at night. As a matter of fact 6 out of 10 sexual assaults occur in the home of the victim or the home of a friend, neighbor or relative. (Greenfeld, 1997) 43.4% of rapes and sexual assaults occur between the hours of 6PM and midnight Greenfeld, 1997) and about two thirds occur between the hours of 6 PM and 6 AM (Greenfeld, 1997).
IThis could explain why weapons were not used often in the assault. However, important to note that only 16% of rapes are reported. I would imagine that the rapes reported would tend to be the more violent onnes. Just speculation.