When defining serial killers, researchers generally use "three or more murders" as the baseline, considering it sufficient to provide a pattern without being overly restrictive. Independent of the number of murders, they need to have been committed at different times, and are usually committed in different places. The lack of a cooling-off period (a significant break between the murders) marks the difference between a spree killer and a serial killer. The category has, however, been found to be of no real value to law enforcement, because of definitional problems relating to the concept of a "cooling-off period". Cases of extended bouts of sequential killings over periods of weeks or months with no apparent "cooling off period" or "return to normality" have caused some experts to suggest a hybrid category of "spree-serial killer".