most prosecutors have a screening process for deciding when to prosecute and when to "noll"; factors:
-most important: not the prosecutor's belief in the guilt of the suspect, but whether there is sufficient evidence for conviction. if prosecutors have strong physical evidence and a number of reliable and believable witnesses, they are quite likely to prosecute
-prosecutors also rely heavily on offense seriousness to guide their priorities, preferring to take on felony offenses rather than misdemeanors. in other words, everything else being equal, a DA will prosecute a rapist instead of a jaywalker because the former presents a greater threat to society than does the latter. a prosecutor will also be more likely to prosecute someone with an extensive record of wrongdoing than a first-time offender
-sometimes, a case is dropped even when it involves a serious crime and a wealth of evidence exists against the suspect. these situations usually involve uncooperative victims. domestic violence cases are particularly difficult to prosecute because the victims may want to keep the matter private, fear reprisals, or have a strong desire to protect their abuser. in some jurisdictions, as many as 80% of domestic violence victims refuse to cooperate with the prosecution.
-unreliability of victims can also affect a charging decision. if the victim in a rape case is a crack addict and a prostitute, while the defendant is a decorated military veteran, prosecutors may be hesitant to have a jury decide which one is more trustworthy
-a prosecutor may be willing to drop a case or reduce the charges against a defendant who is willing to testify against other offenders. federal law encourages this kind of behavior by offering sentencing reductions to defendants who provide "substantial assistance in the investigation or prosecution of another person who has committed an offense"