Collusion is a confidential agreement or understanding, that may be verbal or implied, to cooperate and thereby gain an unfair or illegal advantage that should not otherwise have been available. It involves deception and unfair practices to gain some benefit.
The word conspiracy is often used to replace or supplement the legal meaning of collusion. (see MORAN v. BURBINE, 475 U.S. 412 (1986) at http://laws.findlaw.com/us/475/412.html)
   In law enforcement, comparing stories before giving an official statement, cutting and pasting text between officer’s accounts of an incident or agreeing to present the details of an incident in a way to make an action seem justified, are all evidence of collusion.
When two people give an eyewitness account of any incident, there are inevitably variations of known facts and perceptions that make their stories unique. (For more on perceptions see invisible evidence and suspects) Read the four gospel accounts (Matthew, Mark, Luke and John) of Jesus life in the Bible. They are all the story of Jesus, many of the same facts can be found in multiple books, some facts overlap and some facts are missing from some accounts. Each writer has a different perspective and the accounts are quite unique.
   When two people, independently give identical eyewitness accounts of an incident, they are usually presumed to be guilty of collusion or conspiracy to lie about what happened. Two people could report one fact in an identical manner, but it is almost impossible to report a large number of facts and perceptions identically without having colluded.
   Because defense attorneys are allowed to attack the normal and natural variations in testimony, some law enforcement officers are inclined to make the details of their reports or notes match. A common question in court is “Where was the closest street lighting?” Because of tunnel vision during stress, most new officers cannot answer that question, for most of their cases. Over time they learn to look for such details after the fact, so that they can write it in their notes or report.
   For example, one officer asks his backup officer where the closest street light was so he can include it in his notes. The backup officer honestly but mistakenly provides the wrong information. Both officers include the mistake in their report or notes. Now the first officer, testifying from his notes in court, testifies in error. And the 2nd officer likewise testifies in error. The defense attorney produces a picture of the area showing their error. He then argues that because both officers testified to the same lie, it is proof that they have agreed to lie about what happened.
   Personal integrity means that the law enforcement officer will provide only what he saw in his notes and reports. He will not try to present the facts in a “more favorable light.” He is willing to stand on his own and take the pressure and questioning from the lawyers, no matter how unethically they may act. Integrity will cost you something, but not half the value that you will recieve in return.