Several factors help to establish witness credibility. It must be determined initially that the eyewitness was present and aware of and attentive to surroundings at the crime scene. The officer must distinguish between what the witness actually saw and heard and what he/she believes was observed. The extent of the witness's personal knowledge of the event must be evaluated. This evaluation involves estimating powers of observation and ability to relate information clearly without emotional disturbance. The physical and psychological aspects of the witness must also be assessed by the officer. Vision and hearing impairments must be determined. Any vision or hearing problems are particularly important if adverse conditions, such as inclement weather or loud noise, prevailed during the time of the incident. Evaluating the witness's psychological state involves determining whether the witness is unduly prejudiced or biased toward certain categories of people. A discussion guide accompanies the text. (Author summary modified)