The problem of erroneous elimination usually stems from misinterpretation of differences. Document examiners are trained to recognize similarities, although there are more categories of differences, they are harder to define specifically, and it is often impossible to determine their significance. Some factors that may cause erroneous elimination include natural variations in an individual's handwriting, accidental features, two writing styles (a scrawl and a more formal style), ambidexterity, gradual change in habit, nervousness, disguise, concentration on the writing act, writing conditions, writing surface, health or age of the writer, and influence of alcohol or drugs. 34 notes (Author abstract modified)