Psychological testing is at least as much art as it is science. While the test publishers have made great effort to write tests that will give the most accurate results possible, they are used to measure skills or information in complex human beings. All sorts of things can go wrong.

             If a person is under great stress test performance could suffer. For children this might be a situation at school, such as chronic, severe bullying, or parental divorce, or the effects of trauma of some kind. For an adult it might be that he or she is facing a divorce, or loss of job. Life crises can affect performance, and both children and adults can experience them.

             Physical conditions could contribute to poor test performance, which is why a child's hearing and vision should be checked before proceeding with testing. Some children are easily distracted and need more frequent breaks during testing to do their best. In addition if the child is sleep-deprived, or coming down with an illness, or hungry, these things could interfere. The child may have developed fluid in the ear that sporadically interferes with hearing. The child's background may depress test scores, especially if the deprivation is significant. Young children who haven't been exposed to crayons and books may not even know the names of the colors or the names of letters and numbers. Children who have been exposed to a very enriched environment may know some things other children might not know at their age, such as who George Washington was. .

             If the subject has certain psychological disorders, such as depression or anxiety, these conditions can interfere with test performance.