The first problem, documentation of previously undocumented behavior, is common to all recording instruments. Medical record systems that depend on people to record events, either on paper or computer, are highly filtered. Sometimes this filtering is conscious and intentional, as when personnel attempt to cover up an error by not entering incriminating information into the medical chart. In the worst case, there may even be attempts to change previous entries. This intentional distortion of data, however, is assumed to be relatively infrequent. Most commonly the filtering is unconscious and unintentional. It may occur because the recorder’s memory fades between rendering the care or making the observations and recording them in the medical record. In many situations, it occurs because the nurses do not make their primary entries into the medical record.

While generally discouraged by protocols on medical records management, off- chart records are commonly used by nurses to keep track of things to do, medications given, and patient observations. These temporary records allow the nurses to batch- enter data into medical records rather than keep contemporaneous records. This allows two stages of filtering: first the information is compressed into a minimal temporary record, and then that minimal record is expanded into the permanent chart record. Such filtering makes it easy to transform an item accidentally from the to-do list to the done list without the task’s actually being performed. It also gives the filter an extra chance to remove nonconforming information.

Filters are defined by what they exclude. Human clinical filters tend to exclude things that do not easily fit into the expectations associated with the care of a given patient. This should not be seen as an act of deception or even of carelessness. It is more a smoothing of data that tends to obscure anomalies. The smoothing is aided by the limited amount of information that can be recorded by periodic observations recorded in essentially narrative format. The end result of this process of smoothing and filtering is a medical record that is more often characterized by what it does not contain than by what it does document. This is borne out in litigation where records are usually incriminating because they fail to record what was allegedly done.