Summary: In experimental science, experimenter’s bias is bias towards a result expected by the human experimenter. David Sackett, in a useful review of biases in clinical studies, states that biases can occur in any one of seven stages of research:

1. in reading-up on the field,
2. in specifying and selecting the study sample,
3. in executing the experimental manoeuvre (or exposure),
4. in measuring exposures and outcomes,
5. in analyzing the data,
6. in interpreting the analysis, and
7. in publishing the results.

The inability of a human being to remain completely objective is the ultimate source of this bias. It occurs more often in sociological and medical sciences, for which reason double blind techniques are often employed to combat the bias. But experimenter’s bias can also be found in some physical sciences, where the experimenter rounds off measurements.

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Observer effects are rooted in the universal human tendency to interpret data in a manner consistent with one’s expectations (1). This tendency is particularly likely to distort the results of a scientific test when the underlying data are ambiguous and the scientist is exposed to domain-irrelevant information that engages emotions or desires (2). Despite impressions to the contrary, forensic DNA analysts often must resolve ambiguities, particularly when interpreting difficult evidence samples such as those that contain mixtures of DNA from two or more individuals, degraded or inhibited DNA, or limited quantities of DNA template. The full potential of forensic DNA testing can only be realized if observer effects are minimized. We met n December 1 and 2, 2007 in Washington, D.C. to discuss the implications of observer effects in forensic DNA testing and ways to minimize them.

The interpretation of an evidentiary DNA profile should not be influenced by information about a suspect’s DNA profile (3-6). Each item of evidence must be interpreted independent of other items of evidence or reference samples. Yet forensic analysts are commonly aware of submitted reference profiles when interpreting DNA test results, creating the opportunity for a confirmatory bias, despite the best intentions of the analyst. Furthermore, analysts are sometimes exposed to information about the suspects, such as their history or motives, eyewitness identifications, presence or absence of a confession, and the like. Such information should have no bearing on how the results of a DNA test are interpreted, yet may compound an unintentional confirmatory bias. This bias can result in false inclusions under not uncommon conditions of ambiguity encountered in actual casework. It can also render currently used frequency statistics or likelihood ratios misleading.

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