In experimental science, experimenter's bias is subjective

Subjectivity

Subjectivity refers to the subject and his or her perspective, feelings, beliefs, and desires. In philosophy, the term is usually contrasted with objectivity.-Qualia:...

 bias

Bias

Bias is an inclination to present or hold a partial perspective at the expense of alternatives. Bias can come in many forms.-In judgement and decision making:...

 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 be objective

Objectivity (philosophy)

Objectivity is a central philosophical concept which has been variously defined by sources. A proposition is generally considered to be objectively true when its truth conditions are met and are "mind-independent"—that is, not met by the judgment of a conscious entity or subject.- Objectivism...

 is the ultimate source of this bias. It occurs more often in sociological and medical sciences, where double blind techniques are often employed to combat the bias. But experimenter's bias can also be found in some physical sciences, for instance, where the experimenter rounds off

Rounding

Rounding a numerical value means replacing it by another value that is approximately equal but has a shorter, simpler, or more explicit representation; for example, replacing $23.4476 with $23.45, or the fraction 312/937 with 1/3, or the expression √2 with 1.414.Rounding is often done on purpose to...

 measurements.

Classification of experimenter's biases

Modern electronic or computerized data acquisition techniques have greatly reduced the likelihood of such bias, but it can still be introduced by a poorly designed analysis technique. Experimenter's bias was not well recognized until the 1950s and 60's, and then it was primarily in medical experiments and studies. Sackett (1979) catalogued 56 biases that can arise in sampling and measurement in clinical research, among the above-stated first six stages of research. These are as follows:

1. In reading-up the field
   1. the biases of rhetoric
   2. the all's well literature bias
   3. one-sided reference bias
   4. positive results bias
   5. hot stuff bias
2. In specifying and selecting the study sample
   1. popularity bias
   2. centripetal bias
   3. referral filter bias
   4. diagnostic access bias
   5. diagnostic suspicion bias
   6. unmasking (detection signal) bias
   7. mimicry bias
   8. previous opinion bias
   9. wrong sample size bias
   10. admission rate (Berkson) bias
   11. prevalence-incidence (Neyman) bias
   12. diagnostic vogue bias
   13. diagnostic purity bias
   14. procedure selection bias
   15. missing clinical data bias
   16. non-contemporaneous control bias
   17. starting time bias
   18. unacceptable disease bias
   19. migrator bias
   20. membership bias
   21. non-respondent bias
   22. volunteer bias
3. In executing the experimental manoeuvre (or exposure)
   1. contamination bias
   2. withdrawal bias
   3. compliance bias
   4. therapeutic personality bias
   5. bogus control bias
4. In measuring exposures and outcomes
   1. insensitive measure bias
   2. underlying cause bias (rumination bias)
   3. end-digit preference bias
   4. apprehension bias
   5. unacceptability bias
   6. obsequiousness bias
   7. expectation bias
   8. substitution game
   9. family information bias
   10. exposure suspicion bias
   11. recall bias
   12. attention bias
   13. instrument bias
5. In analyzing the data
   1. post-hoc significance bias
   2. data dredging bias (looking for the pony)
   3. scale degradation bias
   4. tidying-up bias
   5. repeated peeks bias
6. In interpreting the analysis
   1. mistaken identity bias
   2. cognitive dissonance bias
   3. magnitude bias
   4. significance bias
   5. correlation bias
   6. under-exhaustion bias

The effects of bias on experiments in the physical sciences have not always been fully recognized.

Statistical background

In principle, if a measurement has a resolution of , then if the experimenter averages independent measurements the average will have a resolution of (this is the central limit theorem

Central limit theorem

In probability theory, the central limit theorem states conditions under which the mean of a sufficiently large number of independent random variables, each with finite mean and variance, will be approximately normally distributed. The central limit theorem has a number of variants. In its common...

 of statistics). This is an important experimental technique used to reduce the impact of randomness on an experiment's outcome. This requires that the measurements be statistically independent; there are several reasons why they may not be. If independence is not satisfied, then the average may not actually be a better statistic but may merely reflect the correlations among the individual measurements and their non-independent nature.

The most common cause of non-independence is systematic error

Systematic error

Systematic errors are biases in measurement which lead to the situation where the mean of many separate measurements differs significantly from the actual value of the measured attribute. All measurements are prone to systematic errors, often of several different types...

s (errors affecting all measurements equally, causing the different measurements to be highly correlated, so the average is no better than any single measurement). Experimenter bias is another potential cause of non-independence.

Biological and medical sciences

The complexity of living systems and the ethical impossibility of performing fully controlled experiments with certain species of animals and humans provide a rich, and difficult to control, source of experimental bias. The scientific knowledge about the phenomenon under study, and the systematic elimination of probable causes of bias, by detecting confounding factors, is the only way to isolate true cause-effect relationships. It is also in epidemiology

Epidemiology

Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...

 that experimenter bias has been better studied than in other sciences.

A number of studies into Spiritual Healing illustrate how the design of the study can introduce experimenter bias into the results. A comparison of two studies illustrates that subtle differences in the design of the tests can adversely affect the results of one. The difference was due to the intended result: a positive or negative outcome rather than positive or neutral.

A 1995 paper by Hodges & Scofield of spiritual healing used the growth rate of cress

Cress

-Plants:* Alpine Rock Cress* Bulbous Cress* Cedar Glade Cress* Garden cress, a leafy vegetable* Hoary Bitter Cress* Hoary Cress* Indian Cress* Land cress, a biennial herb* Marsh Cress* Peppercress, a mustard* Rockcress, several brassicales...

 seeds as their independent variable

Independent variable

The terms "dependent variable" and "independent variable" are used in similar but subtly different ways in mathematics and statistics as part of the standard terminology in those subjects...

 in order to eliminate a placebo response or participant bias. The study reported positive results as the test results for each sample were consistent with the healers intention that healing should or should not occur. However the healer involved in the experiment was a personal acquaintance of the study authors raising the distinct possibility of experimenter bias. A randomized clinical trial, published in 2001, investigated the efficacy of spiritual healing (both at a distance and face-to-face) on the treatment of chronic pain in 120 patients. Healers were observed by "simulated healers" who then mimicked the healers movements on a control group while silently counting backwards in fives - a neutral rather than should not heal intention. The study found a decrease in pain in all patient groups but "no statistically significant differences between healing and control groups ... it was concluded that a specific effect of face-to-face or distant healing on chronic pain could not be demonstrated."

Physical sciences

If the signal being measured is actually smaller than the rounding error and the data are over-averaged, a positive result for the measurement can be found in the data where none exists (i.e. a more precise experimental apparatus would conclusively show no such signal). If an experiment is searching for a sidereal variation of some measurement, and if the measurement is rounded-off by a human who knows the sidereal time

Sidereal time

Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky...

 of the measurement, and if hundreds of measurements are averaged to extract a "signal" which is smaller than the apparatus' actual resolution, then it should be clear that this "signal" can come from the non-random round-off, and not from the apparatus itself. In such cases a single-blind experimental protocol is required; if the human observer does not know the sidereal time of the measurements, then even though the round-off is non-random it cannot introduce a spurious sidereal variation.

Social sciences

The experimenter may introduce cognitive bias

Cognitive bias

A cognitive bias is a pattern of deviation in judgment that occurs in particular situations. Implicit in the concept of a "pattern of deviation" is a standard of comparison; this may be the judgment of people outside those particular situations, or may be a set of independently verifiable...

 into a study in several ways. First, in what is called the observer-expectancy effect

Observer-expectancy effect

The observer-expectancy effect is a form of reactivity, in which a researcher's cognitive bias causes them to unconsciously influence the participants of an experiment...

, the experimenter may subtly communicate their expectations for the outcome of the study to the participants, causing them to alter their behavior to conform to those expectations. After the data are collected, bias may be introduced during data interpretation and analysis. For example, in deciding which variables to control in analysis, social scientists often face a trade-off between omitted-variable bias

Omitted-variable bias

In statistics, omitted-variable bias occurs when a model is created which incorrectly leaves out one or more important causal factors. The 'bias' is created when the model compensates for the missing factor by over- or under-estimating one of the other factors.More specifically, OVB is the bias...

 and post-treatment bias.

Forensic sciences

Observer effects are rooted in the universal human tendency to interpret data in a manner consistent with one’s expectations. 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. 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.

See also

• Bias
  Bias
  Bias is an inclination to present or hold a partial perspective at the expense of alternatives. Bias can come in many forms.-In judgement and decision making:...
  
  
• Confirmation bias
  Confirmation bias
  Confirmation bias is a tendency for people to favor information that confirms their preconceptions or hypotheses regardless of whether the information is true.David Perkins, a geneticist, coined the term "myside bias" referring to a preference for "my" side of an issue...
  
  
• List of cognitive biases
• Systematic bias
• Funding bias
  Funding bias
  The terms funding bias, sponsorship bias, funding outcome bias, or funding publication bias refer to an observed tendency of the conclusion of a scientific research study to support the interests of the study's financial sponsor. This phenomenon is recognized sufficiently that researchers undertake...