In scientific inquiry, an experiment (Latin: ex- periri, "of (or from) trying") is a method of investigating particular types of research questions or solving particular types of problems. Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. In social relationships an ex (plural exes) is someone with whom a person was once associated The experiment is a cornerstone in the empirical approach to acquiring deeper knowledge about the world and is used in both natural sciences as well as in social sciences. In Philosophy, empiricism is a theory of Knowledge which asserts that knowledge arises from Experience. Knowledge is defined ( Oxford English Dictionary) variously as (i expertise and skills acquired by a person through experience or education the theoretical or practical understanding

Design of experiments

Main article: Design of experiments

An experiment can be thought of as a specific type of method used in scientific inquiries, usually to study causality. Design of experiments, or experimental design, is the design of all information-gathering exercises where variation is present whether under the full control of the experimenter Often the objective is to test a hypothesis: i. A statistical hypothesis test is a method of making statistical decisions using experimental data e. a tentative explanation of a phenomena or mechanism of causality. The essense of an experiment is to introduce a change in a system (the independent variable) and to study the effect of this change (the dependent variable). Two fundamental considerations of experimental design are:

• That the independent variable is the only factor that varies systematically in the experiment; in other words, that the experiment is appropriately controlled - that confounding variables are eliminated; and
• That the dependent variable truly reflects the phenomenon under study (a question of validity) and that the variable can be measured accurately (i. Scientific controls allow Experiments to study one Variable at a time and are a vital part of the Scientific method. In statistics a confounding variable (also confounding factor, lurking variable, a confound, or confounder) is an Extraneous variable In Psychology, validity has two distinct fields of application e. , that various types of experimental error, such as measurement error can be eliminated). Observational error is the difference between a measured value of quantity and its true value

In a very strict application of the experimental method, hypotheses are tested by critical experiments: ones that can falsify the hypothesis in the case of a non-result (i. In the Sciences an experimentum crucis, or critical experiment, is an Experiment capable of decisively determining whether or not a particular Falsifiability (or "refutability" is the logical possibility that an assertion can be shown false by an observation or a physical experiment e. , an experiment showing that the independent variable did not affect the dependent variable as predicted). Such pure applications are rare, however, in part because a result can sometimes be challenged on the basis that an experiment was not sufficiently controlled, that the dependent variable was not valid, or that various forms of error compromised the experiment. The scientific method, as a result, builds in the need for reproducibility (usually termed replication) and convergent evidence (see also: external validity). Reproducibility is one of the main principles of the Scientific method, and refers to the ability of a test or Experiment to be accurately reproduced or replicated Convergent validity is the degree to which an operation is similar to (converges on other operations that it theoretically should also be similar to External validity is the validity of generalized (causal inferences in scientific studies usually based on experiments as experimental validity. ' The design of experiments attempts to balance the requirements and limitations of the field of science in which one works so that the experiment can provide the best conclusion about the hypothesis being tested. In some sciences, such as physics and chemistry, it is relatively easy to meet the requirements that all measurements be made objectively, and that all conditions can be kept controlled across experimental trials. Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties On the other hand, in other cases such as biology, and medicine, it is often hard to ensure that the conditions of an experiment are performed consistently; and in the social sciences, it may even be difficult to determine a method for measuring the outcomes of an experiment in an objective manner. Foundations of modern biology There are five unifying principles Medicine is the art and science of healing It encompasses a range of Health care practices evolved to maintain and restore Human Health by the The social sciences comprise academic disciplines concerned with the study of the social life of human groups and individuals including Anthropology, Communication studies

For this reason, sciences such as physics and several other fields of natural science are sometimes informally referred to as "hard sciences", while social sciences are sometimes informally referred to as "soft sciences"; in an attempt to capture the idea that objective measurements are often far easier in the former, and far more difficult in the latter. In Science, the term natural science refers to a naturalistic approach to the study of the Universe, which is understood as obeying rules or law of The social sciences comprise academic disciplines concerned with the study of the social life of human groups and individuals including Anthropology, Communication studies

In addition, in the social sciences, the requirement for a "controlled situation" may actually work against the utility of the hypothesis in a more general situation. When the desire is to test a hypothesis that works "in general", an experiment may have a great deal of internal validity, in the sense that it is valid in a highly controlled situation, while at the same time lack external validity when the results of the experiment are applied to a real world situation. One of the reasons why this may happen is the Hawthorne effect; another is that partial equilibrium effects may not persist in general equilibrium. The Hawthorne effect is a form of reactivity, and describes a temporary change to behavior or performance in response to a change in the environmental conditions with the response A partial equilibrium is a type of Economic equilibrium, where the clearance on the market of some specific goods is obtained independently from prices and quantities demanded General equilibrium theory is a branch of theoretical Microeconomics.

As a result of these considerations, experimental design in the "hard" sciences tends to focus on the elimination of extraneous effects, while experimental design in the "soft" sciences focuses more on the problems of external validity, often through the use of statistical methods. Statistics is a mathematical science pertaining to the collection analysis interpretation or explanation and presentation of Data. Occasionally events occur naturally from which scientific evidence can be drawn, which is the basis for natural experiments. A natural or Quasi-experiment is a naturally occurring instance of observable phenomena which approximate or duplicate the properties of a controlled Experiment. In such cases the problem of the scientist is to evaluate the natural "design".

Controlled (Laboratory) experiments

Main article: Experimental control

Many hypotheses in sciences such as physics can establish causality by noting that, until some phenomenon occurs, nothing happens; then when the phenomenon occurs, a second phenomenon is observed. Scientific controls allow Experiments to study one Variable at a time and are a vital part of the Scientific method. But often in science, this situation is difficult to obtain.

For example, in the old joke, someone claims that they are snapping their fingers "to keep the tigers away"; and justifies this behavior by saying "see - its working!" While this "experiment" does not falsify the hypothesis "snapping fingers keeps the tigers away", it does not really support the hypothesis - not snapping your fingers keeps the tigers away as well.

To demonstrate a cause and effect hypothesis, an experiment must often show that, for example, a phenomenon occurs after a certain treatment is given to a subject, and that the phenomenon does not occur in the absence of the treatment. (See Baconian method. The Baconian method is the investigative method developed by Francis Bacon. )

Standard curve

A controlled experiment generally compares the results obtained from an experimental sample against a control sample, which is practically identical to the experimental sample except for the one aspect whose effect is being tested. A good example would be a drug trial. The sample or group receiving the drug would be the experimental one; and the one receiving the placebo would be the control one. Placebo is a substance or procedure a patient accepts as medicine or therapy but which has no specific therapeutic activity In many laboratory experiments it is good practice to have several replicate samples for the test being performed and have both a positive control and a negative control. Scientific controls allow Experiments to study one Variable at a time and are a vital part of the Scientific method. The results from replicate samples can often be averaged, or if one of the replicates is obviously inconsistent with the results from the other samples, it can be discarded as being the result of an experimental error (some step of the test procedure may have been mistakenly omitted for that sample). Most often, tests are done in duplicate or triplicate. A positive control is a procedure that is very similar to the actual experimental test but which is known from previous experience to give a positive result. A negative control is known to give a negative result. The positive control confirms that the basic conditions of the experiment were able to produce a positive result, even if none of the actual experimental samples produce a positive result. The negative control demonstrates the base-line result obtained when a test does not produce a measurable positive result; often the value of the negative control is treated as a "background" value to be subtracted from the test sample results. Sometimes the positive control takes the quadrant of a standard curve. A standard curve is a quantitative research tool a method of plotting Assay data that is used to determine the Concentration of a substance particularly Proteins

An example that is often used in teaching laboratories is a controlled protein assay. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl An assay is a procedure where a property or concentration of an Analyte is measured Students might be given a fluid sample containing an unknown (to the student) amount of protein. It is their job to correctly perform a controlled experiment in which they determine the concentration of protein in fluid sample (usually called the "unknown sample"). The teaching lab would be equipped with a protein standard solution with a known protein concentration. Students could make several positive control samples containing various dilutions of the protein standard. Negative control samples would contain all of the reagents for the protein assay but no protein. In this example, all samples are performed in duplicate. The assay is a colorimetric assay in which a spectrophotometer can measure the amount of protein in samples by detecting a colored complex formed by the interaction of protein molecules and molecules of an added dye. In Physics, spectrophotometry is the quantitative study of electromagnetic spectra. In the illustration, the results for the diluted test samples can be compared to the results of the standard curve (the blue line in the illustration) in order to determine an estimate of the amount of protein in the unknown sample.

Controlled experiments can be performed when it is difficult to exactly control all the conditions in an experiment. In this case, the experiment begins by creating two or more sample groups that are probabilistically equivalent, which means that measurements of traits should be similar among the groups and that the groups should respond in the same manner if given the same treatment. This equivalency is determined by statistical methods that take into account the amount of variation between individuals and the number of individuals in each group. Statistics is a mathematical science pertaining to the collection analysis interpretation or explanation and presentation of Data. A number is an Abstract object, tokens of which are Symbols used in Counting and measuring. In fields such as microbiology and chemistry, where there is very little variation between individuals and the group size is easily in the millions, these statistical methods are often bypassed and simply splitting a solution into equal parts is assumed to produce identical sample groups. Microbiology (from Greek grc μῑκρος mīkros, "small" grc βίος bios, " Life " and grc -λογία Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties In Chemistry, a solution is a Homogeneous Mixture composed of two or more substances

Once equivalent groups have been formed, the experimenter tries to treat them identically except for the one variable that he or she wishes to isolate. Human experimentation requires special safeguards against outside variables such as the placebo effect. Human subject research (HSR or human subject use (HSU involves the use of human beings as research subjects Such experiments are generally double blind, meaning that neither the volunteer nor the researcher knows which individuals are in the control group or the experimental group until after all of the data has been collected. This ensures that any effects on the volunteer are due to the treatment itself and are not a response to the knowledge that he is being treated.

In human experiments, a subject (person) may be given a stimulus to which he or she should respond. In Biostatistics or Psychological statistics, a research subject is any object or phenomenon that is observed for purposes of research Stimulation is the action of various agents ( stimuli) on Muscles Nerves or a sensory end organ by which activity is evoked especially the nervous The goal of the experiment is to measure the response to a given stimulus by a test method. Measurement is the process of estimating the magnitude of some attribute of an object such as its length or weight relative to some standard ( unit of measurement) such as A test method is a definitive procedure that produces a test result

Natural experiments

Main article: Natural experiment

The term "experiment" usually implies a controlled experiment, but sometimes controlled experiments are prohibitively difficult or impossible. A natural or Quasi-experiment is a naturally occurring instance of observable phenomena which approximate or duplicate the properties of a controlled Experiment. In this case researchers resort to natural experiments, also called quasi-experiments. Natural experiments rely solely on observations of the variables of the system under study, rather than manipulation of just one or a few variables as occurs in controlled experiments. A variable (ˈvɛərɪəbl is an Attribute of a physical or an abstract System which may change its Value while it is under Observation. System (from Latin systēma, in turn from Greek systēma is a set of interacting or interdependent Entities, real or abstract To the degree possible, they attempt to collect data for the system in such a way that contribution from all variables can be determined, and where the effects of variation in certain variables remain approximately constant so that the effects of other variables can be discerned. The degree to which this is possible depends on the observed correlation between explanatory variables in the observed data. In Probability theory and Statistics, correlation, (often measured as a correlation coefficient) indicates the strength and direction of a linear Dependent variables and independent variables refer to values that change in relationship to each other When these variables are not well correlated, natural experiments can approach the power of controlled experiments. Usually, however, there is some correlation between these variables, which reduces the reliability of natural experiments relative to what could be concluded if a controlled experiment were performed. Also, because natural experiments usually take place in uncontrolled environments, variables from undetected sources are neither measured nor held constant, and these may produce illusory correlations in variables under study.

Much research in several important science disciplines, including economics, political science, geology, paleontology, ecology, meteorology, and astronomy, relies on quasi-experiments. Science (from the Latin scientia, meaning " Knowledge " or "knowing" is the effort to discover, and increase human understanding Economics is the social science that studies the production distribution, and consumption of goods and services. Political science is a branch of Social sciences that deals with the theory and practice of Politics and the description and analysis of Political systems Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit Palaeontology redirects here For the Scientific journal, see Palaeontology (journal. Ecology (from Greek grc οἶκος oikos, "house(hold" and grc -λογία -logia) is the scientific study of Meteorology (from Greek grc μετέωρος metéōros, "high in the sky" and grc -λογία -logia) is the Interdisciplinary Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study For example, in astronomy it is clearly impossible, when testing the hypothesis "suns are collapsed clouds of hydrogen", to start out with a giant cloud of hydrogen, and then perform the experiment of waiting a few billion years for it to form a sun. However, by observing various clouds of hydrogen in various states of collapse, and other implications of the hypothesis (for example, the presence of various spectral emissions from the light of stars), we can collect data we require to support the hypothesis. An early example of this type of experiment was the first verification in the 1600s that light does not travel from place to place instantaneously, but instead has a measurable speed. Observation of the appearance of the moons of Jupiter were slightly delayed when Jupiter was farther from Earth, as opposed to when Jupiter was closer to Earth; and this phenomenon was used to demonstrate that the difference in the time of appearance of the moons was consistent with a measurable speed of light.

Observational studies

Main article: Observational study

Observational studies are very much like controlled experiments except that they lack probabilistic equivalency between groups. In Statistics, an observational study draws inferences about the effect of a treatment on subjects where the assignment of subjects into a treated group versus a Control These types of experiments often arise in the area of medicine where, for ethical reasons, it is not possible to create a truly controlled group. For example, one would not want to deny all forms of treatment for a life-threatening disease from one group of patients to evaluate the effectiveness of another treatment on a different group of patients. The results of observational studies are considered much less convincing than those of designed experiments, as they are much more prone to selection bias. Selection bias is a distortion of evidence or data that arises from the way that the data are collected Researchers attempt to compensate for this with complicated statistical methods such as propensity score matching methods (see hierarchy of evidence). In Statistics, propensity score matching (PSM is one of Quasi-empirical “correction strategies” that corrects for the Selection biases in making estimates Evidence hierarchies reflect the relative authority of various types of Biomedical research. See also quasi-empirical methods

Field experiments

Main article: Field experiment

Field experiments are so named in order to draw a contrast with laboratory experiments. Quasi-empirical methods are applied in science and in mathematics A field experiment applies the scientific method to Experimentally examine an intervention in the real world (or as many experimental economists like to say naturally-occurring Often used in the social sciences, and especially in economic analyses of education and health interventions, field experiments have the advantage that outcomes are observed in a natural setting rather than in a contrived laboratory environment. However, like natural experiments, field experiments suffer from the possibility of contamination: experimental conditions can be controlled with more precision and certainty in the lab.

Examples

• MTT assay
• Colony Formation Assay

Quotes

"We have to learn again that science without contact with experiments is an enterprise which is likely to go completely astray into imaginary conjecture. MTT assay is a laboratory test and a standard colorimetric assay (an assay which measures changes in color for measuring the activity of enzymes that reduce MTT to Science (from the Latin scientia, meaning " Knowledge " or "knowing" is the effort to discover, and increase human understanding " — Hannes Alfven

"Today's scientists have substituted mathematics for experiments, and they wander off through equation after equation, and eventually build a structure which has no relation to reality. Hannes Olof Gösta Alfvén ( May 30, 1908; Norrköping, Sweden &ndash April 2, 1995; Djursholm, Sweden A scientist, in the broadest sense refers to any person that engages in a systematic activity to acquire Knowledge or an individual that engages in such practices Mathematics is the body of Knowledge and Academic discipline that studies such concepts as Quantity, Structure, Space and An equation is a mathematical statement, in symbols, that two things are exactly the same (or equivalent Structure is a fundamental and sometimes Intangible notion covering the Recognition, Observation, nature, and Stability of This article sets out the set-theoretic notion of relation For a more elementary point of view see Binary relations and Triadic relations Reality, in everyday usage means "the state of things as they actually exist" " — Nikola Tesla

See also

• Transplant experiment
• Replication (statistics)

External links

• Lessons In Electric Circuits - Volume VI - Experiments
• [1] Trochim, William M. There have already been discussions about Tesla's ethnicity on the talk page A transplant experiment (also known as a common garden experiment, although this term is more Plant specific is an In Engineering, Science, and Statistics, replication is the repetition of an Experimental condition so that the variability associated with the Experimental Design. The Research Methods Knowledge Base, 2nd Edition. (version current as of July 11, 2006).
• Description of weird experiments (with film clips)
• Concept Development and Experimentation
• Shadish, William R., Thomas D. Cook, and Donald T. Campbell. 2002. Experimental and Quasi-experimental Designs for Generalized Causal Inference. Boston: Houghton Mifflin. 623 p.
• Guide for Understanding and Implementing Defense Experimentation (GUIDEx), The Technical Cooperation Program, 2006