Planning an experiment to obtain appropriate data and drawing inferences from the data concerning any problem under investigation is known as the design and analysis of the experiment or simply the designs of experiment (DOE).
Important Designs of Experiment Terminology are:
EXPERIMENT: An experiment deliberately imposes a treatment on a group of objects or subjects in the interest of observing the response.
EXPERIMENTAL UNIT: The experimental unit is the basic entity or unit on which the experiment is performed. It is an object to which the treatment is applied and in which the variable under investigation is measured and analyzed. For example, the experimental unit may be a single person, animal, plant, manufactured item, or country that belongs to a larger collection of such entities being studied.
Identify the Experimental Units
- A teacher practices the different teaching methods on different groups in her class to see which yields the best results.
- A doctor treats a patient with a skin condition with different creams to see which is most effective.
The experimental unit is the physical entity or subject exposed to the treatment independently of other units. In other words, it is the basic unit on which the experiment is performed (smallest division of experimental material).
TREATMENTS: In experiments, a treatment is something that researchers administer to experimental units. For example, a corn field is divided into four, each part is ‘treated’ with a different fertilizer to see which produces the most corn.
Treatment is an experimental condition whose effect is to be measured and compared. For example, animal diets, temperature, soil types, and brands of tires.
FACTOR: A factor of an experiment is a controlled independent variable; a variable whose levels are set by the experimenter. A factor is a general type or category of treatments. Different treatments constitute different levels of a factor.
EXPERIMENTAL ERROR
It describes the variation among identically and independently treated experimental units. In the designs of experiments, various origins of experimental error include:
- The natural variation among the experimental units.
- Inability to reproduce the treatment conditions exactly from one unit to another.
- Interaction of treatments and experimental units.
- Any other extraneous factors that influence the measured characteristics.
There are two types of errors:
- Systematic Errors
Systematic Errors are caused by a consistent bias in one direction, consistently pushing your results away from the true value. Systematic errors can be caused by a variety of factors, such as a faulty instrument, an incorrect calibration, or an error in the experimental design. Systematic errors will cause data points to shift all in the same direction, away from the true value.
- Random Error
The random error is caused by small and unpredictable variations that occur in every experiment. Random errors can come from a variety of sources, such as slight differences in how a measurement is made, or fluctuations in environmental conditions. Random errors tend to cause data points to scatter randomly around the true value.
The experimental error can be controlled by
- Blocking
- Proper plot technique
- Data Analysis
EXPERIMENTAL DESIGN
An experimental design is a plan to collect the data relevant to the problem under investigation. In such a way as to provide a basis for valid and objective inferences about the stated problem.
The plan usually consists of the selection of the treatments, specifications of the experimental layouts, allocation of the treatments, and collection of observations for analysis.
Hence designs of Experiments are simply a sequence of steps taken ahead of time to ensure that the appropriate data will be obtained in a way that permits an objective analysis leading to a valid analysis concerning the problem.
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