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  • Spotfire Statistica® Variance Estimation and Precision


    Variance Estimation and Precision (VEPAC) contains techniques for analyzing research designs that include both fixed and random effects. With this module, you can obtain estimates of variance components and use those estimated variance components to make precision statements while at the same time comparing fixed effects in the presence of multiple sources of variation.

    Variance Estimation and Precision (VEPAC) contains techniques for analyzing research designs that include both fixed and random effects. With this module, you can obtain estimates of variance components and use those estimated variance components to make precision statements while at the same time comparing fixed effects in the presence of multiple sources of variation. 

    Experimentation is sometimes mistakenly thought to involve only the manipulation of levels of the independent variables and the observation of subsequent responses on the dependent variables. Independent variables whose levels are determined or set by the experimenter are said to have fixed effects.

    There is a second class of effects, however, which is often of great interest to the researcher. Random effects are classification effects where the levels of the effects are assumed to be randomly selected from an infinite population of possible levels. Many independent variables of research interest are not fully amenable to experimental manipulation, but nevertheless can be studied by considering them to have random effects.

    Here are examples of why these techniques are helpful.

    • the genetic makeup of individual members of a species cannot be experimentally manipulated, yet it is of great interest to the geneticist to assess the genetic contribution to an individual variation on outcomes such as health, behavioral characteristics, etc.
    • a manufacturer wants to estimate the components of variation in the characteristics of a product for a random sample of machines operated by a random sample of operators
    • researching the amount of insect damage done to different varieties of wheat and it is not possible to study every possible variety of wheat
    • creating a new teacher pay system; study the variation in achievement attributable to schools, to teachers, and to students

    For more information see Milliken and Johnson (1992).


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