In More Good Reasons to Look at the Data, we looked at data distributions to assess center, shape and spread and described how the validity of many statistical procedures relies on an assumption of approximate normality. But what do we do if our data are not normal? In this article, we’ll cover the difference between parametric and nonparametric procedures. Nonparametric procedures are one possible solution to handle non-normal data.
Definitions
If you’ve ever discussed an analysis plan with a statistician, you’ve probably heard the term “nonparametric” but may not have understood what it means. Parametric and nonparametric are two broad classifications of statistical procedures. The Handbook of Nonparametric Statistics 1 from 1962 (p. 2) says:
“A precise and universally acceptable definition of the term ‘nonparametric’ is not presently available. The viewpoint adopted in this handbook is that a statistical procedure is of a nonparametric type if it has properties which are satisfied to a reasonable approximation when some assumptions that are at least of a moderately general nature hold.”
That definition is not helpful in the least, but it underscores the fact that it is difficult to specifically define the term “nonparametric.” It is generally easier to list examples of each type of procedure (parametric and nonparametric) than to define the terms themselves. For most practical purposes, however, one might define nonparametric statistical procedures as a class of statistical procedures that do not rely on assumptions about the shape or form of the probability distribution from which the data were drawn.
The short
In More Good Reasons to Look at the Data, we looked at data distributions to assess center, shape and spread and described how the validity of many statistical procedures relies on an assumption of approximate normality. But what do we do if our data are not normal? In this article, we’ll cover the difference between parametric and nonparametric procedures. Nonparametric procedures are one possible solution to handle non-normal data.DefinitionsIf you’ve ever discussed an analysis plan with a statistician, you’ve probably heard the term “nonparametric” but may not have understood what it means. Parametric and nonparametric are two broad classifications of statistical procedures. The Handbook of Nonparametric Statistics 1 from 1962 (p. 2) says:“A precise and universally acceptable definition of the term ‘nonparametric’ is not presently available. The viewpoint adopted in this handbook is that a statistical procedure is of a nonparametric type if it has properties which are satisfied to a reasonable approximation when some assumptions that are at least of a moderately general nature hold.”That definition is not helpful in the least, but it underscores the fact that it is difficult to specifically define the term “nonparametric.” It is generally easier to list examples of each type of procedure (parametric and nonparametric) than to define the terms themselves. For most practical purposes, however, one might define nonparametric statistical procedures as a class of statistical procedures that do not rely on assumptions about the shape or form of the probability distribution from which the data were drawn.The short
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