Quasi-variance (qv)
estimates
{{otheruses, Estimate (disambiguation)
In the Westminster system of government, the ''Estimates'' are an outline of government spending for the following fiscal year presented by the cabinet to parliament. The Estimates are drawn up by bureaucrat ...
are a statistical approach that is suitable for communicating the effects of a
categorical explanatory variable within a
statistical model. In standard statistical models the effects of a categorical explanatory variable are assessed by comparing one category (or level) that is set as a benchmark against which all other categories are compared. The benchmark category is usually referred to as the 'reference' or 'base' category. In order for comparisons to be made the reference category is arbitrarily fixed to zero. Statistical data analysis software usually undertakes formal comparisons of whether or not each level of the categorical variable differs from the reference category. These comparisons generate the well known ‘significance values’ of parameter estimates (i.e., coefficients). Whilst it is straightforward to compare any one category with the reference category, it is more difficult to formally compare two other categories (or levels) of an explanatory variable with each other when neither is the reference category. This is known as the reference category problem.
Quasi-variances are approximations of
variance
In probability theory and statistics, variance is the expectation of the squared deviation of a random variable from its population mean or sample mean. Variance is a measure of dispersion, meaning it is a measure of how far a set of numbe ...
s. Quasi-variances are statistics associated with the
parameter estimates (coefficients) of the different levels of categorical explanatory variables within statistical models. Quasi-variances can be presented alongside parameter estimates to enable readers to assess differences between any combinations of parameter estimates for a categorical explanatory variable. The approach is beneficial because such comparisons are not usually possible without access to the full variance-covariance matrix for the estimates.
Using quasi-variance estimates addresses the reference category problem. The underlying idea was first proposed by Ridout but the technique was set out by
David Firth and Renee Menezes. The suitability of this technique for social science data analysis has been demonstrated. An on-line tool for the calculation of quasi-variance estimates i
availableand a short technical description of the methodology i
provided.
Quasi-variances can be calculated in
Stata using the QV module
and can also be calculated in
R using the packag
qvcalc
See also
*
Regression model
In statistical modeling, regression analysis is a set of statistical processes for estimating the relationships between a dependent variable (often called the 'outcome' or 'response' variable, or a 'label' in machine learning parlance) and one ...
*
Generalized linear model
*
Explanatory variable
Dependent and independent variables are variables in mathematical modeling, statistical modeling and experimental sciences. Dependent variables receive this name because, in an experiment, their values are studied under the supposition or deman ...
References
{{Reflist
External links
An extended set of resources with examples in
Stata and
SPSS
SPSS Statistics is a statistical software suite developed by IBM for data management, advanced analytics, multivariate analysis, business intelligence, and criminal investigation. Long produced by SPSS Inc., it was acquired by IBM in 2009. C ...
are als
available
Statistical deviation and dispersion