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ANOVA

In statistics, analysis of variance (ANOVA) is a collection of statistical models and their associated procedures which compare means by splitting the overall observed variance into different parts. The initial techniques of the analysis of variance were pioneered by the statistician and geneticist R. A. Fisher in the 1920s and 1930s, and is sometimes known as Fisher's ANOVA or Fisher's analysis of variance, due to the use of Fisher's F-distribution as part of the test of statistical significance.

Contents

Overview

There are three conceptual classes of such models:

  • Fixed-effects model assumes that the data come from normal populations which differ in their means.
  • Random-effects models assume that the data describe a hierarchy of different populations whose differences are constrained by the hierarchy.
  • Mixed models describe situations where both fixed and random effects are present.

In practice, there are several types of ANOVA depending on the number of treatments and the way they are applied to the subjects in the experiment:

  • One-way ANOVA is used to test for differences among three or more independent groups.
  • One-way ANOVA for repeated measures is used when the subjects are dependent groups; this means that the same subjects are used for each treatment. Note that this method is subject to carryover effects.
  • 2×2 (read: two by two) ANOVA, the most common type of factorial analysis of variance, is used when the experimenter wants to study the effects of two or more treatment variables. Factorial ANOVA can also be 2×2×2, 3×3, etc. but higher numbers of factors is rarely done because the calculations are lengthy and the results are hard to interpret.

Models

Fixed-effects model

The fixed-effects model of analysis of variance applies to situations in which the experimenter has subjected his experimental material to several treatments, each of which affects only the mean of the underlying normal distribution of the "response variable".

Random-effects model

Random effects models are used to describe situations in which incomparable differences in experimental material occur. The simplest example is that of estimating the unknown mean of a population whose individuals differ from each other. In this case, the variation between individuals is confounded with that of the observing instrument.

Assumptions

  • Independence of cases - this is a requirement of the design.
  • Scale of measurement - the dependent variable is interval or ratio.
  • Normality - the distributions in each of the groups are normal (use the Kolmogorov-Smirnov and Shapiro-Wilk normality tests to test it). Note that the F-test is extremely non-robust to deviations from normality (Lindman, 1974).
  • Homogenity of variances - the variance of data in groups should be the same (use Levene's test for homogeneity of variances).

Logic of ANOVA

The fundamental technique is a partitioning of the total sum of squares into components related to the effects in the model used. For example, we show the model for a simplified ANOVA with one type of treatment at different levels. (If the treatment levels are quantitative and the effects are linear, a linear regression analysis may be appropriate.)

SSTotal = SSError + SSTreatments.

The number of degrees of freedom (abbreviated df) can be partitioned in a similar way and specifies the chi-square distribution which describes the associated sums of squares.

dfTotal = dfError + dfTreatments.

Degrees of freedom

Degrees of freedom indicate the effective number of observations which contribute to the sum of squares in an ANOVA, the total number of observations minus the number of linear constraints in the data. The degrees of freedom are the number of participants (for each group) minus 1. This removes the error otherwise produced by the differences in variance of such groups to account for the difference in sample and population variance.

Examples

Group A is given vodka, Group B is given gin, and Group C is given a placebo. All groups are then tested with a memory task. A one-way ANOVA can be used to assess the effect of the various treatments (that is, the vodka, gin, and placebo).

Group A is given vodka and tested on a memory task. The same group is allowed a rest period of five days and then the experiment is repeated with gin. The procedure is repeated using a placebo. A one-way ANOVA with repeated measures can be used to assess the effect of the vodka versus the impact of the placebo.

In an experiment testing the effects of expectations, subjects are randomly assigned to four groups:

  1. expect vodka-receive vodka
  2. expect vodka-receive placebo
  3. expect placebo-receive vodka
  4. expect placebo-receive placebo (the last group is used as the control group)

Each group is then tested on a memory task. The advantage of this design is that multiple variables can be tested at the same time instead of running two different experiments. Also, the experiment can determine whether one variable affects the other variable (known as interaction effects). A factorial ANOVA (2×2) can be used to assess the effect of expecting vodka or the placebo and the actual reception of either.

See also

Additional references

  • King, Bruce M., Minium, Edward W. (2003). Statistical Reasoning in Psychology and Education, Fourth Edition. Hoboken, New Jersey: John Wiley & Sons, Inc. ISBN 0471211877
  • Lindman, H. R. (1974). Analysis of variance in complex experimental designs. San Francisco: W. H. Freeman & Co.