anosim {vegan} | R Documentation

## Analysis of Similarities

### Description

Analysis of similarities (ANOSIM) provides a way to test statistically whether there is a significant difference between two or more groups of sampling units.

### Usage

    anosim(dis, grouping, permutations=1000, strata)

### Arguments

`dis` | Dissimilarity matrix.
`grouping` | Factor for grouping observations.
`permutations` | Number of permutation to assess the significance of the ANOSIM statistic.
`strata` | An integer vector or factor specifying the strata for permutation. If supplied, observations are permuted only within the specified strata.

### Details

Analysis of similarities (ANOSIM) provides a way to test statistically whether there is a significant difference between two or more groups of sampling units. Function `anosim` operates directly on a dissimilarity matrix. A suitable dissimilarity matrix is produced by functions `dist` or `vegdist`. The method is philosophically allied with NMDS ordination (`isoMDS`), in that it uses only the rank order of dissimilarity values.

If two groups of sampling units are really different in their species composition, then compositional dissimilarities between the groups ought to be greater than those within the groups. The `anosim` statistic R is based on the difference of mean ranks between groups (r_B) and within groups (r_W):

R = (r_B - r_W)/(N (N-1) / 4)

The divisor is chosen so that R will be in the interval -1 ... +1, value 0 indicating completely random grouping.

The statistical significance of observed R is assessed by permuting the grouping vector to obtain the empirical distribution of R under null-model.

The function has `summary` and `plot` methods. These both show valuable information to assess the validity of the method: The function assumes that all ranked dissimilarities within groups have about equal median and range. The `plot` method uses `boxplot` with options `notch=TRUE` and `varwidth=TRUE`.

### Value

The function returs a list of class `anosim` with following items:

`call` | Function call.
`statistic` | The value of ANOSIM statistic R
`signif` | Significance from permutation.
`perm` | Permutation values of R
`class.vec` | Factor with value `Between` for dissimilarities between classes and class name for corresponding dissimilarity within class.
`dis.rank` | Rank of dissimilarity entry.
`dissimilarity` | The name of the dissimilarity index: the `"method"` entry of the `dist` object.

### Note

I don't quite trust this method. Somebody should study its performance carefully. The function returns a lot of information to ease further scrutiny. Most `anosim` models could be analysed with `adonis` which seems to be a more robust alternative.

### Author(s)

Jari Oksanen, with a help from Peter R. Minchin.

### References

Clarke, K. R. (1993). Non-parametric multivariate analysis of changes in community structure. Australian Journal of Ecology 18, 117-143.

### See Also

`mrpp` for a similar function using original dissimilarities instead of their ranks. `dist` and `vegdist` for obtaining dissimilarities, and `rank` for ranking real values. For comparing dissimilarities against continuous variables, see `mantel`. Function `adonis` is a more robust alternative that should preferred.

### Examples

    data(dune)
    data(dune.env)
    dune.dist <- vegdist(dune)
    attach(dune.env)
    dune.ano <- anosim(dune.dist, Management)
    summary(dune.ano)
    plot(dune.ano)

[Package vegan version 1.8-8 Index]