Group size measures

A group acts as a social environment of individuals: a flock of nine common cranes.

Many animals, including humans, tend to live in groups, herds, flocks, bands, packs, shoals, or colonies (hereafter: groups) of conspecific individuals. The size of these groups, as expressed by the number of participant individuals, is an important aspect of their social environment. Group size tend to be highly variable even within the same species, thus we often need statistical measures to quantify group size and statistical tests to compare these measures between two or more samples. Group size measures are notoriously hard to handle statistically since groups sizes typically follow an aggregated (right-skewed) distribution: most groups are small, few are large, and a very few are very large.

Statistical measures of group size roughly fall into two categories.

Outsiders' view of group size

Colony size measures for rooks breeding in Normandy. The distribution of colonies (vertical axis above) and the distribution of individuals (vertical axis below) across the size classes of colonies (horizontal axis). The number of individuals is given in pairs. Animal group size data tend to exhibit aggregated (right-skewed) distributions, i.e. most groups are small, a few are large, and a very few are very large. Note that average individuals live in colonies larger than the average colony size. (Data from Normandy, 1999-2000 (smoothed), Debout, 2003)

Insiders' view of group size

As Jarman (1974) pointed out, group sizes are not normally distributed, and thus average individuals live in groups larger than average. Therefore, when we wish to characterize a typical (average) individual’s social environment, we should apply non-parametric estimations of group size. Reiczigel et al. (2008) proposed the following measures:

Example

Imagine a sample with 3 groups, where group sizes are 1, 2, and 6 individuals, respectively, then

mean group size (group sizes averaged over groups) equals (1+2+6)/3=3;
mean crowding (group sizes averaged over individuals) equals (1+2+2+6+6+6+6+6+6)/9=4.555.

Generally speaking, given there are G groups with sizes n1, n2, ..., nG, mean crowding can be calculated as:

mean crowding= \sum_{i=1}^G n^2_i/\sum_{i=1}^G n_i

Statistical methods

Due to the aggregated (right-skewed) distribution of group members among groups, the application of parametric statistics would be misleading. Another problem arises when analyzing crowding values. Crowding data consist of non-independent values, or ties, which show multiple and simultaneous changes due to a single biological event. (Say, all group members' crowding values change simultaneously whenever an individual joins or leaves.)

Reiczigel et al. (2008) discuss the statistical problems associated with group size measures (calculating confidence intervals, 2-sample tests, etc.) and offer a free statistical toolset (Flocker 1.1).

Literature

See also

Size of groups, organizations, and communities

External links

Gallery

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