Some details
Many animals, including humans, tend to live in groups,
herds, flocks, bands, packs, parties, or colonies (hereafter: groups) of
conspecific individuals. The size of these groups, as expressed by the
number of participants, 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. Unfortunately, group size measures are notoriously hard to
handle statistically since groups size values typically exhibit an
aggregated (rightskewed) 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.
1. Outsiders’ view of group size
•
Group size is the number of individuals within a certain
group;
•
Mean group size, i.e. the arithmetic mean of group sizes
averaged across groups;
•
Confidence interval of mean group size;
•
Median group size, i.e. the median of group sizes
calculated across groups;
•
Confidence interval of median group size.
2. Insiders’ view of group size
As Jarman (1974) pointed out, average individuals live in
groups larger than the average – simply because the groups smaller than
average have fewer individuals than the groups larger than average. (Except
for an unrealistic case when all groups are of equal size.) Therefore,
when we wish to characterize a typical (average) individual’s social
environment, we should not apply the outsiders’ view of group size.
Reiczigel et al. (2008) proposed the following measures:
•
Crowding is the number of individuals within a group
(equals to group size: 1 for a solitary individual, 2 for
both individuals in a group of 2, etc.);
•
Mean crowding, i.e. the arithmetic mean of crowding
values averaged across individuals
(this was called "Typical Group Size" in Jarman's
terminology);
•
Confidence interval of mean crowding.
