Social group size
= Social group size
<Social group size> is the number of individuals assigned to a <social group> under a stated membership rule and observation period. A headcount at one moment, a count of regular members and the number present during an experiment can differ. Report which quantity was measured before comparing groups.
The way groups are sampled changes their apparent typical size. Consider two groups containing 2 and 8 animals. Their <arithmetic mean> size is $(2+8)/2=5$. If an animal is instead sampled uniformly from the ten individuals, the expected size of its group is
$$
\frac{2\cdot2+8\cdot8}{2+8}=6.8.
$$
More animals experience the larger group. For group sizes $n_1,\ldots,n_k$, this <individual-weighted mean group size> is $\sum_i n_i^2/\sum_i n_i$, rather than $\sum_i n_i/k$. These are original illustrative calculations, not measured animal data.
<Social group size> also affects the interpretation of <replication>. A finch experiment used 70 <Bengalese finches> in seven groups and 58 <white-rumped munias> in six groups. The groups contained ten birds each except one eight-bird munia group. Those totals describe birds and groups, not interchangeable counts of independent treatment assignments.<Effects of domestication on neophobia: A comparison between the domesticated Bengalese finch and its wild ancestor, the white-rumped munia by Kenta Suzuki and others>[Birds used in this study: cage membership and group counts]{ref} The <experimental unit> must follow the design; a larger headcount alone does not solve <pseudoreplication>.