 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 isMore animals experience the larger group. For group sizes $n_1,\dots ,n_k$, 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.^[ref] The experimental unit must follow the design; a larger headcount alone does not solve pseudoreplication.