Fels et al. (2021).

Population:	Breed unknown, piglets, (Germany).
Sample size:	Before weaning: 16 litters. After weaning: 90 piglets (10 piglets per group). Selection criteria unknown.
Intervention details:	Pre-weaning treatment groups: Group 1 Control (Co): no socialisation, n = 8 litters. Housing design: littermate-only with sow in a farrowing crate until weaning. Group 2 Group housing (Gr): socialisation, n = 8 litters. Socialisation housing design: five farrowing pens with a central common socialisation area which was accessible by the piglets at day 10 onwards. Socialisation group size: five litters per pen.
Study design:	Non-randomised controlled trial.
Outcome studied:	Weaning age: 35 days. Socialisation length: 25 days. Regroup condition: Gr/Gr: five Gr littermate + five Gr littermate, familiar piglets (Not examined in this review because there was no post-weaning mixing of unfamiliar piglets). Gr/Co: five Gr littermate + five Co littermate, unfamiliar piglets. Co/Co: five Co littermate + five Co littermate, unfamiliar piglets. All post-weaning groups were balanced by weight and sex. Post-weaning group size: 10 piglets per group. Familiarity ratio: Gr/Gr: N/A. Gr/Co: 1:1. Co/Co: 1:1. Behavioural indicators: Aggressive interactions (days 35–36) through video recording. Total number of aggressive interactions. Result of aggressive interactions. Total number of aggressive interactions between littermates (LM). Total number of aggressive interactions between non-littermates (non-LM). Skin lesions (before weaning, and day 39).
Main Findings (relevant to PICO question)	Behavioural indicators: Aggressive interactions (days 35–36): Total number of aggressive interactions: Gr/Gr < Gr/Co < Co/Co (trend only). No significant difference between Gr/Co and Co/Co.  Total number of aggressive interactions with unclear results: Co/Co > Gr/Gr or Gr/Co (significant, P < 0.05). Total number of aggressive interactions between LM:  Co/Co > Gr/Co or Gr/Gr (significant, P < 0.05). Total number of aggressive interactions between non-LM: Gr/Co and Co/Co did not differ. Within the Gr/Co group, total number of aggressive interactions, and number of aggressive interactions between LM: Gr > Co (trend only). Skin lesions (before weaning day 39): No significant difference between treatment groups before weaning. Less skin lesions: Gr/Gr < Gr/Co < Co/Co (significant, P < 0.05).
Limitations:	The sample size before weaning was not reported. Treatment group allocation method was not stated. The socialisation group (Gr) had more space allowance than the control group (Co) during lactation, which might be significant. While the total number of piglets for regrouping was specified, the number of groups in each regrouping condition (e.g., Gr/Gr) was not stated. The number of and the identity of the person who assessed the behaviours through videotape was not specified. Aggressive behaviours were not defined clearly. The skin lesion scoring system from 0–3 was vaguely defined as low to high grade.