Schrey et al. (2019).

Population:	Breed unknown, piglets, (Germany).
Sample size:	Before weaning: 34 litters (400 piglets). After weaning: 14 litters (70 piglets).
Intervention details:	Pre-weaning treatment groups: Group 1 conventional individual housing / control / no socialisation (IH): n = 10 litters (126 piglets). Housing design: littermate-only with sow in a farrowing pen until weaning. Group 2 group housing / socialisation (GH):  n = 24 litters (274 piglets). Socialisation housing design: five farrowing pens with a central common area which was made accessible after the first piglet can pass through the flexible steps. Socialisation group size: five litters per pen (socialisation group size is the number of litters that interact with one another during lactation).
Study design:	Non-randomised controlled trial.
Outcome studied:	Weaning age: day 35. Regroup condition: IH: n = six litters (30 piglets). GH: n = eight litters (40 piglets). Five piglets from two unfamiliar litters in the same pre-weaning treatment groups. Balanced by weight and sex. Post-weaning group size: 10 piglets per group. Familiarity ratio: 1:1. Growth performance: Average daily weight gain (ADWG) (days 35, 39 and 63). Behavioural indicators: Skin lesion (before weaning vs day 39).
Main Findings (relevant to PICO question)	Growth performance: ADWG: higher daily weight gain in GH than IH (days 35–39, P = 0.003; days 35–63, P = 0.085). Behavioural indicators: Skin lesion. IH: Skin lesion significantly increased (days 35–39, P = 0.014). GH: No significant increase (days 35–39).
Limitations:	Treatment group allocation method was not stated. The GH had more space allowance than the IH during lactation, which might be significant. The start day of socialisation during lactation was not specified, and based on its description, the start day may have varied in different litters. Socialisation length was unknown. The selection criteria for piglets to proceed to regrouping was not stated (sudden reduction from 400 to 70 piglets). Skin lesions were assessed by one trained observer only.