Verdon et al. (2019).

Population:	Large White × Landrace, piglets, (Australia).
Sample size:	Before weaning: 36 litters (378 piglets). After weaning: 30 litters (198 piglets). 10–14 piglets per pen, five pens per treatment. Selection of piglets was to ensure the balance of the two litters in terms of: Average piglet weight at weaning in each litter CV (the term was not expanded in original study) in piglet weight at weaning. Litter age. Sex.
Intervention details:	Pre-weaning treatment groups: Treatment groups were balanced for sow’s parity, sow weight and litter size. Group 1 farrowing crate for the whole lactation period (no socialisation / control) (FC):  n = 12 litters. Housing design: sows and litters remained in individual farrowing crates for the whole lactation period. Group 2 farrowing crate and then transferred to group lactation (GL) pens at day 7 post-partum (socialisation from day 7) (GL7): n= 12 litters. Socialisation housing design: at average day 7 post-partum, 12 sows and their litters were transferred into either: Five sows / litters group lactation pen. Seven sows / litters group lactation pen. Socialisation group size: five litters or seven litters per pen (socialisation group size is the number of litters that interact with one another during lactation). Group 3 farrowing crate and then transferred to group lactation pens at day 14 post-partum (socialisation from day 14) (GL14): n = 12 litters. Socialisation housing design: At average day 14 post-partum, 12 sows and litters were transferred into either: Five sows / litters group lactation pen. Seven sows / litters group lactation pen. Socialisation group size: five litters or seven litters per pen.
Study design:	Non-randomised controlled trial.
Outcome studied:	Weaning age: average 26.7 days (range from 22–29 days). Socialisation length: 13 days. Regroup condition: Equal proportions from two unfamiliar litters (5–7 piglets from each litter), from the same treatment groups (refer to sample size section for selection criteria). Post-weaning group size: 10–14 piglets. The group size was dependent on: Healthy piglets available per litter. Number of litters available in the smaller group lactation pen (the five litter pen). Group size for each condition: FC: five pens of 14 piglets. GL7: four pens of 14 piglets and one pen of 10 piglets. GL14: two pens of 14, two pens of 12, and one pen of 10 piglets. Familiarity ratio: 1:1. Behavioural indicators: Behavioural recording for 3.5 hours (30 minute interval) after post-weaning mixing. Aggression: frequency of aggressive bouts, frequency and duration (total, average) of fights and bullying events. Dyadic familiarity and aggression: whether each interaction involves a familiar piglet or non-familiar piglet. (The outcome was not relevant to PICO question and not examined in this review).
Main Findings (relevant to PICO question)	Behavioural indicators: Behavioural recording for 3.5 hours (30 min interval) after post-weaning mixing. Aggression Frequency of aggressive bouts:  FC > GL7 (trend only), FC > GL14 (trend only). Frequency of fights: FC > GL7 (significant, P < 0.001), FC > GL14 (significant, P < 0.001). Duration of fights: FC > GL7 (significant, P < 0.001), FC > GL14 (significant, P < 0.001). Frequency of bullying: FC > GL7 (trend only), FC > GL14 (trend only). Aggressive bouts X Time: aggressive bouts delivered by all piglets declined over time, but GL7 and GL14 piglets reached baseline quicker than FC piglets (trend only). During 30–60 minutes post mixing, FC piglets spend more time fighting than GL7 and GL14 (significant, P < 0.05); but no difference among groups found in other time periods.
Limitations:	The socialisation group (GL7 and GL7) had more space allowance than the control group (FC) during lactation, which might be significant. The exact number of piglets in each pre-weaning treatment group was not specified. Group lactation pens for socialisation were not identical in structure: one accommodated five sows and litters while the other accommodated seven sows and litters. This might affect the level of socialisation. The method for allocation of the sows and litters to either five or seven litter group lactation pens was unclear. There was a range of weaning age. Group size after regrouping was not uniform (10–14 piglets), resulting in different space allowance per piglet. The number of observers, and skill level of the observer for behavioural recording were not specified.  The behavioural outcomes were measured for only 3.5 hours.  The baseline for ‘Aggressive bouts X Time’ was compared to but the establishment of such baseline was not specified.