Table 2.

Summary of characteristics of all studies included in the review (n = 18).

PICO variable	Study characteristics
Population	Age range: 4; 4–10 years. Mean $\left(\overline{x}=7.8\mathrm{y}\right)$ Status: Typically developing children (n = 12) Language impaired children (n = 1) Children with reading difficulties (n = 1) Children with identified WM difficulties (n = 1) Children considered at risk of learning difficulties (n = 1) Children with Down’s syndrome (n = 1) Overweight children (n = 1) Studies specifically targeting children from low socio-economic status (SES) (n = 0)
Intervention	Intervention approach: Adapting the environment (n = 1); Direct WM training without strategy instruction (n = 3) Direct WM training with strategy instruction (n = 5); Training skills that indirectly impact on WM (n = 9). – physical activity (n = 5); – phonological awareness skills (n = 2); – fantastical play (n = 1); – inhibition (n = 1). Where the intervention was delivered: via Skype (n = 1) school (n = 12) preschool setting (n = 1) gym (n = 1) No detail given (n = 3) Unit of delivery (grouping of participants): Individual (n = 8) Small group (n = 6) Whole class (n = 2) Combination of individual and small group (n = 2). Dosage: Total duration of intervention ranged from 10 days to 11 months Average total intervention duration in weeks $\left(\overline{x}=13\right)$
Comparison	Study design: Randomised controlled trials (n = 8) Quasi-experimental designs (n = 9) Case series (n = 1) Control group conditions: Active control groups (n = 2) No intervention control groups (n = 14) Own matched controls (n = 2) Sampling: Power analysis reported (n = 0) Sample size range across all included studies (n = 4 to n = 256). Average sample size $\left(\overline{x}=74\right)$
Outcomes	Measurement of WM: Verbal STM (n = 11) Visuospatial STM (n = 7) Verbal ELWM (n = 14) Visuospatial ELWM (n = 4) Studies which measured more than one aspect of WM (n = 7) Studies which implemented a standardised measure of WM (n = 5) Measurement of transfer effects: Near-transfer (n = 4) Far-transfer (n = 7) Durability over time (n = 4)