Table 2. Descriptive Data and Within/Between Cohort Outcome Analysesa.
| Status | Mean (SD) | AMD (95% CI) | P value | Hedges g | ICC | Mean (SD) | AMD (95% CI) | P value | Hedges g | ICC | Mean (SD) | AMD (95% CI) | P value | Hedges g | ICC | |||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| CES-D T3 | CES-D T4 | SDQ T3 | SDQ T4 | WEMWBS T3 | WEMWBS T4 | |||||||||||||
| Cohort 1, prepandemicb | 15.80 (11.66) | 17.29 (12.04) | 1.52 (0.84 to 2.20) | <.001 | 0.16 | 0.02 | 12.33 (6.70) | 13.19 (6.62) | 0.86 (0.53 to 1.20) | <.001 | 0.18 | 0.04 | 48.26 (9.49) | 47.40 (9.39) | −0.85 (−1.45 to −0.24) | .006 | −0.10 | 0.02 |
| Cohort 2, pandemicc | 16.80 (11.67) | 20.26 (12.44) | 3.43 (3.04 to 3.83) | <.001 | 0.33 | 0.02 | 12.69 (6.63) | 14.32 (6.55) | 1.62 (1.42 to 1.82) | <.001 | 0.30 | 0.02 | 47.83 (9.83) | 44.87 (9.80) | −2.92 (−3.25 to −2.59) | <.001 | −0.32 | 0.02 |
| Between cohort | NA | NA | 1.91 (1.07 to 2.76) | <.001 | 0.17 | 0.02 | NA | NA | 0.76 (0.33 to 1.18) | <.001 | 0.12 | 0.02 | NA | NA | −2.08 (−2.80 to −1.36) | <.001 | −0.22 | 0.02 |
Abbreviations: AMD, adjusted mean difference; CES-D, Center for Epidemiological Studies for Depression scale; ICC, intracluster correlation coefficient ; NA, not applicable; SDQ, Strengths and Difficulties Questionnaire; T3, time point 3; T4, time point 4; WEMWBS, Warwick-Edinburgh Mental Well-Being Scale.
Mixed-effects linear regressions with maximum likelihood estimation were fitted, including schools (clusters) as random effects and adjusted for the country, school size, coeducation, allocation group, and the time difference (days) between T3 and T4. Descriptive data are raw. The degree of school-level clustering, using the ICC for the 3 mental health outcomes, was evaluated. Unconditional means models were fitted, using 2-level mixed-effects linear regressions at T3 to estimate ICCs at the school level. All differences remained significant when the multiple testing correction was applied. The CES-D range is 6 to 60; SDQ, 0 to 40; and WEMWBS, 14 to 70. T3 for cohort 1 was from September 21, 2018, to February 14, 2019, and for cohort 2 was September 4, 2019, to January 17, 2020. T4 for cohort 1 was from September 10, 2019, to January 15, 2020, and for cohort 2 was February 23, 2021, to June 21, 2021.
In cohort 1, of 769 students providing data, 764 provided data on the CES-D, 763 on the SDQ, and 768 on the WEMWBS.
In cohort 2, of 2958 students providing data, 2934 provided data on the CES-D, 2903 on the SDQ, and 2948 on the WEMWBS.