Risk criteria at entry
The prevalence of risk criteria met in each study is shown in Table . There was an elevated prevalence in the CHAMP high risk study on each risk criterion with the exception of marital problems and chronic difficulties, which were elevated in the ROOTS epidemiological study. This pattern was replicated with only high risk individuals, except there were no differences within the familial psychiatric illness and high emotionality risk criteria.
Proportions of individuals exhibiting each risk criterion in the CHAMP and the ROOTS study (table shows the full sample and those considered at high risk)
There was a higher level of depressive symptoms in the CHAMP compared to the ROOTS study (Table ).
Means (and SD) of the MFQ total scores for the CHAMP and ROOTS studies for the full sample and the high and low risk samples only
Comparing only those at high risk in both studies revealed no differences in mean depression scores. Similarly, there was no difference between the studies among those classified as not high risk. The study by risk status interaction term was not significant, p = .44, indicating that the relationship between high and not high risk was similar in both samples when looking at depressive symptoms.
Each of the individual risk criteria (with differing rates of prevalence across the two studies) may have a different relationship to depressive symptoms in the two studies. To test this, we conducted regressions with risk criterion by study interaction terms, as reported in Table .
Means (and SD) of the MFQ total scores for the CHAMP and ROOTS studies by the presence or absence of each individual risk criterion for the full sample and high risk individuals only
For all individuals, there were significant interaction terms with the following risk criteria: familial psychiatric illness, negative emotionality and chronic difficulties. For family psychiatric illness, this interaction was due to no difference in depressive symptoms between those with affected relatives and those without in the CHAMP study. Within the ROOTS study, individuals with familial psychiatric illness had increased levels of depressive symptoms. There were increased depressive symptoms in those with high negative emotionality scores for both samples. In CHAMP those not exposed to chronic difficulties had more depressive symptoms, whereas the expected pattern was observed in the ROOTS study.
The results were replicated with only high risk individuals from both studies and no significant interaction terms were found between the individual risk criteria and the study variable. Overall, the relationship between depressive symptoms and the individual risk criteria was not significantly different for high risk participants in the CHAMP and ROOTS studies.
Thus, the studies have a sufficiently similar association between risk criteria and depressive symptoms.
Longitudinal trajectory of depressive symptoms
Combining the samples allowed us to use both cross sectional and longitudinal data to investigate the trajectory of depressive symptoms. Within the CHAMP high risk study, each age treated as a distinct age cohort. For reference, previous research has found that there is good specificity at a cut-off of 29 on MFQ for differentiating individuals a major depressive disorder [27
Figure displays the average total score of depressive symptoms by each age group at baseline for the CHAMP study and the longitudinal trajectory of the ROOTS study. Within the CHAMP study, there were no age differences in depressive symptoms for the female participants, ps > .3. For the male participants, there was a significant decrease of between 2.5 to just over 4.5 points on the MFQ from age 12 to ages 13, 14 and 15, β = −2.53, 95% CI (−4.69, −.37), p < .05; β = −3.11, 95% CI (−5.53, −.68), p < .05; and β = −4.63, 95% CI (−7.84, −1.41), p < .01, respectively. There were no other differences between the age cohorts, ps > .1.
Average total score for depressive symptoms at age 12, 13, 14 and 15 CHAMP cohorts (n = 868) and the longitudinal trajectories in the ROOTS sample (n = 1,181) for female and male participants
We analysed differences between the genders in each age cohort separately. There was no significant difference between the genders at age 12, p = .71. At age 13 and 14, females had significantly more depressive symptoms than males, β = 3.22, 95% CI (1.45, 5.00), p < .001 and β = 4.56, 95% CI (2.05, 7.08), p < .001, respectively. However, there was no difference at age 15, p = .10, where the sample size was smaller.
Within the ROOTS epidemiological study, there were significant linear and quadratic differences from age 14 to 17, β = 1.85, 95% CI (.55, 3.16), p < .01 and β = −.71, 95% CI (−1.15, −.28), p < .005, respectively. When tested categorically, it was found that depressive symptoms were higher at age 15.5 than at either age 14 or 17, β = 1.18, 95% CI (.14, 2.22), p < .05 and β = 2.02, 95% CI (.86, 3.18), p < .005, respectively. There was no difference between 14 and 17 years, p > .10. The pattern of results differed for male participants, with no significant linear and quadratic trends from age 14 to 17, ps > .5.
Elevated levels of depressive symptoms were found in female compared to male ROOTS participants, β = 4.48, 95% CI (3.42, 5.54), p < .001. This was found to be independent of time point, as the interaction term between age and gender was not significant, p = .85.
The results when only looking at the CHAMP male and female high risk individuals were replicated from the full sample analysis and are shown in Fig. .
Average MFQ total score for age 12, 13, 14 and 15 CHAMP cohorts (n = 678) and the longitudinal trajectories in the ROOTS study (n = 399) for high risk female and male participants
Within the ROOTS study, there were no linear or quadratic differences in high risk female depressive symptoms between the ages of 14 and 17, ps > .09, which contrasts with the quadratic differences when considering the full sample shown in Fig. . The results for the male ROOTS high risk individuals were replicated from the full sample as were the gender differences in both studies.