Prevalence of MDE across gender and age groups
The prevalence estimates for 1-year and lifetime MDE in the total sample were 1.8% (95% CI 1.2-2.4%) and 3.6% (95% CI 2.8-4.4%) respectively. The ratio of 1-year:lifetime prevalence was approximately 0.5 (). Males in the 35-49 years age group were estimated to have a higher lifetime prevalence of MDE than females in the corresponding age group at trend level (3.6% v. 2.3%, p=0.06). The two genders did not differ in either lifetime or 1-year prevalence in any other age group. Regarding age, there was no significant difference among age groups in 1-year prevalence. Controlling for age, gender and time-varying factors such as education and marital status, cohort was a significant predictor of lifetime risk of MDE (Wald χ2=54.8, p<0.01), with risk significantly elevated among more recently born respondents (those born in 1967 or later compared with earlier cohorts).
Prevalence of 1-year and lifetime MDE
Sociodemographic correlates of 1-year MDE
None of the sociodemographic variables we considered was independently associated with MDE in the total sample (). However, significant associations were found within age group. Separated, widowed and divorced respondents were more likely to have MDE than those married in the 35-49 years age groups; this subgroup also had higher prevalence of MDE than other age groups and the marital subgroup. For respondents having low-average education or above, those in the younger age group (18-34 years) had higher prevalence of MDE than older age groups. By contrast, for respondents in the 50-64 years age group, those with high-average or low education seemed to have higher risk of MDE. As for income, respondents having low-average income exhibited an elevated risk of MDE in the total age group sample but significant likelihood was not shown for particular age groups.
Sociodemographic correlates of 1-year MDE by agea
History of depression across gender and age groups
The mean age of onset of MDE for the entire sample was 30.3 years (). While the age of onset was generally later for older age groups, there was no significant difference between genders and among age groups on the mean number of years having MDE or the mean number of lifetime MDE episodes, except for the following. First, females with 1-year MDE in young adulthood (18-24 years) had later age of onset than males in the same age group. Second, those in the middle age group (35-49 years), for both males and females, had a larger number of lifetime episodes. Third, the group of five elderly females (≥65 years) with 1-year MDE had the largest mean number of years having MDE in their lifetime and largest mean number of lifetime episodes.
History of depression between genders and among age groups
Symptom severity, role impairment, and suicidality
The distribution of severity among respondents with 1-year MDE was 15.7 % (s.e.=6.7%) mild, 51.8% (s.e.=8.0%) moderate, 25.3% (s.e.=5.8%) severe and 6.4% (s.e.=3.1%) very severe. The mean number of days out of role because of MDE in the year before interview among 1-year cases was 27.5 days. The number of days out of role is associated with the QIDS-SR measure of symptom severity: mild, 7.4 days, moderate, 23.2 days, severe, 31.7 days, very severe, 95.5 days. Percentage of respondents who reported severe or very severe impairment in at least one of the role domains on the SDS was 37.6% among those with mild QIDS-SR scores; 35.0 % among moderate, 46.6% among severe, and 62.0% among very severe (p=0.2). The overall percentage of 1-year cases with severe or very severe SDS scores was 40.1%. Respondents with different symptom severities did not differ in terms of having one or more co-morbid 1-year DSM-IV disorders (total average, 26.9 %; mild, 22.4 %; moderate, 34.0%; severe, 21.8 %; very severe, 0 %).
Suicidality was common. Among those reporting 1-year MDE, 9.8% (s.e.=5.4%) reported suicide ideation (OR 3.2 compared with respondents who were not depressed), 12.9% (s.e.=4.9%) reported suicide plans (OR 5.8) and 11.0% (s.e.=3.8%) reported suicide attempts (OR 3.8) in their lifetime. The number in each cell was too small for cross-tabulation analysis of age group and suicidality. Suicidal ideation and middle adulthood (35-49 years) included enough sample in the cell and showed significant association (OR 4.7, 95% CI 1.8-12.2, Wald χ2=10.7, p<0.01).
Role impairment by 1-year MDE across gender and age groups
All respondents with 1-year MDE reported some degree of role impairment in at least one domain on the SDS because of depression (). With regard to global functioning in the 35-49 years age group, males were not only more impaired than females, but also more impaired than males in other age groups. For close relationships, females reported more impairment than males in the older age groups(50-64, ≥65 years). Females in the 50-64 years age group and males in the 18-34 years age group reported more impairment than other age groups in their own gender respectively. Regarding social functioning, both genders in the 50-64 years age group reported more impairment than other age groups. Likewise, both males and females in the 50-64 years age group reported more impairment at home than other age groups. Moreover, males in middle adulthood (age 35-49 years) reported more impairment at home than middle-age females. For the domain of work, there was no significant difference between genders and among age groups.
Role impairment by 1-year MDE shown by mean Sheehan scorea by age×gender category
We also cross-tabulated gender and age group for those reporting 1-year MDE and severe impairment as indicated by an SDS score of ≥7 (results available upon request). Males in the 35-49 years age group were more likely than their female counterparts to report severe global impairment and impairment in the home domain; females in the 50-64 years age group were more likely to report impairment in close relationships than females in other age groups. As for mean days out of role, respondents in the 50-64 years age group reported substantially more impairment (108.3 days) than other age groups; the difference was significant at trend level (p=0.06).
Mental and physical co-morbidity of 1-year MDE
Co-morbidity was high between 1-year MDE and any other DSM-IV disorders (OR 22.0, 95% CI 9.8-49.7, Wald χ2=58.3, p<0.01). Over half (52.5%, s.e.=8.3%) of those with 1-year MDE reported any other mental disorder(s). Anxiety disorders were the most commonly associated group of disorders (OR 28.8, 95% CI 11.9-69.6, Wald χ2=58.3, p<0.01); 39% (s.e.=8.9%) of those reporting 1-year MDE reported any of them. The association strengthened [χ2(2)=34.6, p<0.01] with increasing age (age 18-34 years, 22.6%, OR 10.2; age 35-49 years, 36.9%, OR 43.7; age 50-64 years, 67.4%, OR 70.7). Specifically, generalized anxiety disorder (OR 8.9) and specific phobia (OR 3.7) were associated with MDE, especially in the age range of 35-49 years (OR 20.3 and OR 4.6 respectively). Substance-use disorders were significantly associated with 1-year MDE (OR 7.5), as was intermittent explosive disorder in the age range of 18-34 years (OR 6.7, Wald χ2=6.6, p=0.01). There was no other significant association between MDE and other mental disorders (detailed cross-tabulation for all surveyed mental disorders in all age groups is available upon request).
One-year MDE was also associated with any physical conditions (OR 3.2, CI 1.3-8.0, Wald χ2=6.3, p=0.01). Of those with MDE, 76.8% (s.e.=6.7%) reported at least one physical condition. The youngest age group (18-34 years) reported the largest number of physical conditions including frequent or severe headache (17.5%, OR 6.2), back or neck pain (46.9%, OR 4.8), other chronic pain (31.1%, OR 7.7) and seasonal allergy (23.9%, OR 3.7). They were also more likely to report three or more physical conditions (28.4%, OR 12.2). There were specific associations between particular age groups and physical conditions, such as middle adulthood and heart disease (16.1%, OR 2.9), or late adulthood and arthritis or rheumatism (67.2%, OR 8.0). Headache and MDE were associated in all age groups and the association increased with age [χ2(2)=17.2, p<0.01]. The strength of association between MDE and physical conditions was not as strong as between MDE and other mental disorder, but such associations were of a greater variety (detailed cross-tabulation is available upon request).
One-year treatment among age groups and sociodemographic correlates of receiving treatment
Only 22.7% (s.e.=9.7%) of those with any severity of 1-year MDE sought treatment in the same year. Specifically, 18%, 5.2% and 12.4% were treated in the general medical, mental health, and complementary-alternative medical sectors respectively. Respondents in middle adulthood (35-49) were the least likely to be treated [χ2(3)=13.7, p<0.01]. Among the 29 respondents who reported severe impairment, only one received treatment in the complementary-alternative medical sector. None was treated in any other sector.
Because of small cell sizes for treatment and no respondent with severe impairment received treatment from the mental health sector, only a few associations were found between receiving any treatment and sociodemographic correlates. Respondents with 1-year MDE and low-average income (40.4%, OR 23.3) were more likely to receive any treatment in the same year compared with higher income groups [χ2(2)=8.6, p=0.01].