Review: Selection Effects in Psychiatric Epidemiology

The paper reviewed here is a Norwegian study ”Selection Effects in Psychiatric Epidemiology’ by Nygard and colleagues. In the abstract, the authors conclude that

Studies of predictors ought to have a long follow-up, as several years might pass before differences in mortality are revealed

In the introduction the researchers discuss some of the varied results from studies looking at the association between mental illnesses and mortality. Some have shown a reduction in mortality, others an increase and still others no difference with samples from the general population. The researchers used a longitudinal design here with an assessment instrument -the Hopkins Symptom Check List 25 – to compare a sample – the OsLof sample with a sample of the general population (from the Norwegian registry database).


The researchers identified a random sample of 5000 people in Norway, over the age of 18. Letters were sent out and in 2726 ‘contact was obtained’. 712 declined to take part and 24 weren’t matched with controls (from a registry study from which 1% of the Norwegian population were sampled). Both samples were followed up between 1990 and 2003. The OsLof sample was stratified according to the Hopkins Symptom Check List 25 score – those above and below a threshold of 1.75.


I won’t comment on all of the results here, just the two I found most interesting. Firstly they found that HSCL-25 scores were a predictor of mortality in the OsLof sample but this only became significant at 12 years (close to the study end) compared to the population mortality rates – 1.57 times higher mortality rates (I think this corresponded to the hazards ratio for men with a confidence interval of 1.26-1.97) . So essentially for 12/15 of the study years there was no significant difference.  The second result of interest was the in the first few years of the study, the OsLof sample had lower mortality rates than the registry sample, the implication being that those included in the OsLof were likely to be healthier supporting some of the other cited studies which examine the health of subjects that aren’t included in studies at the initial stages.


The authors conclude that relative to the healthy sample, those with a score on the HSCL of >= 1.75 showed a signifiacntly increased risk of mortality only after 15 years. They suggested that those who did not participate in the study may have had higher rates of metnal distress on the basis of other cited studies.

Returning to the issue of multiple copmparisons, i’m not sure if it applies here but I thought it could do. My reasoning is as follows. In the results section, the analysis the researchers have calculated the differences in mortality at several points in this longitudinal study. The more points at which this comparison takes place, the higher is the likelihood of a false positive necessitating a correction (e.g the Bonferroni correction). The other point here is the choice of the threshold of 1.75. There are obviously a large number of alternative thresholds that could be used. However I suspect this threshold was used because of  a previous study using this threshold for women and a threshold of 1.67 for men as optimum cut-off point (Sandanger et al, 1998).

The other point of interest here is to what degree a cross-sectional use of a symptom checklist should be used as predictor. The HSCL is being used as a proxy marker of anxiety and depression but there is an argument for a clinical diagnosis to support the results on the HSCL-25. The symptoms themselves may fluctuate over time. Additionally another study provides evidence that the scale should be validated in the specific cultures in which it is to be used (Ventevogel et al, 2007).

In summary, the study shows evidence of a selection effect for health in those included in the study relative to a sample from the population registry. It would be interesting to see the study replicated with a clinical diagnosis supported by quantitative measure of illness severity.  A difficulty in these types of epidemiological studies is that it is difficult to generalise to mental illnesses because this covers such a broad spectrum. The pending changes to the DSM-IV and ICD-10 will no doubt introduce a number of conditions not previously considered and so studies of this type could focus on specific illnesses as a proxy to the broader group of mental illnesses.


Nygard J, Klungsoyr O, Sandanger I and Svensson E. Selection Effects in Psychiatric Epidemiology. A 14-year prospective study of the Hopkins Symptom Checklist 25 as a predictor of mortality in the Norwegian general population. Soc Psychiat Epidemiol. 2009. 44. 881-886.

Sandanger I, Moum T, Ingebrigtsen G, Dalgard OS, Sørensen T, Bruusgaard D. Soc Psychiatry Psychiatr Epidemiol. 1998 Jul;33(7):345-54.Concordance between symptom screening and diagnostic procedure: the Hopkins Symptom Checklist-25 and the Composite International Diagnostic Interview I.

Ventevogel P, De Vries G, Scholte WF, Shinwari NR, Faiz H, Nassery R, van den Brink W, Olff M. Soc Psychiatry Psychiatr Epidemiol. 2007 Apr;42(4):328-35. Epub 2007 Feb 13. Properties of the Hopkins Symptom Checklist-25 (HSCL-25) and the Self-Reporting Questionnaire (SRQ-20) as screening instruments used in primary care in Afghanistan.


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The comments made here represent the opinions of the author and do not represent the profession or any body/organisation. The comments made here are not meant as a source of medical advice and those seeking medical advice are advised to consult with their own doctor. The author is not responsible for the contents of any external sites that are linked to in this blog.


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