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Adjustment for income?
Posted by Amcunningham on 27 Jan 2011 at 01:02 GMT
I'm surprised that there was no adjustment for income. Low income is associated with increased risk of depression, and of food high in transfats as they are cheaper.
The study was based only on university graduates. So, we consider that the sample is homogenous regarding socioeconomic-status (medium-high).
Thank you for your response. Yes, I was aware that the study was based only on graduates but there is no reason to believe that graduates have homogenous incomes. Reports in the UK suggest that graduate income in Spain can vary significantly with degree (http://www.prospects.ac.u...). Do you have any data on the income of respondents at baseline or at any other time point?
Recent research in Spain has shown that income is associated with depression after adjustment for educational level (http://www.springerlink.c...).
You are right when you say that, in general, there are no reasons to believe that graduates have homogenous income. This is true in general.
However, the participants in our cohort do represent a fairly homogeneous group of graduates (responsible, highly motivated, and many of them former students of a private university charging higher fees than the usual Spanish public universities).
In fact, when we used years of university education (continouous) as a surrogate of income no association with incident depression was found.
The idea that you mentioned was that the higher the degree, the higher the income, and the lower the risk of depression.
In our cohort, the age-, and sex-adjusted association of the variable level of educational attainment (years) with the risk of depression was not in the direction that you suggested. The association was not inverse, but direct. However, it was nonsignificant and basically null (RR= 1.03, 95% CI: 0.98-1.10 for each additional year of educational attainment).
Therefore, there is no basis to consider this as an important confounder in our study (we acknowledge that it could be an important confounder in studies that do not use restriction).
Restriction is an excellent technique for preventing or at least reducing confounding by known factors, and it is recommended by methodologists because restriction is usually more effective than multivariable adjustment to control for potential confounding (see for example Rothman, Greenland and Lash, Modern Epidemiology, 3rd edition, page 169).
Thanks for you kind comments,
I'm sorry but you have misunderstood my query. I was suggesting that it is not reasonable to assume that graduates have similar incomes, and I can find no evidence to suggest that this is the case in Spain. I have not made any comment about the length of degree or attaining a higher degree. I do not assume that those who attain a doctorate have higher incomes that those who do not which is what you seem to think I have suggested.
I agree that restriction is a good way of dealing with confounding variables. My concern is that you are assuming that all graduates from the same university have similar incomes and are not providing any evidence to justify this.
Thank you for your response,
Dr Anne Marie Cunningham
Primary Care and Public Health
Dear Ann Marie:
Thanks for your interest in our work. I think I did understand well your query. You textually stated that "graduate income in Spain can vary significantly with degree". I answered to your sentence by showing the data on depression by degree. There was no difference in depression incidence according to years of educational attainment in our cohort.
The reference that you provided to support differences in salaries in the Spanish general population (http://www.prospects.ac.u...) is not any scientific peer-review source; it is just a very short statement in a general information source. It cannot be specifically applied to graduates of our University. Our University is ranked according to a set of different standards of quality as the first private university in the country, please check:
The key issue here is if there is any substantial amount of residual confounding by income after applying two classical epidemiological methods for the control of confounding, a) restriction to university graduates; b) multivariable adjustments for sex, age, smoking, physical activity during leisure time, total energy intake, baseline body mass index (BMI), and adherence to the Mediterranean dietary pattern.
I acknowledge that I do not have evidence on the amount of variability in income within the tiny strata that are constructed by breaking down the whole cohort according to the several categories of all these variables considered together. However, it is very difficult to figure out that a wide variability in income might exist within them. In addition, it is very difficult to conceive that a substantial variability in depression risk might exist according to the potential residual variability in income within these tiny strata. Furthermore, there is no evidence to think that a sufficient amount of heterogeneity in trans fat intake according to income might exist within these tiny strata.
These explanations might account for the fact that income is seldom used in nutritional epidemiology to adjust for. The multivariable models commonly used in nutritional epidemiology adjust only for education but not for income.
An additional issue is that in contrast to US or UK, a direct question about income in a questionnaire is usually not regarded as reliable in the Spanish culture because of social desirability bias and other issues. Therefore, when we designed our cohort study we took into account that once education was accounted for, income was not included as a relevant confounder in the best known cohorts of nutritional epidemiology. This is the reason why we do not have evidence about income variability.
To support that residual confounding by income is not a likely explanation of our findings I can add that I am not aware of:
a) any longitudinal study in Spain showing that income is a stronger risk factor for depression than education. The paper that you quote in the "Journal of Happiness Studies" only uses cross-sectional data and reverse causality cannot be excluded (i.e. depressed people at baseline are more likely to exhibit a worse job performance and reduce their income as a consequence of depression).
b) any longitudinal study showing that baseline income is a better predictor of trans fat intake than education.
c) any epidemiological cohort study in Spain assessing the relationship between dietary habits and chronic disease that has adjusted for income instead of adjusting for education (please see, for example, Buckland et al. Am J Epidemiol 2009;170:1518-29 or Arriola et al. Heart 2010;96:124–130 that are cohorts with a wider variability in income and educational attainment than the SUN cohort).
Another cohort study has longitudinally assessed the relationship between dietary patterns and depressive symptoms and has adjusted for employment grade and education, the paper by Akbaraly (Br J Psychiatry 2009;195:408–413.). As you probably are aware of, the British civil service employment grade is defined on the basis of salary, social status and level of responsibility and consists of three levels, with grade 1 representing the highest level and grade 3 the lowest. Therefore they considered the salaries. The results of this British cohort are very consistent with our findings.
All this is reassuring that residual confounding by income is not a likely explanation of our findings.
This is really interesting - I wonder whether you have come across http://www.adzuna.com/job.... This is a new online classifieds search-engine that has fantastic data on graduate incomes. Their blog has very interesting articles on data/trends http://www.adzuna.co.uk/b...