Dear Editor

The results of the study by Fairley and Leyland [1] of changing social class inequalities in perinatal outcomes in Scotland must be interpreted in light of the statistical tendency whereby the rarer an outcome the greater the relative differences in experiencing the outcome and the smaller the relative difference in avoiding the outcome.[2-6] In times of declines in adverse outcome (the more common situation in recent decades), the relative difference in experiencing the outcome will tend to increase solely as a consequence of the decline in prevalence. But such increases in relative difference in experiencing the outcome – which usually are attended by declines in the relative difference in rates of avoiding the outcome as well as declines in the absolute difference between rates of experiencing (or avoiding) – ought not to be regarded as a meaningful worsening of the relative situation of disadvantaged groups. A departure from the pattern in the 1980s, however, might suggest an improvement in that situation. [2,6]

The same tendency influences the differing patterns observed among groups categorized by characteristics that are related to overall risk, such as are shown in Tables 3-5 of the study. The authors note that inequalities among lone mothers are smaller than among married mothers despite the fact that lone mothers suffer greater socioeconomic disadvantage and ill health than married mothers. But one should expect to find greater relative social class differences in adverse perinatal outcome rates among married mothers simply because adverse outcome rates are lower among married mothers. The relative difference in avoiding these outcomes, however, generally would be smaller.[2,4,6]

For the same reason, inequalities measured in relative rates of experiencing adverse perinatal outcomes will tend to be greater among the (lower risk) age 20-34 group than the (higher risk) under 20 group. That the relative difference is greater among the 35 or above group than the 20 -34 group, assuming that the 35 or above group is at higher risk of adverse outcomes than the 20-34 group, does not mean the statistical tendency is not present. Rather, it merely suggests that certain factors, possibly including the implications of smoking noted by the authors, outweigh the statistical tendency.

The above observations apply as well to differences in the relative index of inequality, which measure is largely a function of relative differences in rates of experiencing an outcome.[2,6] The odds ratios shown in the tables raise somewhat different issues. In the case of the outcome frequencies at issue here, odds ratios tend to approximate relative risks of experiencing the outcome, and the observations regarding the sizes of such relative risks are unlikely to be less pertinent because the figures shown in the tables are odds ratio. It is true that relative difference patterns measured in odds ratios will not vary depending on whether one examines the adverse or the favorable outcome. But that does not mean that differences between odds ratios are in some manner reflecting differences between the sizes of inequalities that are not solely the function of differences in prevalence in the various settings, whether defined temporally or demographically, that are being compared. Like other measures of differences between the rates at which two groups experience or avoid such outcomes, odds ratios tend to change solely as a result of changes in prevalence, though less predictably than changes in relative risks.[2,6]

James P. Scanlan

References

1. Fairley L, Leyland AH. Social class inequalities in perinatal outcomes: Scotland 1980-2000. J Epidemiol Community Health 2006;601:31- 36.

2. Scanlan JP. Can we actually measure health disparities? Chance 2006:19(2):47-51: http://www.jpscanlan.com/images/Can_We_Actually_Measure_Health_Disparities.pdf.

3. Scanlan JP. Measuring health disparities. J Public Health Manag Pract 2006;12(3):294 [Lttr]: http://www.nursingcenter.com/library/JournalArticle.asp?Article_ID=641470.

4. Scanlan JP. Race and Mortality. Society. 2000;37(2):19-35: http://www.jpscanlan.com/images/Race_and_Mortality.pdf.

5. Scanlan JP. Divining difference. Chance. 1994;7(4):38-9,48: http://jpscanlan.com/images/Divining_Difference.pdf.

6. Scanlan JP. The misinterpretation of health inequalities in the United Kingdom. Paper presented at: British Society for Population Studies Annual Conference 2006, Southampton, England, Sept. 18-20, 2006: http://www.jpscanlan.com/images/BSPS_2006_Complete_Paper.pdf.