This study group reviewed the evidence that month of birth varies significantly with geographical location and over time in the population and considered the implication of this variation for case-control studies considering month of birth as a risk factor for the development of MS. They assessed the evidence for seasonal variations in birth rate and tested the extent to which these are subject to regional and temporal variation using national birth statistics from two continents.
From this they then established the age and regional origin distribution for a typical MS case collection. They then determined the false positive rate expected when comparing such a collection with birth rates estimated by averaging population specific national statistics.
They found that seasonality in birth rate is ubiquitous and subject to highly significant regional and temporal variations. Birth rates seen in typical case collections are highly likely to deviate from those obtained by the simple un-weighted averaging of national statistics. The apparent seasonal patterns for month of birth suggested to be specific for MS (increased in the spring and reduced in the winter) are probably false positives due to the absence of adequate control for confounding factors, like year and place of birth.
These observations serve as a reminder that risk factors that are easy to determine and seemingly homogenous, like date of birth, may yet be heterogenous within the general population and therefore generate false positive signals if cases and controls are not adequately matched.
Authors: Fiddes B, Wason J
Source: Ann Neurol. 2013 Jun 6.[Epub ahead of print]
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