Confounding effects

No neurobehavioral effects were seen in 288 randomly selected males who were occupationally exposed to lead as compared to 181 demographically similar controls (Ryan et al. 1987). The mean PbB level in the exposed workers was 40.1 pg/dL and that of the controls was 7.2 pg/dL. Nineteen tests of neuropsychological performance were conducted. The lead-exposed workers performed no differently from controls on all measures except psychomotor speed and manual dexterity. The authors discounted this difference due to the observation that conflicting results were obtained in two different tests of motor speed and manual dexterity and possible confounding effects of age. There was no evidence that history of previous very high exposure had any effect on performance. 

In comparative studies of organic and conventional fruit quality it is important that the fruit samples compared originate from trees with an optimum or equivalent fruit load (e.g. see Weibel et al., 2000), to avoid the confounding effects of fruit load on quality. Unfortunately, in most comparative studies on fruit quality this factor was not addressed (Bourn and Prescott, 2002 Harker, 2004). 

Materials required As described in the principles, a researcher needs access to a flow cytometer plus fluorescent dyes generally, these are sold as intact emits, though the fluids used in the injection wells can differ - but they should not unduly alter the current between the electrodes lest confounding effects occur. 

Arteel and Sies (1999) examined procyanidin oligomers of different size, isolated from the seeds of Theobroma cacao, for their ability to protect against nitration of tyrosine. Serraino and others (2003) investigated antioxidant activity of the blackberry juice and cyanidin-3-O-glucoside on endothelial dysfunction in cells and in vascular rings exposed to peroxynitrite. However, more work is needed in this area, and the confounding effects of oxidized protein/amino acids in the diet need to be elucidated. 

As both elements are important environmental contaminants whose mobility and environmental impact depend on redox reactions, we expect that Cr and Se isotope analyses will be used widely as indicators of oxyanion reduction. It may also be possible to trace the origin of the Se or Cr via isotopic signatures, but only if the possible confounding effects of reduction can be constrained. 

The confounding effect has also been quantified (8). f allele frequency differences are large between two subgroups, then a 50% increase of population relative risk in one population may result in an odds ratio of approx. 1.5. 

One conventional approach that tackles the confounding effect is family-based design. A typical family-based design involves genetic information for both parents and an affected child. A transmission disequilibrium test (TDT) can be used to examine the association of an allele with a phenotype by testing whether the allele is over- or undertransmitted to the affected offspring with a statistic (12). 

The i.v. approach has the distinct advantage of measurements being carried out under the most physiological conditions. On the other hand, the caveats include confounding effects of peripheral metabolism, which may give rise to artifactual brain uptake of degradation products. To exclude such a possibility the application of suitable analytical techniques to tissue and plasma samples is required. 

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