Retically happen in any of the cohort studies, particularly those with longer follow-up periods. viagra canada This might result in bias if the smokers at highest risk of developing amd were also those at highest risk of dropout because of serious ill health or death (eg, the heaviest smokers and smokers with multiple risk factors for coronary heart and neoplastic disease). generic viagra This would result in the selective loss to the study of smokers at highest risk of amd, leading to a reduction in the observed association between amd and smoking. what is viagra pills used for Whether differential loss to follow-up because of death or illness is a cause of bias is not clear. different mg of viagra However, it is highly plausible as there is a mass of evidence from prospective studies that smokers have greatly increased mortality and morbidity, and this is greatest in heavy and prolonged smokers. 36 hour viagra canada The effect of such a bias is likely to increase with prolonged follow-up. The beaver dam eye study investigators did not find that amd was associated with increased mortality in the analysis at 5 years. viagra pills 36 however, this contrasts with the findings of increased mortality among patients with amd in the 10-year follow-up of the rotterdam study 48 and 6. buy viagra canada 5-year follow-up in the areds. buy viagra 49 finally, in the blue mountains eye study, current-smokers at the baseline examination who were still alive at follow-up were less likely to re-participate at 5 years. viagra safe for teenagers 33 lower participation rates among current-smokers compared with nonsmokers is well described in epidemiological studies. order viagra This would also be likely to result in an under-estimation of the strength of any smoking–amd association if, as seems plausible, smokers at highest risk of amd (ie, heavier smokers) were selectively dropping out in these studies. what is viagra pills used for A further potential bias in longitudinal studies examining disease associations with smoking is that nondifferential misclassification of exposure status may occur where smokers and nonsmokers are categorised at baseline only. This will not allow for the effect of changed exposure status, such as quitting smoking during follow-up. Such changes in exposure status are especially problematic with prolonged follow-up, and will bias the measure of association towards a null finding. An example of this problem comes from a recent negative study of the effects of passive smoking, which classified exposure largely by spousal smoking at baseline and attracted considerable critical comme. generic viagra canada review what is viagra pills used for Benvido     Bienvenido

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