Original sentence: This and other observational studies are subject to residual confounding and biasEthical concerns regarding the withholding of potentially beneficial treatment to patients with life-threatening illness hinder conducting randomized clinical tirals in this patient group
Edited sentence: As with all observational studies, residual confounding and bias can occur.Covariates not included in the propensity score, such as socioeconomic factors that could have affected health-care seeking and access in some study sites, might not be balanced and residual bias might remain.In the period October 2008 till September 2009 resistance for oseltamivir to seasonal influenza A (H1N1) was 99,6% in the US; there was almost no (0,6%) resistance to 2009 Influenza A (H1N1), a pandemic flu with low mortality.Severity of illness at presentation is an important predictor of mortality, but was measured differently in each study.in which infection rates have been observed to be very low in the oldest age group , approximately 90% of deaths occurred in those under 65 years of age and overall case fatality rate among hospitalized patients appeared to have been highest among those 50 years of age or older and lowest among children