The more assumptions that are made, the more uncertain estimates of risk related to pathogens will be.  However, even with considerable uncertainty, QMRAs are a good way to compare different risk scenarios.  In a study comparing estimated health risks from exposures to recreational waters impacted by human and non-human sources of fecal contamination, QMRA determined that the risk of gastrointestinal illness from exposure to waters impacted by cattle were similar to those impacted by human waste, and these were higher than for waters impacted by gull, chicken, or pig faeces. Such studies could be useful to risk managers for determining how best to focus their limited resources, however, risk managers must be aware of the limitations of data used in these calculations.  For example, this study used data describing concentrations of Salmonella in chicken feces published in 1969.   Methods for quantifying bacteria, changes in animal housing practices and sanitation, and many other factors may have changed the prevalence of Salmonella since that time.  Also, such an approach often ignores the complicated fate and transport processes that determine bacteria concentrations from the source to the point of exposure.