However, it is a tricky subject to study as with observational data we often can not separate the effects of drug treatment from other risk factors such as maternal illnesses. (sciencemediacentre.org)
A second evaluation found that those districts reporting student gains couldn't separate the effect of the project from those of other factors. (discovermagazine.com)
Using a 3D atmospheric model, the researchers separated the effect of the chemicals from those of weather and volcanic emissions, which can also destroy ozone. (science.sciencemag.org)