a large study in the uk was done to determine if there was a link between cell phone usage and brain cancer. the study set out to determine if using a cell phone more, increased the likelihood of getting brain cancer. the study took about 800,000 women and asked them to report their cell phone use in 1999, 2005, and again in 2009. in 2009, they recorded the proportion of women who had developed brain cancer. their conclusion was "in this large prospective study, mobile phone use was not associated with increased incidence of glioma, meningioma or non-cns cancers." is this an observational study or an experiment? justify your answer.