pet owners vs non pet owners

I would tell my grandmother that the experiment mentioned in the newspaper is not 100% proof that having a pet will help a person to live longer.  There are a lot of factors and variables that are unknown.  The experiment does specify where the different groups of people come from.  There are other possible variables that could be affecting the groups of people with or without pets.  The researchers in the article are making a correlation of pet ownership to living longer as if it were the cause of the latter.  What the researchers failed to mention was that there are possibly other variables that would be causing people to live longer.  I would tell my grandmother that there is not enough scientific evidence to prove that owning a pet would cause a person to live longer.  For instance, what if the people in the experiment who did not have pets lived in an environment that had long term health risks associated with them such as industrialized pollution as opposed to the other group which may have lived in rural areas that were cleaner and healthier?  There are a lot of unknown and untested factors that could be causing the different test results among pet owners and non pet owners.