The population of this survey is everyone in Canada. In my opinion, they used a sample because it would be almost impossible to survey everyone in Canada. I think it was a stratified sample, which means they split the population into multiple groups, males and females depending on their age and randomly asked a certain number of each.

Some influencing factors could have been timing, depending on that week if it was good or bad, time, because they could have been in a certain mood that would affect their answer (for example, tired and not willing to answer properly) and privacy, since they minght want to seem less stressed than they are in front of their friends or others.

My predictions of the population using the results of this survey is that people aged 35-44 experience the most days that are the most stressful. I can also predict that females usually find thier days more stressful than males of the same age.

Survey source: http://www.statcan.gc.ca/pub/82-625-x/2014001/article/14023-eng.htm