It is important to keep in mind what your population is throughout designing, conducting, and finally making conclusions for your study. If you want your conclusions to apply to all pet owners, you can't just sample from dog owners. You must include cat owners, fish owners, bird owners, snake owners, etc. When we claim that our conclusions made on a sample hold for the population, we are making an inference from the sample to the population. We go to a local mall and ask people who walk by whether they are prochoice or prolife on the abortion issue.
Some people freely respond to our question and others profess to have no opinion.
We work for a company that makes paint. We believe our paint dries faster than another brand. In order to prove this, we take 8 plywood boards and paint them. We paint 4 with our paint and 4 with our competitor's. We measure the time for each board to dry.