For instance, if we want to know what percent of the population would vote for a Democratic or a Republican in the next presidential election, we are simply interested in describing something. When a study is designed to look at the relationships between two or more variables. A public opinion poll that compares what proportion of males and females say they would vote for a Democratic or a Republican candidate in the next presidential election is essentially studying the relationship between gender and voting preference. The three question types can be viewed as cumulative. And, a causal study assumes that you can describe both the cause and effect variables and that you can show that they are related to each other.
Causal studies are probably the most demanding of the three.