We have to ask if our design allows us to say our independent variable makes our dependent variable vary systematically as it changes and that those changes in the dependent variable are not due to so...We have to ask if our design allows us to say our independent variable makes our dependent variable vary systematically as it changes and that those changes in the dependent variable are not due to some third or extraneous variable/factor. The quality of the sampling method you choose is directly tied to your ability to generalize the findings of one particular study to the entire population.
