Research designs, no matter how well executed, must deal with questions about one's ability to draw certain conclusions based on the results they produce.
These questions concern the validity of the research design.
Types of Validity
Internal Validity
Deals with one's ability to make conclusions about the causal nature of the relationship between the independent variable and dependent variable.
Simply put...
Is there some other factor influencing the dependent variable that the researcher has not controlled for?
External Validity
Deals with one's ability to generalize the results of the study to the broader population or "the real world."
In other words...
Is the sample of cases in the study representative of the population you wish to generalize to, and/or does the context the participants encounter in the study resemble "real life" conditions?