Notes on Qualitative Research


 Qualitative vs. Quantitative Research



Qualitative Research

Quantitative Research

   Quantitative research measures differences in number for variables, and usually studies a large number of cases (Large “N”).

Qualitative research measures differences in kind for variables, and usually studies a small number of cases (Small “N”). 

Because it covers a broad range of cases, quantitative research yields conclusions that can be generalized (it has the strongest external validity).

Because it looks closely at a few cases and traces causal pathways, qualitative research outperforms quantitative research in its measurement validity and internal validity.

When selecting cases for your quantitative research sample, it is imperative that you use random selection.

In qualitative research, “selection must be done in an intentional fashion, consistent with research objectives and strategy.” (King, Keohane, and Verba, 1994, p.139)



l   Qualitative and quantitative studies are both types of observational studies.



A. Selecting Cases on the Independent Variable

l   “Selecting on the independent variable” means “selecting your cases according to the values of the independent variable that  they take on.” 

            l   In order to do this, you have to know a little bit about all of your potential cases.

            l   In order to do this right, you cannot act as if you also know the values that the dependent variable takes on.


The Most Similar Systems

The Most Different Systems

The Most Similar Systems method selects cases that take on similar values of confounding variables, but different values of a key independent variable.

            l   This “holds constant” the confounds because they take on the same values in all of the cases.

            l   This is the design recommended by King, Keohane, and Verba. 

l   The cases that you might select in a Most Similar Systems design will differ based on the research question you ask.

            l   USA, Canada, and Britain in a study of life expectancy.

            l   Bill Clinton, Jimmy Carter, and JFK in a study of regional appeal in voting.

            l   Genghis Kahn, Hannibal, and Hitler in a study of how to defend invasions.


   The Most Different Systems method selects cases that take on very different values for multiple independent variables.


            l   If it turns out that these cases all take on the same value of a dependent variable, then we can rule out the independent variables as causes of the dependent variable.

            l   Less useful since it can only disprove a hypothesis.


Example: Income Inequality and Civil War





B. Selecting Cases on the Dependent Variable


l   Selecting cases according to the value of the dependent variable that they take on is more controversial than selecting on the independent variable.

l   It allows you to look at extreme values or divergent cases.

l   “However, if this design is to lead to meaningful … causal inferences, it is crucial to select observations without regard to values of the explanatory variables. K.K.V.”


Method of Agreement

Method of Difference

 When you use Mills’ Method of Agreement, you select cases that take on the same values of the dependent variable.


l   This helps you to rule out possible causes, because independent variables that vary over these cases can’t cause the dependent var.


l   This method can only disprove a hypothesis, because it can’t find a correlation. 

When you use Mills’ Method of Difference, you select cases that take on different values of the dependent variable.

l   After you have selected your cases, you determine what values they take on for some independent variables.

l   Perhaps one independent variable will vary across your cases, and explain the D.V. 




Example: Clem Miller  

l   This Congressman writing home to his supporters tells the story of two different lobbying efforts on behalf of farmers.


l   Let’s assume that his process of case selection came in the same order that his narrative is written.


l   If so, he selected on independent variables and used the “Most Similar Systems” design.

Example: Dreze and Sen

l   Both countries began a new political regime at mid-century with large populations and little wealth.


l   They have diverged since then: “There is little doubt that as far a morbidity, mortality, and longevity are concerned, China has a large and decisive lead over India. (p. 205)”


l   “What has brought about that lead is a matter of very considerable interest. (p. 206)”

Comparing Many Research Methods: Measurement Validity

l   Measurement validity judges the gap between your conceptual definition and your operational definition.

l   Qualitative – Highest measurement validity

l   The measurement validity of other research methods (Lab experiments, quasi/natural experiments, and quantitative research) really depends on what you are trying to measure.


Comparing Many Research Methods:

Internal Validity External Validity

Internal validity judges how well a research design has tested a causal relationship, in the cases examined. Random assignment is the key.


Lab Experiments – highest  Quasi/Natural experiments – medium

Qualitative research – medium

Quantitative research – lowest


Comparing Many Research Methods:  External validity is how confident we can be that a causal relationship identified in our cases can be generalized to the outside world.  Random sampling is key


Quantitative research – highest

Quasi/Natural experiments – medium

Lab experiments – low

Qualitative research – low



l  The best research design depends on your research question and the particular problems that it poses.


l  The best research uses a mixture of methods to test a hypothesis.