Dear Joseph:

First of all, both theoretical sampling and snowball sampling correspond to what is usually called "sampling strategies" in qualitative research.

1) Theoretical sampling makes up the classical sampling strategy of grounded theory approach. According to Staruss & Corbin (1998, p. 73), it involves "sampling on the basis of the emerging concepts, with the aim being to explore the dimensional range or varied conditions along which the properties of concepts vary". conduced samplig", which is another strategy where the researcher samples inidents, slices of life, time periods, or people on the basis of their potential manifestation or representation of a theoretical construct (i.e.: "neurotic", "resilient", etc.).

2) The "snowball or chain sampling" is a sampling strategy for locating information-rich key informants in respect to your research aims. It works by asking well situated people "who knows a lot about ...(the topic you are intersted in)?", "whom should you talk with?". By asking a number of people these kind of questions, the snowball gets bigger an bigger in terms of information-rich cases.

You may find a clear classification and explanation of close to 16 sampling strategies in qualitative research in the classical book of Patton:

 Patton, M. (2002). Qualitative evaluation and research methods. Newbury Park: Sage.

Of course, for theoretical sampling I strongly recommend you the also classical work of Strauss & Corbin::

Strauss, A. & Corbin, J. (1998). Basics of qualitative research: Techniques and procedures for developing grounded theory. Thousand Oaks: Sage.

Additionally, for a classification of the more properly samplig strategies to be used according to the qualitative tradition you are working with, I recommend you:

Creswell, J. (2007). Qualitative inquiry and research design: Choosing among five approaches. Thousand Oaks: Sage.

Good luck!!!

Francisco Donoso-Maluf

Dear Joseph:

First of all, both theoretical sampling and snowball sampling correspond to what is usually called "sampling strategies" in qualitative research.

1) Theoretical sampling makes up the classical sampling strategy of grounded theory approach. According to Staruss & Corbin (1998, p. 73), it involves "sampling on the basis of the emerging concepts, with the aim being to explore the dimensional range or varied conditions along which the properties of concepts vary". conduced samplig", which is another strategy where the researcher samples inidents, slices of life, time periods, or people on the basis of their potential manifestation or representation of a theoretical construct (i.e.: "neurotic", "resilient", etc.).

2) The "snowball or chain sampling" is a sampling strategy for locating information-rich key informants in respect to your research aims. It works by asking well situated people "who knows a lot about ...(the topic you are intersted in)?", "whom should you talk with?". By asking a number of people these kind of questions, the snowball gets bigger an bigger in terms of information-rich cases.

You may find a clear classification and explanation of close to 16 sampling strategies in qualitative research in the classical book of Patton:

 Patton, M. (2002). Qualitative evaluation and research methods. Newbury Park: Sage.

Of course, for theoretical sampling I strongly recommend you the also classical work of Strauss & Corbin::

Strauss, A. & Corbin, J. (1998). Basics of qualitative research: Techniques and procedures for developing grounded theory. Thousand Oaks: Sage.

Additionally, for a classification of the more properly samplig strategies to be used according to the qualitative tradition you are working with, I recommend you:

Creswell, J. (2007). Qualitative inquiry and research design: Choosing among five approaches. Thousand Oaks: Sage.

Good luck!!!

Francisco Donoso-Maluf

Thank you Francisco. Your explanation and references really helpful. It is clear now to me that in theoretical sampling, researcher only collects data (an incidents, an information, documents, etc) based on, and in order to give more explanatory power to, his/her emerging concepts or theory.  And as you said above, snowball sampling is only a sampling strategy for locating an information-rich informants. So, I think in Grounded Theory, whether the researcher realize it or not, he/she probably use snowball sampling to serve theoretical sampling he/she conducted. And if the researcher doesn't properly differentiate those two kinds of sampling, then his/her audience, especially those who are only trained in general qualitative methods, will regard those two kinds of sampling as similar to each other.

Dear Joseph:

Excuse me for my delay in responding. Yes, you are right: Eventually, snowball sampling could serve theoretical sampling in the same way as a random revision of, in example, some organizational documents, could serve as well. The essence of theoretical sampling -which is a tool of Grounded Theory approach- is the purposeful sampling of rich-information informants who could give you useful information for the theory you are building in order to respond to your research question. As I told you, in Patton's classical book you will find an extense list, explanation and examples about purposeful samplig strategies in qualitative research (including theoretical and snowball ones).

Good luck!!! 

They both seem the same essentially. But theoretical sampling is usually used in Grounded theory studies.