Step 3: Prepare a set of probe questions to use when the interviewee's response indicates confusion or is not sufficiently detailed.

Probing questions are follow-up questions to an open-ended interview question. Follow-up questions are advisable when the interviewer does not fully understand a response or when the interviewer wants to obtain a more in-depth response. Although it is impossible to plan ahead of time how to handle every such situation, there nonetheless are general approaches that can be planned in advance. Two broad circumstances that invite probing are considered in turn.

Probing for Clarification. Occasionally, an interviewee's response will indicate that he or she did not understand the question (in effect, answering another question that you did not ask). For example, if you ask a teacher to talk about his or her fall classes, the teacher could end up talking about his or her summer classes instead. Unless there is a serious time constraint, let the interviewee finish the response and then rephrase the original question to elicit the intended response. This tactic is preferable to interrupting or correcting the interviewee.

Another situation that may call for clarification is if the interviewee appears to give contradictory information, either within or across responses. The best approach here is to paraphrase back what you have understood the interviewee to say, and then ask the interviewee to confirm or clarify what he or she meant.

Alternatively, the interviewee may start talking about someone or something you are not familiar with. Again, the best approach is to be direct. Let the interviewee finish the answer and then ask him or her to explain the reference.

Probing for Greater Depth. People vary considerably in the extent to which they like to talk and share information. Even when you have done your utmost to make the interviewee comfortable (see Administering Interviews), it still is possible to encounter someone whose natural tendency is to give terse responses, thereby providing less information than you would like. When this occurs, it is necessary to follow up the responses with techniques designed to elicit more information. One such technique that often gets overlooked is silence . Sometimes simply by waiting for several seconds after the interviewee finishes an answer, you signal that you are giving him or her time to think of additional information. If waiting does not produce an extended response, then it is necessary to use probes. Here, it is advisable to have some general probes planned for this purpose.

There also may be instances in a more structured interview when very specific topic probes are needed to get exact information from all respondents. These should be focused questions, some of which ask for information quite directly (unlike the open-ended wordings recommended above). For example, let's say that for an interview of math teachers, you want to find out about their math background. For the interview protocol, you start with a general, open-ended question: "Please tell me about your educational math background prior to becoming a math teacher." You find that most teachers, when asked this question, give you the necessary information about how many math courses they have taken and the levels of these courses. For a teacher who does not provide this information, you should have more direct probes "in waiting" (e.g., "So, please summarize for me the number of math classes you took in high school and college."). These specific probes do not have to appear as part of the formal protocol, but you may wish to include notation in the protocol reminding you to ask them if necessary, as shown below:

A less structured interview also is likely to involve considerable probing. Recall that in this type of interview, because you allow the respondent's accounts and views to guide the direction that much of the interview will take, you may have to use a number of unique probes to follow up on responses and get the desired depth. Thus, you may not be able to plan "in-waiting" probe questions in advance, other than the general probes suggested above.

Regardless of how structured the interview, probing for depth is especially important at the beginning because this sets the expectation level for the entire interview. If a nontalkative interviewee realizes that every brief response to an open-ended question is going to be followed by a set of "extra" direct questions, the interviewee may begin to talk at greater depth without prompting after a few such experiences.