This prompts the recommendation that a tape recorder be used to ensure that data sets are accurately reported and that, additionally, the findings and analysis may be cross-checked against actual recorded evidence of the exchange and intonations thereby produced. External validity is somewhat more complex, because it demands that the outcome of our research is not just illuminating of findings concerning the specific resource officer and district interviewed, but can also be applied to our broader understanding of the subject. The best way to accomplish this is to conduct an array of interviews all guided by a very similar line of questions, differentiated only by the individual directions into which individual subjects will tend to steer questions. The result should be multiple sets of data allowing for comparison and synthesis of qualitative findings.

This interest is also useful in achieving a better understanding of the reliability...

According to what we understand about external validity, "the quality of a research is related to generalizability of the result and thereby to the testing and increasing the validity or trustworthiness of the research." (Golafshani, 603) Trustworthiness as a measure of reliability must somehow be testable in order to be regarded as verifiable. This is a complex challenge where qualitative studies are concerned. However, with respect to the interview, it might also be appropriate to construct a preliminary likert scale survey as a way of creating numerical representations which can be tested objectively or empirically for consistency across interview subjects.
Works Cited:

Golafshani, N. (2003). Understanding Reliability and Validity in Qualitative Research. The Qualitative Report, 8(4), 597-607/

Patton, M.Q. (2002). Qualitative Research and Evaluation Methods. Sage Publications.