Quantitative/Qualitative research; Reliability/Validity

In the field of psychology, scientific researchers use the nomothetic approach to study groups of people in an experimentally controlled way. They are more apt to use large scale (rather than individual) studies via survey questionnaires to gather their quantitative data, and to compare differences and similarities between groups of people. Quantitative data results in numerical, statistical data that can provide a measure of association and causes between variables in a precise, mathematical way. Qualitative data, on the other hand, produces verbal summaries of research findings, with no numerical or statistical summaries that can be mathematically compared.

Seeing as that reliability and validity are represented in quantitative data as numerical figures, how can they apply to qualitative data in which there are no numerical figures to work with? It is hard if not impossible to determine the reliability and validity of qualitative research, so they have adapted other terms such as "precision", "credibility", "transferability", "trustworthiness", "dependability" and "goodness" to provide the criteria for determining the reliability and validity of qualitative research (Golafshani, 2003). Another problem with trying to explain the reliability and validity of qualitative research is that the researcher is a highly involved instrument of measurement, rather than some anonymous, scientific machine, so the results are always measured in a subjective rather than an objective light.