Quantitative and Qualitative Data

Quantitative Data

The term qualitative data is used to describe a type of information that can be counted or expressed numerically. This type of data is often collected in experiments, manipulated and statistically analyzed. Quantitative data can be represented visually in graphs, histograms, tables and charts.
Some examples of quantitative data include exact counts ('there were 789 students who attended the rally') or a type of measurement ('it was 78 degree Fahrenheit yesterday at 2 PM').

Qualitative Data

Qualitative data is extremely varied in nature. It includes virtually any information that can be captured that is not numerical in nature. Here are some of the major categories or types:
In-Depth Interviews: In-Depth Interviews include both individual interviews (e.g., one-on-one) as well as "group" interviews (including focus groups). The data can be recorded in a wide variety of ways including stenography, audio recording, video recording or written notes. In depth interviews differ from direct observation primarily in the nature of the interaction. In interviews it is assumed that there is a questioner and one or more interviewees. The purpose of the interview is to probe the ideas of the interviewees about the phenomenon of interest.
Direct Observation: Direct observation is meant very broadly here. It differs from interviewing in that the observer does not actively query the respondent. It can include everything from field research where one lives in another context or culture for a period of time to photographs that illustrate some aspect of the phenomenon. The data can be recorded in many of the same ways as interviews (stenography, audio, and video) and through pictures, photos or drawings (e.g., those courtroom drawings of witnesses are a form of direct observation).
Written Documents: Usually this refers to existing documents (as opposed transcripts of interviews conducted for the research). It can include newspapers, magazines, books, websites, memos, transcripts of conversations, annual reports, and so on. Usually written documents are analyzed with some form of content analysis.

Quantitative Vs. Qualitative Data

Some methods provide data which are quantitative and some methods data which are qualitative.  Quantitative methods are those which focus on numbers and frequencies rather than on meaning and experience.  Quantitative methods (e.g. experiments, questionnaires and psychometric tests) provide information which is easy to analyze statistically and fairly reliable.  Quantitative methods are associated with the scientific and experimental approach and are criticized for not providing an in depth description.
Qualitative methods are ways of collecting data which are concerned with describing meaning, rather than with drawing statistical inferences.  What qualitative methods (e.g. case studies and interviews) lose on reliability they gain in terms of validity?  They provide a more in depth and rich description.
Quantitative methods have come under considerable criticism.  In modern research, most psychologists tend to adopt a combination of qualitative and quantitative approaches, which allow statistically reliable information obtained from numerical measurement to be backed up by and enriched by information about the research participants' explanations.