We have provided an overview of techniques used to assess variables in the applied behavioral sciences. Most of the methods are used by both quantitative/positivist and qualitative/constructivist researchers but to different extents. Qualitative researchers prefer more open-ended, less structured data collection techniques than do quantitative researchers. Direct observation of participants is common in experimental and qualitative research; it is less common in so-called survey research, which tends to use self-report questionnaires. It is important that investigators use instruments that are reliable and valid for the population and purpose for which they will be used. Standardized instruments have manuals that provide norms and indexes of reliability and validity. However, if the populations and purpose on which these data are based are different from yours, it may be necessary for you to develop your own instrument or provide new evidence of reliability and validity.