Some authors have written articles based on their own observations and experiences as coaches. These articles usually are more anecdotal than based on theory and few have met the standards of reliable methodology. A case in point is the Harvard Business Review article on peer coaching entitled The Young and the Clueless.  The article discusses many examples, presumably from the authors’ coaching experiences, but does not provide any detailed analysis, leaving the evidence to remain anecdotal. Case studies have been used by some practitioners to document their successes, in an attempt to share their knowledge and methods with other practitioners. However, it is still difficult to find case studies that show specifically what organizations have done in measuring ROI. The ASTD used case studies to develop their evidence on ROI; however, the ASTD study was not done on specific coaching engagements, but rather on training programs in general. 
One of the best examples of reliable methodology is the study undertaken by Lahti and Beyerlein. In their article, the authors used subsections for each important research criteria and systematically described the purpose of their study, the participants, the procedures, the results, and their implications. They started the article with a literature review to ground their study in theory.  This is one article that does a solid job of describing the research study in such detail that any reader could replicate it.
For most of the twentieth century, quantitative research has been the dominant method in the social sciences.  However, quantitative research with its numbered variables analyzed with statistical procedures has not been used much in the research of coaching. To date, most research has fallen into the qualitative realm. Authors Sheila Maher and Suzi Pomerantz quote several quantitative studies showing the estimates of return on investment in coaching to diverge more than 500%.  Authors Talkington, Voss, and Wise cite several quantitative studies to make their case in favor of executive coaching.  According to Dr. Anthony M. Grant, “It would be useful to see an increasing emphasis on objective quantitative outcome measures as well as investigating the relative efficacy of different approaches to coaching.” 
Some researchers have combined both methods in an attempt to give a multi-dimensional facet to the results. In his unique approach to determine if evidence based coaching could increase ROI, Dr. Otto Laske used a multiple assessment strategy he called “action science”. First he gave cognitive assessments to the executives and coaches, followed by interviews of the executives conducted by the coaches. This study follows the mental-emotional growth of six executives coached over a period of 14 months and correlates the coaching outcomes with the developmental level of the coaches and the participants. 
Some research never clearly states what method was used to produce the findings. This is fairly common in articles found in the popular press. At times, the authors allude to a study but never really explain the procedures they used to gather their data. In other instances, the author will mention statistics from a study done by another author but does not provide the reference for the reader. Such an example is found in the article What an Executive Coach Can Do for You by Paul Michelman which quotes a 2004 study by Right Management Consultants that stated 86% of companies are now using coaching to develop future organizational leaders. Download Article 1K Club