From past experience I know that the news media often leaves out important details of a study and can be bad at interpreting the results of the studies. I thought I would take a look Jacobs et al. (2013) original article which has a lot more detail to allow one to replicate the study. In the methods sections they state that there was a treatment group of 30 for those who participated in the retreat and a control group of 30 who were wait listed for the retreat using stratified random sampling to control for any potential confounding variables such as BMI (Body Mass Index which is weight divided by height squared and is a crude measure of obesity), handedness and IQ. The wait listed group did receive the mindfulness intervention after the treatment group.
The authors do not discuss if there is a difference between the wait list and non waitlist groups in cortisol or mindfulness levels especially during the time where the wait list group was not receiving the intervention. This may not have been feasible during the study as the individuals were probably scattered all over the US and measurements of cortisol and mindfulness could not be taken without great cost. Participants were paid during the retreat. Three participants had complete data and were excluded from the study.
The results showed no overall effect of cortisol decrease from pre to post measurements but did significantly decrease for BMI. Mindfulness significantly increased between the pre and post. The pre and post cortisol levels were significantly negatively correlated with mindfulness as measured by a 37 item questionnaire which was previously validated. Negatively correlated means that as mindfulness increases cortisol levels decrease. This effect was still significant after adjusting for age and BMI.
The authors acknowledge that this study is correlational and does not establish a cause and effect relationship between meditation, mindfulness, and stress. The article in the Huffington Post seems to suggest the same thing by stating that there is no control group. Because of the difficulty in doing this type of research, the need for more of theses types of studies is established. Experimental studies with a well defined control and treatment group with all other confounding variables are adjusted for are ideal for establishing cause and effect relationships. The ideal of research is seldom met. When the situation is less than ideal, converging validation with many different methods is needed to accomplish the same cause-effect relationship.