7.2.3 Meta-analysis A different form of secondary analysis occurs when data from several studies are combined to provide evidence for the exploration of a theory. Such studies are usually referred to as meta-analyses.
A meta-analysis might collect together several statistical studies exploring the same thing and then in some way combine the data and analyse it afresh to see to what extent the conclusions of the separate studies provide more substantive support for a particular theory.
Meta-analyses might also take similar studies from different time periods to see if there has been an increase of decrease in a particular phenomenon; or take data from different countries or cultures for a comparative analysis.
Meta-analysis is more than just a literature search reporting what is already known, it involves a degree of reworking of the original empirical evidence from the studies that are being combined.
For example, researchers at Karolinska Institutetís Clinical Epidemiology Unit and the Obesity Centre at Karolinska University Hospital (Johansson et al., 2013) undertook a meta-analysis of 20 published scientific studies including a total of 3,017 participants, who were either obese or overweight at the start of the weight loss process, with a view to identifying what strategies aided maintenance of weight loss.
The studies were identified through a systematic review by using MEDLINE, the Cochrane Controlled Trial Register, and EMBASE from January 1981 to February 2013. The studies included were those with randomised controlled trials that evaluated weight-loss maintenance strategies. Two of the authors performed independent data extraction by using a predefined data template. (SeeCASE STUDY Weight-loss maintenance for details.)
The various studies examined the effects of (a) drugs, (b) meal replacements, (c) high protein diets, (d) dietary supplements and (e) exercise on rebound weight gain after an intensive weight loss, low-calorie diet (less the 1,000 calories a day). Even though the study shows that rebound weight gain is more the rule than the exception, the researchers found that several strategies obviously helped to reduce the unwanted effect (that is, aided maintenance of weight loss). First, anti-obesity drugs had an impact but they carry a risk of adverse reaction and most have been withdrawn. Second, powdered meal replacements and, third, a high protein diet were both effective and both of these are available to everyone. Low glycaemic index (GI) food was also effective, although the data in that case came from a single study, which the researchers claimed made the conclusions less reliable.
Exercise had no clear effect on weight loss maintenance. The reason, however, might be that one of the included trials studied relatively sick patients with serious arthrosis who had been prescribed special physiotherapy. In another study, in which the participants exercised in a more normal way, the effect was similar to eating a high protein diet.
This research was part-financed by a grant from Cambridge Manufacturing Company Limited, a British company working with weight-loss products and dietary advice, which may raise questions about the analysis and the presuppositions of the researchers.