Nevertheless, narrative reviews, when written in a balanced way, can be a helpful way for readers to get an overall impression of the relevant research questions and evidence base of a field. Overall, this can lead to biases favoring the opinion of the author. There are also no fixed rules on how to draw conclusions from the reviewed evidence. There are no strict rules on how studies in a narrative review have to be selected, and how to define the scope of the review. Narrative reviews are often written by experts and authorities of a research field. Until way into the 1980s, narrative reviews were the most common way to summarize a research field. There are at least three distinct ways through which evidence from multiple studies can be synthesized ( Pim Cuijpers 2016). However, it is only one method to do this. The aim of meta-analysis is to combine, summarize and interpret all available evidence pertaining to a clearly defined research field or research question ( Lipsey and Wilson 2001, chap. In meta-analysis, primary studies themselves become the elements of our analysis. In conventional studies, the units of analysis are a number of people, specimens, countries, or objects. This simple definition already tells us a lot. Glass, described meta-analysis as an “analysis of analyses” ( Glass 1976). Meta-analysis can be enormously helpful in achieving this, as long as we acknowledge its own limitations and biases. With unprecedented amounts of research findings produced each day, it is even more important to view and critically appraise bodies of evidence in their entirety. It is an inconvenient truth that the scientific process, when left to its own devices, will not automatically move us to the best of all possible worlds. Many theories, Meehl argued, are not continuously improved or refuted, they simply “fade away” when people start to lose interest in them ( Meehl 1978). He argues that research fields can often exist in a state where an immense research output is produced on a particular topic or theory, but where fundamental fallacies remain unchallenged and are only perpetuated.īack in the 1970s, the brilliant psychologist Paul Meehl already observed that in some research disciplines, there is a close resemblance between theories and fashion trends. His article has the fitting title “Why Science Is Not Necessarily Self-Correcting” ( Ioannidis 2012). In a highly influential paper, John Ioannidis of Stanford criticized the notion that science is automatically cumulative and constantly improving. This should allow us to build more powerful theories, and to dismantle fallacies of the past. If science is cumulative, more published research equals more evidence. In principle, this development should make us enthusiastic about the prospects of science. These numbers rise substantially for the following decades, and since the beginning of the 21 st century, they skyrocket (see Figure 1.1).įigure 1.1: Articles indexed in PubMed by year, 1781-2019 Until the middle of the 20 th century, only a few hundred research articles are listed for each year. The number of articles indexed for each year in one of the largest bibliographical databases, PubMed, symbolizes this in an exemplary fashion. The amount of published research findings is also increasing almost exponentially. In biomedicine alone, more than one million peer-reviewed articles are published each year ( Björk, Roos, and Lauri 2008). Petabytes of research findings are produced every day all around the world. Never in history did we have access to more evidence in the form of published research articles than we do today. Many of us are fascinated by science because it is progressive, furthering our understanding of the world, and helping us to make better decisions.Īt least by the numbers alone, this sentiment may be justified. A famous quote by Isaac Newton stresses that if we want to see further, we can do so by standing on the “shoulders of giants”. In their scientific endeavors, researchers build on the evidence compiled by generations of scientists who came before them. Cience is generally assumed to be a cumulative process.
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