From The Conversation
From social to natural and applied sciences, overall scientific output has been growing worldwide – it doubles every nine years.
Traditionally, researchers solve a problem by conducting new experiments. With the ever-growing body of scientific literature, though, it is becoming more common to make a discovery based on the vast number of already-published journal articles. Researchers synthesize the findings from previous studies to develop a more complete understanding of a phenomenon. Making sense of this explosion of studies is critical for scientists not only to build on previous work but also to push research fields forward.
My colleagues Hazhir Rahmandad and Kamran Paynabar and I have developed a new, more robust way to pull together all the prior research on a particular topic. In a five-year joint project between MIT and Georgia Tech, we worked to create a new technique for research aggregation. Our recently published paper in PLOS ONE introduces a flexible method that helps synthesize findings from prior studies, even potentially those with diverse methods and diverging results. We call it generalized model aggregation, or GMA.
Pulling it all together
Narrative reviews of the literature have long been a key component of scientific publications. The need for more comprehensive approaches has led to the emergence of two other very useful methods: systematic review and meta-analysis.
In a systematic review, an author finds and critiques all prior studies around a similar research question. The idea is to bring a reader up to speed on the current state of affairs around a particular research topic.
In a meta-analysis, researchers go one step further and synthesize the findings quantitatively. Essentially, it takes a weighted average of the findings of several studies on one topic. Pooling results from multiple studies is meant to generate a more reliable finding than that of any single study. This is crucially helpful when prior studies reported diverging findings and conclusions. And the rise in the publications of meta-analysis has shot up over the last decade, underscoring their importance across research communities.
Meta-analysis has been helpful in increasing our understanding of many scientific problems. But it has some challenges. A typical meta-analysis combines just one explanatory variable (that is, a treatment controlled by the experimenter) and one response variable (for instance, a health outcome). Also, a researcher has to be very careful not to lump apples and oranges together in the meta-analysis. She must be selective and make sure to include only previous work that shared a very similar study design.
Here is where our simple and flexible generalized model aggregation method comes in. Using GMA, the prior studies do not necessarily need to have the same study design or method. They can also have different explanatory variables. As long as they are all answering a similar research question, GMA can synthesize them.
Pooling findings from across a field
Consider an example from the health literature. Obesity and nutrition researchers need reliable equations that estimate basal metabolic rate (BMR) – the amount of energy the human body spends at complete rest. Understanding BMR has big implications for real-world questions of weight management.
Researchers often estimate BMR as a function of different attributes: age, height, weight, fat mass and fat-free mass. The challenge is that current publications in research journals provide over 200 such equations estimated for different samples and age groups. These equations also include different subsets of those attributes.
For example, one of these equations included weight and age, but another included only fat-free mass. Another equation considered the impact of all these attributes, but the sample size was too small to make it reliable. More interestingly, and confusingly, there have been several studies with similar samples and variables but they have reported very different equations to explain the relationships.
So which equations are you going to choose to accurately estimate BMR? How do you ensure that your selected equation is more reliable than the rest?
In order to address these questions, we identified 27 published BMR equations for white males from published studies. Then we used GMA to aggregate them into a single equation, which we called a meta-model.
Read the full post at The Conversation.
Mohammad Jalali is a research scientist at the MIT Sloan School of Management.