Most research on health interventions aims to find evidence to support better causal inferences about those interventions. However, for decades, a majority of this research has been criticised for inadequate control of bias and overconfident conclusions that do not reflect the uncertainty. Yet, despite the need for improvement, clear signs of progress have not appeared, suggesting the need for new ideas on ways to reduce bias and improve the quality of research.
With the aim of understanding why bias has been difficult to reduce, we first explore the concepts of causal inference, bias and uncertainty as they relate to health intervention research. We propose a useful definition of ‘a causal inference’ as: ‘a conclusion that the evidence available supports either the existence, or the non-existence, of a causal effect’.
We used this definition in a methodological review that compared the statistical methods used in health intervention cohort studies with the strength of causal language expressed in each study’s conclusions. Studies that used simple instead of multivariable methods, or did not conduct a sensitivity analysis, were more likely to contain overconfident conclusions and potentially mislead readers. The review also examined how the strength of causal language can be judged, including an attempt to create an automatic rating algorithm that we ultimately deemed cannot succeed.
This review also found that a third of the articles (94/288) used a propensity score method, highlighting the popularity of a method developed specifically for causal inference. On the other hand, 11% of the articles did not adjust for any confounders, relying on methods such as t-tests and chi-squared tests. This suggests that many researchers still lack an understanding of how likely it is that confounding affects their results.
Drawing on knowledge from statistics, philosophy, linguistics, cognitive psychology, and all areas of health research, the central importance of how people think and make decisions is examined in relation to bias in research. This reveals the many hard-wired cognitive biases that, aside from confirmation bias, are mostly unknown to statisticians and researchers in health. This is partly because they mostly occur without conscious awareness, yet everyone is susceptible. But while the existence of biases such as overconfidence bias, anchoring, and failure to account for the base rate have been raised in the health research literature, we examine biases that have not been raised in health, or we discuss them from a different perspective. This includes a tendency of people to accept the first explanation that comes to mind (called take-the-first heuristic); how we tend to believe that other people are more susceptible to cognitive biases than we are (bias blind spot); a tendency to seek arguments that defend our beliefs, rather than seeking the objective truth (myside bias); a bias for causal explanations (various names including the causality heuristic); and our desire to avoid cognitive effort (many names including the ‘law of least mental effort’).
This knowledge and understanding also suggest methods that might counter these biases and improve the quality of research. This includes any technique that encourages the consideration of alternative explanations of the results. We provide novel arguments for a number of methods that might help, such as the deliberate listing of alternative explanations, but also some novel ideas including a form of adversarial collaboration.
Another method that encourages the researcher to consider alternative explanations is causal diagrams. However, we introduce them in a way that differs from the more formal presentation that is currently the norm, avoiding most of the terminology to focus instead on their use as an intuitive framework, helping the researcher to understand the biases that may lead to different conclusions.
We also present a case study where we analysed the data for a pragmatic randomised controlled trial of a telemonitoring service. Considerable missing data hampered the forming of conclusions; however, this enabled an exploration of methods to better understand, reduce and communicate the uncertainty that remained after the analysis. Methods used included multiple imputation, causal diagrams, a listing of alternative explanations, and the parametric g-formula to handle bias from time-dependent confounding.
Finally, we suggest strategies, resources and tools that may overcome some of the barriers to better control of bias and improvements in causal inference, based on the knowledge and ideas presented in this thesis. This includes a proposed online searchable causal diagram database, to make causal diagrams themselves easier to learn and use.