Established scientific models are supported by multiple independent lines of evidence.

A scientific model can be relied on when it’s had its tyres kicked, its log book examined, its mechanics tested, its body checked and it’s been taken for many test-drives.

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In short…

Scientists have most confidence in models that have been tested from as many different angles as possible. The great bulk of established scientific knowledge consists of such models. Much of the work at the frontiers of science consists of performing additional tests on the more sparsely-supported models.

In order to reach a reliable conclusion about how the world operates it is better to have multiple independent lines of evidence converging on one conclusion. That, of course, requires multiple studies.

Steven Novella, American neurologist, 2012.

What is meant by multiple lines of independent evidence.

In science, no model is accepted until it is tested against the real world. But a single line of evidence (for example, from one particular study or experiment) is not usually sufficient to establish the model beyond reasonable doubt.

The gold standard of testing is ‘multiple independent lines of evidence’, meaning that the model has been thoroughly tested and supported by many different investigations. If the evidence from these converges and indicates the model is correct, we can have great confidence in the model. In this case, the consensus of the scientific community swings behind the model and it becomes the accepted explanation.

Sometimes, there is a lot of evidence in support of a model, and some other evidence that fails to support it. As long as there is no falsifying evidence, scientists would say that the weight of evidence supports the model.

Why multiple lines independent evidence increase reliability of models.

Convergence of evidence is not the same as replication. Replication is a kind of checking of results, in which an investigation is repeated by researchers who are independent of the original investigating team. Convergence requires that the different lines of evidence come from different types of tests, sometimes from completely different fields of science. The more different and less connected the test techniques, the greater the support they provide by convergence.

Examples

• The discovery that CFCs were responsible for depleting the Ozone layer was made after multiple independent lines of evidence converged. The evidence came from measurements  taken at ground stations, via balloons and aircraft, satellite data,  and chemical studies. 
  
• The Plate Tectonic model of the Earth is supported by multiple independent lines of evidence – magnetic stripes in rocks showing sea-floor spreading, the global distribution of earthquakes and volcanoes, comparable fossils found on widely separated continents and satellite measurements.
• The Anthropogenic Global Warming model proposes that the current global warming and associated climate change are caused principally by human emissions of greenhouse gases. This is supported by multiple independent lines of evidence. For more details, see here.
• The biological evolutionary model is supported by evidence from many different fields:  paleontology, geology, genetics, molecular biology, biogeography, comparative anatomy and many others. Richard Dawkins relates some of them in this podcast.
• During the latter half of the 19th century, there was a remarkable lack of convergence in scientist’s estimates of the age of the Earth. Geologists argued for an age of perhaps billions of years and evolutionary biologists agreed that such timescales were needed for present lifeforms to evolve. However, physicists had other ideas. The influential physicist William Thomson (later Lord Kelvin) used calculations based on energy flows to estimate the age of the Sun at 20 – 60 million years, and the age of Earth at about 100 million years. The disagreement persisted until the discovery of nuclear reactions at the turn of the century. The realization that nuclear energy is responsible for both the Sun’s energy and the heat within the Earth changed the nature of the calculations. All evidence now converges on an age of 4.5 billion years for the Earth.
• The Big Bang model for the origin of the universe is supported by multiple lines of evidence: the expansion of space as described by Hubble’s Law, the existence of the cosmic microwave background radiation, the distribution and evolution of galaxies, the abundance of light elements in the universe, the large-scale structure of the universe, and many others.
  

Steven Novella’s quote is from A Study Showed…., JREF Swift blog, 2012.

The Climate system graphic is from the Australian Government Department of Industry, Innovation, Climate Change, Science, Research and Tertiary Education.

This is one of ScienceOrNot’s Hallmarks of science. See them all here.

Updated: 2013/09/09