The concept of ‘evidence-based policy’ – one of the defining features of the century – refers to the idea that policies should be based on, or informed by, high quality evidence, as opposed to ideology or personal judgment.

But putting the principle into practise raises a host of complexities. Two key questions are: how much evidence is enough to make a decision? And what kind of evidence counts as strong and relevant?

Among those of us who specialise in methodology there are heated debates on these questions and there exists no universally agreed way of reviewing evidence.

We can see that to review the evidence on face masks, Hiqa used a model that is a “copy and paste” from the field of medicine.

This might sound logical except mandating face masks is not a medicine. It is a social policy and, for a variety of reasons, there is debate about whether this model is always appropriate.

In the medical model adopted by Hiqa the strongest evidence is assumed to come from a particular type of research study called a randomised controlled trial (RCT).

A randomly selected group of people is given the medicine and another randomly selected group is not. If the group who were given the medicine on average do better, we say the medicine works.

In this model, observational studies (which are like RCTs except participants aren’t randomly selected) can also be considered but are seen as a lesser quality evidence.

According to this framework, without positive RCTs, a medicine won’t be declared effective.

The Hiqa protocol shows their review of face coverings adopted this model and so only included evidence from RCTs, observational studies and systematic reviews, which are reviews of RCTs and observational studies. All other studies are excluded.

There are no RCTs on the effect of face masks or face coverings worn by the public during the Covid-19 pandemic.

A Royal Society review found only two RCTs on the use of face masks to reduce transmission of the regular flu.

These studies had flaws but did suggest face masks can reduce infection.

For policy makers who follow the medical model in which good quality RCTs are the only form of strong evidence, this means they conclude, as the WHO has, that “currently there is not enough evidence for or against the use of masks (medical or other) for healthy individuals in the wider community”.

Dr Tony Holohan, the chief medical officer, has stated that “the evidence isn’t very strong in relation to the value [of face masks in stopping the spread of Covid-19]”.

They are concluding, not that face masks are ineffective, but that they don’t know and they don’t have enough evidence to fully endorse their use.

Yet if we look outside of the RCT-focused medical model there is evidence which could help to inform a decision.

This comes from physics, from laboratory studies, natural experimental and mathematical modelling and it can logically be supplemented with broader social science evidence.

To give an example, the highly prestigious ‘New England Journal of Medicine’ recently published the video for an experiment demonstrating how a cloth cover over the mouth effectively blocks droplets that come out of our mouths when we speak.

The point here is to generate evidence that helps us to understand how covering the mouth might help to prevent transmission.

A growing body of evidence suggests face coverings help because people are contagious without having symptoms and when we speak droplets harbouring the virus can come out of our mouths.

The cloth covering acts as a barrier, catching the droplets while they are large and preventing them evaporating to become small particles that linger in the air.

Of course, evidence that face coverings are physically effective is no guarantee they will be effective in practise. This is because how effective anything is depends on how it is used. It depends on human behaviour.

A key concern in Ireland seems to be that if the Government fully endorses face coverings, the public may neglect other important preventative measures like physical distancing and hand hygiene because they will feel an exaggerated sense of security.

Again, on this question we simply don’t have the evidence for how people will behave in a Covid-19 context. However, using social science evidence from other policies, we can make a good guess. We have a wealth of evidence from other policies that evoked the same type of concern, from seat belts to helmets.

At the population level, these have led to increased safety and even increased safety-oriented behaviour. As the highly regarded Royal Society recently concluded, there is simply no evidence that individuals will engage in risky behaviour during the pandemic if face coverings are strongly endorsed.

Similarly, there seems to be an assumption that the public will use face coverings incorrectly. But again, there is no evidence of this.

All in all, assuming the public will act irresponsibility or incapably is just that, an assumption, and an unfounded one in terms of evidence.

The stakes are too high to wait for certainty. As we move out of lockdown, many countries and top experts at home and abroad are concluding face coverings are a vital means of protection.

The Government does not have the luxury of solid, unambiguous evidence but we can make best use of the evidence we have by drawing on a wider range of quality evidence. In the context of Covid-19 we need decisive action.

For now, I’ll be wearing my home-crafted face covering to protect others and hoping the Government urgently mandates masks to protect me.