It is now generally accepted that R, the reproduction number, is one of the keys to understanding the progress of the Covid-19 epidemic.
When R is less than 1, the number of cases, hospital admissions and deaths is in decline and we can be generally said to have the epidemic under a degree of control.
However, if R increases to above 1, it is an indication that the number of cases, hospital admissions and deaths is rising and that further measures to deal with the epidemic may be required.
Every time we release some of the current restrictions, it will take around three weeks before we can be confident that R will remain below 1.
However, R is not the only number which is an important indicator in relation to Covid-19. An R value below 1, no matter how far it falls, is not enough on its own to indicate that it is safe to relax the social distancing and other restrictions which are in place. Other numbers also matter.
Chief among the other important numbers are those which indicate the current level of activity of the epidemic - the number of new cases per day, the number of patients being admitted to hospital, the number of patients who need critical care treatment, the number of cases and outbreaks in our care homes, and the number of deaths.
While R is below 1, all of these other numbers will be falling slowly, but we need to be confident that they are at a sufficiently low level before we consider releasing further restrictions.
We need the number of new cases each day to be low, so that 'test, trace, isolate and support' can ensure that every significant contact of each new case can be informed of their need to self-isolate until it is clear that they do not have the infection and cannot pass it on to their family or friends. This is the key to doing our best to keep R below 1 as we move through the stages of the Executive's recovery plan over the next few months.
We need the number of hospital admissions and patients who need critical care treatment to be low to ensure that our Health and Social Care system can deliver high-quality care for Covid-19 patients while normal services in all other areas are restored.
And we want the number of cases and outbreaks in our care homes to be as low as possible to keep both patients and staff safe and healthy. While there has rightly been a great deal of emphasis on R, its value and how it is calculated, the other numbers will be just as important as we move forwards through the epidemic.
R is an important number, but it is not the only number which matters.
Professor Ian Young is Northern Ireland's Chief Scientific Advisor