With Covid-19 the numbers are coming at us all constantly. Every night there's an update, every day a further briefing.

Many of us use the numbers of 'confirmed positives' to gauge how the country is handling the crisis. But not every update is equal - on Tuesday and Wednesday of this week the first results for testing done under a new case definition will begin to emerge into the nightly numbers.

That will make it difficult for epidemiologists and statisticians to compare last week's updates to this week. The change in case definition focused testing on groups of people more likely to test positive than others.

According to the experts Prime Time spoke to, any day-to-day comparisons will need to come with a proviso, until we see how the trend develops under the new definition.

What appears to be a spike or a dip, may only be a spike or dip in the cases we're finding, not a spike or dip in the number of infections.

But that's just one of a number of qualifications or provisos worth keeping in mind if you're using the nightly confirmed positive figure as a measure of how we're dealing with the pandemic. Others include the delay between infection and and the appearance of symptoms, and time it takes to access and get test results. 

In this edition of Explained by Prime Time we try to break down what you need to know about the nightly confirmed positive number. The series will focus on COVID-19 for the foreseeable future, and be available on RTE Player, Youtube.com/RTE, the Prime Time Facebook page.