Illustrating the Positive Rate

Five months ago we were experiencing the peak of COVID-19 in Ireland. Back then testing was limited and no one was sure exactly how many cases there were.

Since then things have thankfully improved and testing rates are much higher. But what does this relationship between the amount of testing and the case numbers look like?

Well, here’s a view of how the number of tests has varied with positive results.

Looking at NI first – you can see the numbers were low in June/July and in August the number of tests increased at approximately the same rate as the positive results.

However, in September this changed – and rates are now increasing.


Here’s the same graphic for ROI.

While testing increased in July, and the positive results stayed low, but from August the number of positive results have increased.

Here’s both of these, side by side.

A similar pattern here as the positive rate pushes past 2% in the South, and close to 3% in the North.

Finally, if you want to see what things looked like before June – this shows the numbers from March.

Back at the early stages of the outbreak, testing rates were low, and the percentage of positive cases peaked at over 20%. It’s noticeable that the number of positive cases in NI is a lot lower, but you can also see that the number of tests are also a lot lower. It’s not until June that both parts of the island start to follow a similar path.


NI Source:

ROI Source:

The positive rate % is based on the positive results aligned with the test data and so the dates may not align with the reported ‘case’ numbers. In NI the % rate is based on tests per individual, whereas in ROI this is based on the full volume of tests.

The dates from these 2 datasets have been aligned to show the latest data available when this was compiled on the 21st September and so dates have been adjusted to show a common ‘reported date’. Also, be aware that the historical numbers reported in NI are updated daily and are not fixed.

Referencing the image below also (and link to the R code here). I initially used this, but couldn’t get the formatting they way I wanted it – so went in a different direction.