Generally, the number of positive COVID-19 cases increases when the testing rate increases. However, this is not always the case, as data from Gujarat show.
Who gets tested determines the number of positive cases more than how many get tested. In Tamil Nadu, the number of positive cases went up when more people from the Nizamuddin cluster were tested and went down when more people outside the cluster were tested.
Cases, tests in India
The chart plots the daily samples tested (blue line, left axis) and daily cases recorded (yellow line, right axis) in India since April 3.
(If chart doesn’t appear in full, click here)
As seen above, after April 9, a somewhat linear relationship emerges between samples tested daily and the number of new cases recorded.
Cases, tests in Gujarat
However, the above is not always the case, as the following example from Gujarat shows. The chart plots the daily samples tested (blue line, left axis) and daily cases recorded (yellow line, right axis) in Gujarat since April 3.
Testing clusters
The graph shows daily cases recorded in Tamil Nadu (bar, right axis), daily samples tested of those in the Nizamuddin cluster (red line, left axis) and daily samples tested of those not in the cluster (blue line, left axis).
Tests vs positives
The graph shows the % of positive cases among the samples tested of those in the Nizamuddin cluster (red line) and those outside the cluster in Tamil Nadu (blue line).
As the probability of positives inside clusters is high, on days when a State tests more people who belong to clusters, the case count is higher. And the days on which a State tests more people from outside clusters, where the probability of positives is much lower, the case count is also lower.