The curious case of Tamil Nadu’s steady Covid numbers

For the last few weeks, Tamil Nadu’s daily Covid bulletin is causing some confusion in the state. Like Sachin Tendulkar, the joke goes, TN appears to be suffering from “nervous 90s”.

For nine straight days during late August, the state reported over 5,900 cases each day but never touched 6,000 cases. More specifically, while new cases rise across the country, Tamil Nadu has had a remarkably – or suspiciously – stable month. Tamil Nadu reported 5,976 new corona positive cases on Friday. The TN case tally now stands at 451,827 with 7,687 deaths in the state.

The standard deviation is a statistical measure used to understand how much a range of values deviates from the average. A high standard deviation means that a state might show large changes each day, while a low value shows that the number of new cases each day hovers near the same value. Tamil Nadu has had the least deviation in any state for the last month.

This means that each day, new cases have only been up to 4% lower or higher than the average value. In Maharashtra, for comparison, the standard deviation has been 20% for the last month. Even in smaller states like Delhi and Kerala with lower reported cases, the standard deviation was in the 28-30% range.

Other states have experienced such periods of remarkable stability too – between July 22 and August 22, Gujarat saw very little change in daily reported cases and its standard deviation in cases too was just 4%, as Tamil Nadu’s is now.

What could be driving this phenomenon? Neither Tamil Nadu nor Gujarat has seen a concurrent flattening of testing, so that’s not the explanation – the two states are testing more people each day, but still appear to be discovering a stable number of positive people.

In the case of deaths, it is Rajasthan that shows very little daily variation, although at much lower values. Over the last month, Rajasthan has reported virtually the same number of deaths each day. Both Rajasthan and Bihar have reported the same average number of deaths over the period. But while Bihar’s count shows a daily fluctuation, Rajasthan’s is remarkably stable.

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