Variation among states in rate of coronavirus spread

The corona virus, COVID-19, has been spreading rapidly across the USA since early March, but at a decreasing rate, where the rate r is defined as the exponential increase. I modeled the way the rate of increase y = ln(e^r − 1) has declined through time in each of the 51 states with the goal of determining how quickly the rate has declined, whether the decline has changed, and whether states differ. A piecewise linear regression was used, with a single break point. This model can identify whether there was a change in the rate of decline, when the change happened, and which states have shown the greatest improvement in reducing the spread of COVID-19. The piecewise model identified a significant breakpoint on 24 Mar for all states combined, and all states had nearly the same breakpoint. Prior to 24 Mar, the average change in y was −0.013 d⁻¹, meaning a reduction in the rate of spread from 23.5 pct. d⁻¹ to 19.5 pct. d⁻¹; after 24 Mar, the average change in y was −0.070 d⁻¹, a reduction from 19.5 pct. d⁻¹ to 7.5 pct. d⁻¹. Prior to 24 Mar there was no significant variation among states in the decline in y, but after 24 Mar there was substantial variation. Montana, Idaho, and Vermont showed the greatest improvement, while Nebraska, South Dakota, and Iowa the least. The improvement as measured by the reduction after 24 Mar did not correlate with case density in a state, nor state population. The next question is whether it correlates with differences among states in the health measures taken to combat the spread.