New COVID-19 Cases in Los Angeles - Fun with Numbers

Earlier today, I wrote about the Los Angeles County Department of Public Health COVID-19 Dashboard which shows the number of daily COVID-19 cases, and I provided a figure in my blog to illustrate the trend. However, I felt unsatisfied with the dramatic day-to-day variation in the number of cases, so I decided to calculate rolling averages over the prior 2, 3, 4, 5, 6, and 7 days. Here’s what it looks like:



Based on a calculated 7-day rolling average, we reached a peak of 897 cases on May 11, 2020 and have seen a decline since then. The last day for which we have data are for May 21, 2020 where the rolling 7-day average was 547 cases.

Not surprisingly, I noticed that the dramatic peaks and valleys of the original data follow a 7-day cycle. For each 7-day period, the nadir usually falls on a Sunday (e.g, April 26, May 3, May 10, and May 17), and the peaks occur the following Monday and/or Tuesday which are likely to be the cases that were not reported over the weekend. I can think of a couple of possible reasons for this. First, there may be less staffing on weekends which may result in less case reporting. However, given that the nadir usually occurs on Sundays, I also wonder if patients seek healthcare less frequently on Sundays, perhaps because they are attending church or other activities. Regardless, a 7-day rolling average may be a good way to look at statistical trends because it smooths out the daily variation in case reporting.

I hope that we continue seeing a downward trend of daily cases as we continue reopening businesses.