When doubling isn't doubling. Strange math for NM COVID-19 cases

NM Department of Health Math


- It took New Mexico 7 days to double our numbers, not 4.5 days. This is a big deal because that means NM DOH and the citizens of New Mexico are doing a stellar job of following the social distancing rules and slowing the virus in New Mexico.


When the COVID-19 virus began I decided I would not write about it because I had not written about COVID 1 through 18 (shout out to Rush Limbaugh and Fox News). Seriously, I didn’t want to write about it as I knew the mainstream media would beat and repeat this story day after day, week after week. I didn’t want to regurgitate what was already being done by other news outlets.


That changed on April 15 when Governor Michele Lujan Grisham and New Mexico Department of Health Dr. David Scrase held their news conference update for the virus. During this news conference Dr. Scrase stated that the number of COVID-19 cases were doubling every 4.6 days.


I have been keeping track of the NMDOH daily news releases regarding the current number of COVID-19 cases and what Dr. Scrase stated just didn’t correspond with what the NMDOH has been telling us.


For example, on April 6, 2020 NMDOH reported that the total COVID-19 cases in New Mexico were 686 (you can find this at the NM DOH website). Doubling this number would be 1,372 cases. On April 13, 2020, NMDOH issued press release stated that the number of cases in New Mexico was 1,345 (27 cases below the doubling from April 6).


That means it took New Mexico 7 days to double our numbers, not 4.5 days. This is a big deal because that means NM DOH and the citizens of New Mexico are doing a stellar job of following the social distancing rules and slowing the virus in New Mexico.


The doubling numbers get even better when you look at the NMDOH release for April 9, stating 989 total cases in New Mexico. The doubling of this number did not occur until April 20, when NM DOH reported 1978 total cases.


It took 11 days for the number of cases to double! Good job New Mexico!


But wait, when I asked NMDOH for the doubling numbers on, April 21, 2020, I was told by David Morgan of NMDOH that the doubling case growth rate is now 5.1 days. Huh?


A few more emails (social distancing remember) to NMDOH and we got the explanation as to the discrepancy in doubling numbers. The best way to explain this is by quoting Jodi McGinnis-Porter, spokeswoman for David Scrase, MD, Secretary for New Mexico Human Services Department. McGinnis-Porter stated that this is how Doctor Scrase explains the difference:


“The official epidemiologic doubling time is calculated by taking the average of all the daily growth rates (x) since the epidemic started and then calculating the doubling time using this equation: =LN(2)/LN(1+x).


A simpler method is to just look at more recent data and that does show a longer doubling time. But we use the epidemiologic standard which allows us to compare how NM is doing compared to other states. We have also been using date of sample collection rather than report date to calculate the doubling time, which explains the five-day lag period in doubling time.


Interestingly, Johns Hopkins changed to this formula in mid-stream and we adapted. So, there was a change there in the second week.


For a more scientific explanation of doubling time, see here: https://en.wikipedia.org/wiki/Doubling_time


I know what you are thinking, “clear as mud,” right?


Well this does make sense. Basically, both ways to figure doubling are correct. One method is what I was taught in the Topeka Public School system, 4 doubled is 8, 8 doubled is 16, etc. What NMDOH is using is also correct but they are looking at doubling in a different fashion.


Epidemiology, as defined by the CDC, “is a method used to find the causes of health outcomes and diseases in populations. In epidemiology the patient is the community and the individuals are viewed collectively. Epidemiology is the study (scientific, systematic and data driven) of the distribution (frequency, pattern) and the determinants (causes, risk factors) of health-related events in specified populations.”


Simply put, NMDOH is trying to capture a lot more than simple doubling and their formula is including a comparison with other states. They are also using the day the sample was collected, not the report date. NMDOH says there can be a five-day lag period between collection and reported.


Another question that I had involved the number of cases at the Navajo Nation.


The Navajo Nation encompasses Utah, Arizona and New Mexico. Were the Navajo Nation number of infected persons separate from New Mexico? Or do they overlap?


Back to David Morgan with NMDOH, “Members of the Navajo Nation testing positive for COVID-19, on the portion of the nation overlapping with the State of New Mexico, are included in the NMDOH case count.”


On April 21, the Navajo Nation stated they had 1,321 cases. 523 of these cases occurred within New Mexico. On the same date NMDOH announced 2,072 of cases in New Mexico. Of these New Mexico cases, 523 overlap (are counted by) the Navajo Nation.


Numbers and statistics are important, especially during a pandemic. Citizens need to know how numbers are being calculated and if they are being counted by two different government entities.


When the DOH announces doubling dates the citizens now know that they are using a different formula, to catch different data, and they are not simply looking at basic doubling numbers. It’s an important difference and should be explained every time these statistics are discussed.


The takeaway from all of this is, New Mexico is doing very well in the fight with COVID0-19, those at NMDOH are working hard to protect us and citizens are interested and want to be informed. That’s bodes well for New Mexico.



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