Something doesn’t add up

I was watching the San Diego County media briefing yesterday (video) and when Dr. Wooten showed the latest slide related to the county’s triggers (full slide deck), I was surprised to see that our 14-day case rate was 105.7 (per 100,000). This is the metric that the state used to place the county on the County Monitoring List. We need to get that under 100 per 100,000 to get off the list, which will allow businesses and schools to reopen.

In late July, that 14-day case rate was in the 150s. And Dr. Wooten said at several media briefings that in order for San Diego County to get off the monitoring list (i.e. to get the number under 100 per 100k), we would need to have fewer than 240 new cases per day for 14 days.

But we haven’t had fewer than 240 cases since June 22. Our daily case numbers have been trending down, but have generally been bouncing around between the high 200s and low 600s for the last 2 weeks. So it didn’t make sense that we were already down to 105.7, but I wasn’t sure how it was calculated and filed that thought away for the duration of the briefing.

At the end of the county’s briefing, Tarryn Mento from KPBS asked a question directly about this calculation and pointed out that it didn’t match her own calculations or what the state had posted for San Diego County on their website (her coverage of the briefing is here, but only briefly mentions the glaring discrepancy in case rates).

Dr. Wooten explained how they calculate the number. She said (and I’ll include a transcript from the video at the end of this blog post) that they use a 3 day lag time, meaning they add up the number of cases for the 14-day period starting 3 days ago. Then they divide that by the population and multiply by 100,000. She said they use 3.37M for the calculation. (The state lists the population of San Diego County as 3,370,418.)

Curious, I went to the state’s website after Ms. Mento’s question. At that time, the state showed a 14-day case rate of about 177. That is quite a difference from 105.7.

To make sure I understood the calculation, I used the 240 number that Dr. Wooten has repeated as the number we’d need to be under for 14 days to get off the monitoring list.

240 cases x 14 days = 3,360
3,360 cases ÷ 3,370,418 people in the county = 0.0009969
0.0009969 x 100,000 people = 99.69

So 240 cases per day for 14 days would put us just under 100 cases per 100,000 people. Ok, this makes sense so far.

Today, I took the county’s COVID 19 stats from the open data portal, which at the time I downloaded it only went through August 3. I manually added the total reported new cases (348) for August 4. And I also noticed that for August 1, they listed 256 new cases, but the Percent Positive chart provided by the county and the state data (which I confirmed in the state’s dataset) show 306 cases reported that day. So I updated that in my downloaded dataset.

Then I calculated the 14 day case rate in my own spreadsheet, first by simply adding up the new cases from the most recent 14 days (i.e. not using the 3 day lag) and got 5757, which when divided by the population and multiplied by 100,000 gave me 170.8 for August 4th. (I used formulas in my spreadsheet so I could calculate the values back to July 6, when the county officially changed the first trigger on our Triggers dashboard to use the 14-day case rate).

The screenshot below which I took today (Aug 6, 2020) shows that my numbers match the state’s calculation exactly.

So what is going on? Dr. Wooten mentioned a 3 day lag, but since the last few days have been lower than before, I wouldn’t expect a lower case rate if I started 3 days ago. But I calculated it anyway. And as expected, the rate is higher – 183.98 per 100,000 for August 4.

I also saw that calculations for previous days were over 200. But I never saw a 14-day case rate on the Triggers Dashboard higher than 158.5.

Although the dashboard only displays the current metrics, the SANDAG Open Data Portal has a dataset of the COVID 19 Triggers, so I downloaded that and copied the values for Trigger 1 into my spreadsheet to compare. The numbers from the Triggers Dashboard are so far off from the state’s values and even further off from what it would be using the calculation described by Dr. Wooten at the press briefing yesterday, I’m at a complete loss to understand this discrepancy.

So in my spreadsheet, I added two more columns. One is the number of cases that the 14 day total would need to be to get the county’s 14-day case rate, and one that is the difference in number of cases between that and what the 14-day total (using the 3 day lag calculation) would actually be. The numbers of cases are off by thousands.

I’ve posted my spreadsheet here.

I went to the Triggers Dashboard this evening, hoping it would be cleared up, and this is now what it shows:

I still don’t know where the 110.1 rate comes from and I don’t know how they arrived at the 112.4 case rate for the state.

The only possible clue I see is that it says in the trigger explanation “measured using the date of illness onset with a 3-day lag”.

Illness onset may be different than the cases reported each day. Test results have been repeatedly delayed. It seems like nearly every column in the COVID 19 Percent Positive chart has an asterisk to indicate this. The county does provide another chart showing Covid cases by date of illness onset, but it is very difficult to establish the exact numbers each day and these values are not, as far as I can see, provided on the county’s open data portal.

The county really needs to clear up this discrepancy. People are frustrated – both those who think the state is going to far in trying to slow the spread and those who think we still aren’t doing enough. Transparency is essential. And between this and the fact that Gavin Newsom has not adequately addressed the issue of the missing test results from commercial labs in the state, it is becoming difficult to have confidence that we are getting an accurate count of new cases in the county.

The following is the transcript from the relevant portion of the media briefing video.

Tarryn Mento, KPBS:
Dr. Wooten, I apologize, but I’m trying to understand a little bit more about how you get to the case rate of 105 that you are reporting today. Can you clarify the total amount of cases over the 14 day period that you’re looking at? Because if you look back, it’s about 5700 cases and dividing that by the population and then using the formula you used, I’m not getting 105.7.

Dr. Wilma Wooten:
So let me explain again. First of all, you have to go back 3 days. The 3 day lag. And then from that point, back 14 days, it’s the number of cases for each of those days. So you have to be very clear on the dates that you’re using. And you add that, divided by the population. And you have to be very clear on what population you’re using. The state is using 3.37 [million]. And then you multiply that by 100,000. And that’s the way it’s calculated.

Ms. Mento:
Can you provide the total number of cases that you are including in that 14 day period? Because also the number you reported today and on the state’s website, for case rate, are also different. I checked it earlier today, it was 134.2 I believe that the state posted.

Dr. Wooten:
And the state’s going to be different. The Governor’s website is different. Again, it’s very clear on what days you use. So exactly what I just described to you. I don’t have the number of cases for each day for 14 days past the 3 day lag. We can certainly send it to you via email to show how we calculate that.

Ms. Mento:
I guess then my question is if our calculation and the state’s is different, but the case rate matters according to the state, why are we following our calculation and not the state? Because it’s the state that will allow us to get off the monitoring list.

Dr. Wooten:
Well for the case rate, our case rate, and if you saw the case rate today, is that for today’s, with our 398 or whatever the number. It wouldn’t be with those numbers. It would be a 3 day lag. But again, I can send you the information and an explanation of why our numbers, uh. Our numbers are exactly how I have just described them. Again, I don’t know what dates the state has used.

Ms. Mento:
I guess then, my final question is just going to be why are we not aligning with the state’s calculation if the state is setting the metric that allows us to get off the monitoring list.

Dr. Wooten:
We are aligning with the state’s, but it depends on what point in time. Again, I have not looked at the state’s website today. But again, we can show you and provide information as to how we’ve calculated the numbers and how it aligns with the state. We are using – and the state has a webpage where you go and calculate the numbers and it’s right there on the website – we are using the same calculations.