New State Tiers

In late August, California announced a new state-wide tier system called Blueprint for a Safer Economy to standardize how counties decide what and when businesses can reopen. Previously, we had been using a 14-day case rate with a goal of under 100 per 100k to get off the state’s monitoring list, which required the closure of certain indoor operations.

This binary system (on or off the state monitoring list) has been replaced with are now 4 tiers based on the average daily case rate (per 100k) over a 7 day period and testing positivity rate. Exceeding either threshold places a county in a more restrictive tier. Counties can only move up one tier at a time and must wait 3 weeks before moving to a new tier, even if our case rate and testing positivity rates would place us in a less restrictive tier.

You can find more details on these new guidelines on the California Department of Public Health website:

The tiers

Image source:

Most counties in the state are in the most restrictive tier, with widespread community transmission. The new metric for moving from Tier 1 (purple) to Tier 2 (red) is essentially the same as the previous metric; that is, getting to under 7 per 100k for a daily average case rate is comparable to under 100 per 100k over a 2 week period (it corresponds to 98 per 100k in a 14-day range). So when this new system was introduced, shortly after San Diego County came off the state monitoring list, we landed in the red tier. (Our positive test rates are well under the threshold for that tier.)

The state provides specific guidelines for businesses on if and how they can operate depending on which tier the county falls within.

How it works

Like the previous 14-day metric, the new 7-day daily average case rate is calculated using the date of illness onset data, not the test reporting date.

Detailed instructions how how the number is calculated are available on the metadata page for the relevant data set, but here is the relevant excerpt:

To calculate San Diego County’s current case rate following the method described by the California Department of Public, do the following:

1. Download the data.
2. Filter for the most recent “update date.”
3. Ignore the seven most recent episode dates. 
4. Sum the case counts excluding prison inmate cases of the seven most recent episode dates prior to that and divide by 7.
5. Divide by 3,370,418 (State of California Department of Finance 2020 Population Projection for San Diego County:
6. Multiply by 100,000.

When the county first announced this new metric, I set up my Tableau Public file to calculate and display this metric. The county only updates the value once per week, although I’m able to calculate and display it for each day.

I noticed that when they posted the updated case rate yesterday, it was 6.9 per 100k, which just barely keeps us in Tier 2 (or the red tier). My calculations showed we hadn’t been at 6.9 since August 30. That’s when I noticed that the instructions say “Sum the case counts excluding prison inmate cases“. And sure enough, I looked at the dataset and saw they had added a column for the total number of cases by illness onset date excluding prisoners. So I adjusted my calculations accordingly.

My calculations

In Tableau, I created a calculated field that sums up 7 days, starting with 7 days ago.

Then I take that value, divide by 7, divide by 3,370,418, then multiply by 100,000, per the instructions from the county.

And then I chart it over time.

You can see that I am getting 7.29 using the data through September 7, while the county reported 6.9.

To be fair, I am charting past dates based on the most recent Update Date, whereas the county does not go back and recalculate previous case rates when the data for illness onset is updated for past dates. The 7-day lag for calculating the case rate is designed to account for the delay in illness onset data reporting. However, the most recent case rate I am calculating, using the September 8th update date values should match the county’s value for September 8th.

It’s complicated

I looked again at the state’s page on their tier framework and noticed that they use a case rate adjustment factor based on testing rates. Since San Diego County is reporting 133.2 tests per 100k people, and the state’s average testing rate is 217.9 per 100k, we have been assigned an adjustment factor of 1.155.

To explore the data county by county, the page linked above includes a link to an Excel spreadsheet tracking data for the tier framework, which you can also access here:

Their spreadsheet shows a rate of 6.9 for San Diego, the same as the county reported. And with the adjustment factor, it is a 7.9. However, we are still listed as in Tier 2.

So I’m still unclear where the county’s case rate of 6.9 per 100k comes from. If you see an error in my calculation, please let me know.

For my current charts of cases by illness onset and the case rate, see