Explainer: Here's why Ontario's daily COVID-19 case numbers may not match those released by Waterloo Region
Daily COVID-19 case counts released every morning by the Ministry of Health may differ from those reported by the Region of Waterloo in the afternoons.
The difference is due to timing; specifically, when the case counts are reported by the province and in the region.
Ontario's data, which is released every day at 10:30 a.m., is current from the afternoon of the day before. Waterloo Region, however, has a reporting cycle from 12 a.m. to 11:59 p.m. from the day prior. That information is released on the region's COVID-19 dashboard daily at 1:30 p.m. Earlier this year, the region sent out a tweet saying it needs time to collate data before releasing its numbers.
A post on the province's COVID-19 website says differences in numbers may also be due to time lags for data entry in the Public Health Case and Contact Management Solution (CCM). Public health units report COVID-19 data to the CCM every day.
"The Ministry of Health receives the data once a day in the evening. This time lag is usually no more than one day, but means that Public Health Unit data is often the most up-to-date," the website says in part.
The province also says there can be differences due to reporting metrics.
"We report COVID-19 cases and their status (active, resolved or death) according to the date those cases or status changes happened. That means case numbers for days in the past are continuously being updated as more information is received," the website says. "Some organizations use a different approach where case information is reported by the day it is received instead of the day it happened."
Local health units are also continually cleaning up their data, correcting for missing or overcounted cases and deaths.
"These corrections can result in data spikes, negative numbers and current totals being different from previously reported case and death counts," the province says.
The website says the different reporting approaches have both advantages and disadvantages.
"It’s important to be aware of these differences when comparing data between sources," the province says.