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Ottawa Public Health（OPH）的数据不是大家用的“通常的 confirmed cases” vs. date 曲线，而是 epidemic curve (epi curve or epidemiological curve)，是 scientific community/research health professional 用的，OPH的图标也清楚标明“Cumulative epidemiological curve of Ottawa residents with confirmed COVID-19, by the EARLIEST of onset date, test and reported date”。举个例子，一个病人3月11日出现症状，过了一周后觉得可能是 covid 19，3月18日去测试，3月28日结果出来阳性确诊，那么在通常的统计中，这个病例算入3月28的数据，但是在 OPH 的 epi curve 中，这个病例最后会算入3月11日的数据中去。
下面是来自CDC网站的关于epidemic curve的解释：
Interpretation of Epidemic (Epi) Curves during Ongoing Outbreak Investigations | Foodborne Outbreaks | Food Safety | CDC
An epidemic curve (epi curve) shows progression of illnesses in an outbreak over time. Epi curves depict when people became ill by day, week, or month.
An epidemic curve (epi curve) shows progression of illnesses in an outbreak over time. Epi curves depict when people became ill by day, week, or month. In foodborne outbreak investigations, this information is often shown by the week people became ill. The horizontal axis (x-axis) is the week when a person became ill, also called the week of illness onset. The vertical axis (y-axis) is the number of persons with illness onset each week. During ongoing outbreak investigations, the epi curve is updated as new data become available. There are several important issues in understanding and interpreting epi curves during ongoing outbreak investigations.
- There is an inherent delay between the date that an illness starts and the date that the case is reported to public health authorities. This delay typically takes 2-3 weeks for Salmonella infections. Thus, for example, someone who got sick with a Salmonella infection last week is very unlikely to have their infection reported to public health authorities by now, and someone who got sick 3 weeks ago may just be reported now.
- Some background cases of illness are likely to occur that would have happened even without an outbreak. This makes it difficult to say exactly which case is the first in an outbreak. Epidemiologists typically focus on the first recognized cluster (group) of illnesses rather than the very first case. Due to the inherent reporting delay described above, the cluster is sometimes not detected until several weeks after the persons became ill.
- For some cases, the date of illness onset is not known because it takes time before someone from the health department can do an interview to ask for this information. Sometimes, this interview never occurs. If investigators know the date that a specimen from an ill person arrived in the laboratory for testing, they may estimate the date of illness onset as 3 days before the specimen submission date.
- It can be difficult to determine when cases start to decline because of the reporting delay. This information can become clearer as time passes.
波士顿大学一段关于epidemic curve的内有说根据epidemic curve可以判断是那一种outbreak：Point source outbreaks (epidemics) ，Continuous common source epidemics or Propagated (or progressive source) epidemic.
