Different Speed of Reporting Process Leads to Different Incidences
Consider two fictitious health departments A and B. Health department A reports all cases to the RKI by the following day, health department B only manages to report half of the cases on the following day and needs another day for the other half. For simplicity, both counties have 100,000 inhabitants. In addition, they also each recorded a constant 10 cases in the days from May 1 to May 7. What incidences will the RKI report on May 8?
At Health Office A, it knew all 70 cases for the relevant period, and will announce the incidence 70. For Health Office B, the RKI knew only 65 of the 70 cases (because only half of the May 7 cases were actually reported by May 8). Thus, the incidence reported by the RKI is 65 for county B, a good 7% less, than in county A, even though the situation is the same. Unfortunately, this effect does not simply average away.
Due to the delayed reporting of a portion of the cases, this portion of the cases will only ever be included in six incidence calculations: The five cases with GA reporting dates of May 7, which do not become known to the RKI until May 9, will only factor into the incidence calculations for May 9-14. Accordingly, these cases will receive less weight. Health department B will therefore always push a few cases ahead of itself and, in comparable situations, will be certified by the RKI as having a lower incidence than health department A. Since the incidences are also not subsequently adjusted by subsequent notifications, in practice this can lead to county B being able to open earlier than county A.
How strong can this effect be? Let us take a closer look at the difference between frozen and corrected incidences.