Protecting Confidentiality: A False Global Premise

1,912 staff tested (of whom 1,351 were teachers) – *fewer than 5 were Covid-19 positive
1,510 15-18-year olds tested – *fewer than 5 were Covid-19 positive

(*When a figure is less than 5, we do not publish the exact number to protect confidentiality)

It is annoying that the school attendance figures do now show the number of staff or pupils that are Covid-19 positive These are very small numbers, and taken from a large dataset, with no other characteristic - for instance a breakdown by age or gender or school. 

If the numbers were small, identification would be a risk factor. For example, if a breakdown gave schools as an added dimension, it would break down the figures to the point where identifications would be easier, although even there the people most likely to know from the statistics that someone is away because of Covid will probably know that fact anyway.

But with large numbers, there is no way someone can easily be identified. If I was told that 1,2 or 3 teachers had tested positive for Covid, out of all the teachers in Jersey, how could I identify them? And close colleagues will already have been notified as direct or indirect contacts, so they will know anyway. 

If - to take another example - the statistics gave a breakdown by school, and one teacher was positive at St Trinians, then students and parents alike could probably discover who that was, even if it didn't affect them at all. 

But as it currently stands, it is another example of a blanket policy which requires no statistical judgment and in fact is not made on any sound basis whatsoever.

The Office of National Statistics in the UK adopts a far more nuanced approach, considering a variety of factors. The Jersey figures are what they would term "single dimension" statistics, and given the numbers, they would almost certainly give an exact figure.

Statistics:

School attendance figures on 11 January 2021
Primary schools
The average figures for primary schools show that 93.6% of primary school students attended on Monday, 11 January 2021.
Authorised absence: 2%
Unauthorised absence: 1.9%
Covid-19% absence: 2.5%
Secondary Schools
The figures for secondary school show that 83% of secondary school students attended on 11 January 2021.
Authorised absence: 3%
Unauthorised absence: 4.1%
Covid-19% absence: 4.7%
PCR Covid-19 swabbing data
The figures for secondary school students and nursery, school and college staff for the voluntary PCR testing undertaken between 1 January – 10 January:

1,912 staff tested (of whom 1,351 were teachers) – *fewer than 5 were Covid-19 positive
1,510 15-18-year olds tested – *fewer than 5 were Covid-19 positive

(*When a figure is less than 5, we do not publish the exact number to protect confidentiality)

Compare with the UK office of national statistics:

https://www.ons.gov.uk/methodology/methodologytopicsandstatisticalconcepts/disclosurecontrol/policyonprotectingconfidentialityintablesofbirthanddeathstatistics

Example:

Deaths from cystic fibrosis, England, 1990 to 2013
Three dimensions (individual age by year by sex at national level).
Number of deaths from unusual cause of death.
Relatively sparse table.
Most deaths occur in late childhood or early adulthood.

There are many low counts where there is some possibility of identification but little or no risk of attribute disclosure. This table can be published.