In the run-up to the British census in 2021, the response options to questions were optimized using empirical methods. For the question concerning one's own national identity, this was done via a card sorting pretest. The resulting order was British, English, Welsh, Scottish, Northern Irish, and Other. Compared to the 2011 census, the first two options swapped places. The results of the survey then showed that the values for British had risen sharply, while those for English had fallen. This resulted in intensive commentary both in the scientific community and the press. The 2021 data are not comparable to the 2011 data due to the sequence sorting effect which led to a discontinuity in the time series with all its troubling implications.

 

 

In principle, it makes sense to avoid structural breaks in panel data as far as possible. Therefore, it is advisable to keep both the questions and the order of answers constant for each wave of a survey. For interoperability with third-party data sets, detailed documentation of the procedure or the use of standardized variables is also advantageous. The fact that the U.K. Office of National Statistics both documented the pretest procedure and published warnings regarding the interpretability of the data along with it represents a best practice in dealing with the problem of discontinuity in the time series.