Special events are events that are identifiable; they do not recur on a regular cycle and have at least the potential to have an impact on statistics.
It is not possible to give rigorous criteria, but ONS will determine whether to treat an event as 'special' with regard to:
whether it has a general effect across a number of outputs
whether it is restricted to one (or at most very few) periods
whether the effect is, or is likely to be, noticeable (specifically, any effects that are difficult to distinguish from the normal variation in a series will not be special)
the views of users
It is expected that special events will be uncommon.
ONS will routinely identify events that may affect the series it publishes, and provide commentary on them alongside published figures as appropriate.
In the case of a special event, ONS will continue to provide commentary to help users to understand the data.
In addition, there will be co-ordination across the affected outputs to gather and summarise the available information.
ONS will not attempt to quantify the impact on any directly affected statistical outputs based mainly on movements in the time series (because of the difficulty of distinguishing them from other factors).
ONS will consider whether any of the following procedures should be used to provide further supporting information:
in exceptional circumstances where the available data supports a direct estimate of sufficient quality, an effect of the impact of the special event may be made
collection of additional qualitative data to provide contextual information on the effect of an event
analysis of paradata (data about the data collection process) to provide contextual information on the effect of an event
'what if' analysis based on a reasonable assumption about how the special event influenced conditions; the assumptions will be clearly identified
Some time after a special event, when more data periods are available for the affected outputs, ONS may produce an analytical article interpreting how the relevant series moved in the affected period.
When a special event potentially affects data that are normally nowcast (a forecast of the latest period needed because the data are not yet available), any variation in the assumptions used for nowcasting will be explained alongside the published statistics.
ONS will maintain a list of candidate events in the special events calendar on the ONS website, with links to publications that have been affected.
An article 'Developing a policy on special events' explaining this policy in more detail and highlighting some of the reasoning and implications is available on the website.