Each extract is cumulative and contains data submitted for the financial year so far, that is to say, month 1 will only contain the data submitted with an activity date in April, but month 6 will contain data submitted with an activity date from April to September. One of the reasons for this is that additional data may be needed to update patient records from earlier in the year. For example, an episode may potentially run for several months or an amendment may need to be made as clinical coding takes place on discharge.
HES data is a snapshot of data taken on the Reconciliation and Post-Reconciliation dates for each month. The SUS database is continually updated so the totals in SUS for a month will only match HES if they are queried on the day the snapshot is taken.