About Hospital Episode Statistics Weighted Activity Units (HES WAU) (In-Year Cost Per WAU)

This article explains:

·      the HES weighted activity unit (HES WAU) and how it can be used to see productivity progress over time, including progress within a financial year (‘in-year’)

·      the difference between the HES weighted activity unit (HES WAU) and the standard weighted activity unit (WAU)

·      how you can use these metrics to understand and improve your trust’s efficiency

The cost per HES weighted activity unit (HES WAU) is a measure of absolute efficiency. It’s available monthly from the Model Hospital and allows you to monitor your trust’s progress over time.


The cost per weighted activity unit (WAU) on the other hand is a measure of relative efficiency. It’s published on the Model Hospital annually and is used to quantify how much it costs to deliver a standardised unit of activity, compared to other trusts.

These two measures can be used to assess different aspects of your trust’s efficiency. But you need to be careful about using them together because they are calculated using different data sources and they sometimes give different signals because they seek to answer different questions:


·      HES WAU: what is a trust’s progress over time (‘absolute’ efficiency)

·      WAU: what is its performance relative to others (‘relative’ efficiency)

Type of
Absolute efficiency – best for
  tracking the progress of your trust over time
Relative efficiency – best for
  comparing your trust to other trusts
HES and monthly finance returns Reference Costs and annual
  finance returns
Frequency Monthly Annual
Time lag
  between data submission and publication on Model Hospital (approximate)
3 months 6 months +
Constraints · Less activity is covered – the data collected monthly is less comprehensive
than the annual data.
· You can’t compare progress between
trusts, because they report slightly differently.
· You can’t compare progress over time, because the definition of activity
· Reference costs are not collected
frequently enough to be useful.
· There’s a long time lag before

The cost per HES WAU is used for comparison of efficiency over time. It’s based on HES (Hospital Episode Statistics) data and monthly finance returns, which means it can be refreshed more regularly – up to monthly. It gives a more accurate representation of the absolute efficiency improvement between years by applying a single categorisation to patient-level data over time.


The cost per WAU is based on the annual Reference Cost collection. It allows for a simpler comparison of relative efficiency between trusts because there is a common reporting standard and less chance of differences arising from local accounting treatments.


However, because the cost per WAU uses the national average cost of each category, any year-on-year changes in average costs and in provider’s mix of activity mean you can’t compare the cost per WAU over time. It also has a long lag in reporting (the Model Hospital doesn’t get the data until about 6 months after the end of each financial year), which means it’s less useful for tracking progress.


Unlike the cost per WAU, the cost per HES WAU is not suitable for comparing efficiency between trusts. This is because it is constructed from HES data and monthly finance returns – each collected independently – and there is potential for mismatch in coverage between cost and activity for each trust.


As opposed to the yearly cost per WAU measure, cost per HES WAU is available monthly. While there is a delay between the end of the month and the availability of this information – up to 3 months – this is still a much better tool for monitoring progress.


The WAU also covers more types of acute activity than the HES WAU, including both ‘unbundled services’ (e.g. chemotherapy, critical care) and some community and mental health activity. This means that for any one trust, their quantity of HES WAU will be lower than their quantity of WAU

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