Methodology
Use of Funnel Plots for Reporting Performance
Indicator Results
What is a funnel plot?
A funnel plot is a way of visualizing data for an indicator. It consists of 2 main parts: a funnel and a scatter plot.
In essence, the funnel plot consists of the funnel being superimposed on the scatter plot.
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The scatter plot represents indicator results for health organizations. Each dot represents a hospital’s or
a health region’s indicator result, relative to its denominator size or service volume.
The funnel is a mathematical creation (also known as statistical margins of error) that illustrates
boundaries signifying either 2 or 3 standard deviations above and below a calculated average, typically
the national average. The outside funnel represents 3 standard deviations from the average and is
equal to a 99.8% confidence limit. The inside funnel represents 2 standard deviations from the average
and is equal to a 95% confidence limit. The denominator size (either cases or service volumes) upon
which an indicator result is calculated is taken into account when calculating the funnels; therefore, the
funnel boundaries are wider for smaller organizations (i.e., lower service volumes) and narrower for
larger organizations (i.e., larger service volumes).
The vertical axis (y-axis) represents the indicator value. Dots higher up the axis show hospitals or health
regions with a higher indicator value, regardless of the directionality of an indicator. For some indicators, a
higher value means good performance whereas for other indicators (e.g., mortality), a higher value is not
desirable.
The horizontal axis (x-axis) shows the number of persons served by the hospital (patient volume), the number
of people residing in the health region (census population) or the statistically expected number of people that
may experience the event of interest (such as hospital readmissions or deaths). Dots that appear closer to the
right of the graph show hospitals that serve more people (higher volume) or health regions that have more
residents (larger population).
The solid horizontal line in the middle of the funnel represents the national value for the indicator.
Note: Funnel plots can be provided for only those indicators that have well-developed measures of variation.
This enables a clear presentation of the indicator results in relation to an appropriate size measure for a
hospital or health region, as well as the creation of the funnel around the na tional average. For these reasons,
funnel plots were not created for all indicators.
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Methodology
How is a funnel plot interpreted?
Funnel plots discourage inappropriate ranking because they take into consideration the service volume or
denominator upon which an indicator result is calculated. Typically, lower numerator/denominator indicators
create results that are subject to more variation than those created by larger numerator/denominator indicators.
However, they provide a strong visual indication of indicator results that are statistically higher or lower than
the national average despite service volumes.
Indicator values outside of the funnel indicate out-of-the-ordinary results after the size of a hospital or a health
region has been taken into account. Health regions or hospitals with indicator values inside the funnel have
results within an expected variation, given their size. To interpret indicator results as they relate to the funnel,
we must consider the directionality of the indicator.
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When lower values for the indicator are desirable (e.g., mortality rates), hospitals or health regions with
rates lying above the upper 95% control limits (funnel) can be considered to be in the “warning” zone,
while those above the upper 99.8% control limits (funnel) are in the “concern” zone. Such results may
warrant further investigation.
When higher values for the indicator are desirable (e.g., rates of hip fracture repair within 48 hours),
hospitals or health regions with rates lying below the lower 95% control limits (funnel) are in the
“warning” zone, while those below the lower 99.8% control limits (funnel) are in the “concern” zone.
Bibliography
Spiegelhalter DJ. Funnel plots for comparing institutional performance. Statistics in Medicine 30;24(8):11851202.
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