7
Rations and percentages
Having decided on a topic for a
measure, for example surgical site
infections, we now need to decide
how it should be expressed. Do we
want to express it as a percentage of
patients seen, the rate per 1000
patients or simply as a count (the
number of infections)? What follows
are some guidelines to help you
decide which option to use.
Use Counts when the target
population (for example number of
patients on a ward) does not change
much. It has the advantage of
simplicity but it can be difficult to
compare with others or even with
yourself over time. So, expressing our
measure as the number of infections
per month is fine as long as the
patient population we are treating
remains reasonably constant over time.
Use Ratios or rates when you want
to relate the infections to some other
factor such as patients or bed days.
If your target population numbers are
quite variable a simple count is not
sufficient without the context. In this
case the measure would be infections
per 100 patients or infections per
1000 bed days. Now a ratio is simply
one number divided by another
(infections divided by patients) and
statisticians use specific words to
describe the two numbers that
comprise a ratio. They would call the
infections number the ‘numerator’
and the patients number the
‘denominator’.
Use Percentages when you want to
make your focus more specific.
For example, if you want to learn
about patient falls in your
organisation is your focus on the
occurrence of falls or the result of
falls in terms of patient harm? If your
focus is falls then you would measure
this as a rate or ratio. If your focus is
on what has happened to the patient
you might select a measure as the %
of patients who were harmed by their
fall. In our infection example, the
measure would be percentage of
patients who had a surgical site
infection that met your pre determined
criteria for infection. In both examples
you would probably be gathering the
same information - just expressing it a
different way. Notice that we have
moved away from counting infections
now to counting patients who had an
infection to allow us to frame the
measure as a percentage - if we were
counting the former we could not
express this as a percentage because
some patients may already have more
than one and statistically that means
it would be possible to end up with a
number that is greater than 100%!
Use ‘time between’ or ‘cases
between’ when you are tracking a
‘rare’ event, say one that occurs less
than once a week on average.
If surgical infections occur this
infrequently then measures expressed
as rates or percentages become less
useful. A count of monthy infections
might look something like:
2,3,3,3,2,3,4,3,3,2,2,4. A change of
1 infection is quite a percentage shift
and therefore our run chart would
vary wildly but based only on 1 more
or less infection. Clearly this is not
very helpful. In this case express the
measure as the number of cases since
the last infection. We might now get
values such as 75, 57, 82, 34 cases
between infections. When charted
this gives us something more useful
to look at and it is not affected by the
‘small number’ problem that can
impair rates and percentages.
The How-to guide for measurement for improvement