joint strategic needs assessment foundation profile - JSNA

Interative Hull Atlas: www.hullpublichealth.org/Pages/hull_atlas.htm More information: www.jsnaonline.org and www.hullpublichealth.org
an underlying increase. Whereas if the 95% CI ranged from 132 to 148 then we would
be more confident in stating there was a higher mortality rate locally as the lower
estimate of the SMR is still more than 30% higher than the mortality rate nationally.
When dealing with small numbers of events (see section 12.7 on page 776), it is very
important to consider the implications of this and present and assess the width of CIs to
determine how much confidence there is in the estimate presented.
12.6 Moving Average
A moving average is an average or mean value over a number of years, with the years
„moving‟ over time. A three-year moving average is very common (where the value
presented is the mean value over three years). A moving average is very useful in
summarising data where the number of events are small on an annual basis and there
are potentially large fluctuations in the rate of events (see section 12.7 on page 776).
Calculating the moving average smoothes out the fluctuations and makes interpretation
easier so that the overall trend can be better seen. For instance, if we had the mortality
rate for each of the eleven years from 1994 to 2004 and the annual number of deaths is
small resulting in large fluctuations over the years, then it would be better to calculate
and present nine three-year moving averages rather than the mortality rates for the
individual years. The first three-year moving average mortality rate would be the mean
or average mortality rate for the years 1994, 1995 and 1996. The second three-year
moving average value would be the mean mortality rate for 1995, 1996 and 1997, and
so on, with the final three-year moving average being the mean mortality rate for the
years 2002, 2003 and 2004.
Nevertheless, this does not guarantee a smooth „trend‟ line. For women in Hull, there
was a particularly high number of deaths in 1998 and a particularly low number in 2001.
As these were three years apart, this dramatically influenced the life expectancy trend in
Hull for women over time (there was also a slightly increase in 2004). Life expectancy
increased dramatically between 1998-2000 and 1999-2001 as the high mortality rate
year (1998) was removed and the low mortality rate (2001) was added.
12.7 Small Number of Events
When comparing the mortality rates for specific relatively rare cancers, for example, skin
cancer, differences in the mortality rates can occur which appear to be large, but are
actually only based on a very small number of deaths.
For example, if there are two geographical areas both with populations of 1,200 people
and one death in area A giving a standardised mortality rate (see section 12.3 on page
773) of 83 per 100,000 persons and two deaths in the area B giving a mortality rate of
167 per 100,000 persons, then the number of deaths and the mortality rate is twice as
high in area B compared to area A. However, if the mortality rates only are compared
the difference looks dramatic, but it is misleading as the differences in the total number
Joint Strategic Needs Assessment Foundation Profile – Hull Health Profile: Release 3. March 2011. 776