HIGHLIGHTS
- Introduction
- Approaches to Standardization
- Direct Standardization
- Indirect Standardization
- Abuse of Standardization
- Conclusion
INTRODUCTION
Rates according to Webster’s dictionary of English
language is the amount of something in relation to some other thing. It is a
fixed ratio between two things or quantities. Rate is the most important tool
in measuring disease or death or measuring morbidity or mortality Onwasigwe
(2006). Rate is used to measure events that are related to the population or
subgroups in which they occur. A rate is usually expressed as per standard
population size which could be 1000, 10,000 or even 100,000.
Standardization or adjustment of
rates is used to enable the valid comparison of groups that differ regarding an
important health determinant most commonly age. It is in fact a specific
application of the general methods to control confounding factors. Historically
the need for age standardization was recognized well before the general concept
of confounding can be applied to standardization.
APPROACHES TO STANDARDIZATION
There are two major approach
to standardization, they include the Direct standardization, and the indirect
standardization.
DIRECT STANDARDIZATION
This
is used when the study population is large enough that the age specific rates
within the population is stable. The variables of interest that is the
age-specific rates in two or more populations are applied to the chosen
population of age structure called the standard population. In as much as any
standard population may be selected, it is better to utilize one in which the
age distribution equals the those of the population under study.
Direct standardization is commonly
used in reports of vital statistics as in mortality or disease incidence trends
eg cancer incidence.
The standard to be the used should
be one that is relevant at that particular point in time and are being used. A
direct method of standardization can be shown in the following table.
Age group (year)
|
|
|
0-5
years
|
5-19
years
|
20
years
|
Total
|
|
No
1
|
Number
of deaths
|
84
|
75
|
16
|
175
|
|
|
Number
of population
|
3000
|
5000
|
12000
|
20,000
|
|
|
Mortality
per 1000
|
84
|
40
|
4
|
17.5
|
|
N0
2
|
Number
of deaths
|
180
|
168
|
16
|
364
|
|
|
Number
of population
|
5000
|
7000
|
8000
|
20,000
|
|
|
Mortality
per 100
|
96
|
50
|
4
|
36.4
|
In the above table, there were two
population groups, group I and group 2. It shows the number of deaths in the
two groups. In group I, the population is 20,000 individuals, of various age
groups. There were 175 deaths with mortality rate of 17.5.
In no 2, the population is also
20,000 with a total no of 364 deaths and mortality rate of 36-4.
The mortality rate in group 2 is
almost double of that of group I.
In all, the mortality rate varies
between the various age groups. It is seen to be highest in the younger age
groups. i.e 0-5years and lowest in the higher age group 20 years+.
Also the age distribution of the population are different. Group 2 has more
deaths than group I.
The more deaths in group 2 are as a
result of the difference in the age structure of the population involved. This
problems is solved by direct standardization where the mortality in each
population is adjusted to allow for the difference in the age structure.
The comparison of mortality or
mortality rates in different population
groups requires the calculation of standardized rate, sex as well can also be
standardized to give the standard rates in males and females separately or when
combined to give the sex/age standardized rates.
The standard approach to explaining
standardization involves the concept of expected and (Observed) courts in
direct standardization and one estimates the rate that would have been observed
if the study population had the same age structure as the reference group.
INDIRECT STANDARDIZATION
Indirect standardization has played
a major role in studies of occupational diseases of studies of place and time
–limited environmental catastrophes. Indirect standardization was introduced as
a tool before direct standardization.
Two populations are compared in
indirect method. Due to small members one of the populations are compared in
indirect method. Due to small members one of the populations has a highly
variable or unknown age specific rates.
To get the expected rates of the
smaller study group, the more stable rates of the larger population is applied.
The conventional method of indirect standardization is to calculate the standardized
mortality ratio (SMR). The standard mortality ratio compares the mortality or
morbidity which occurred in a designated group with that of a standard
population.
The number of deaths or diseased
persons expected in a particular age/sex groups is obtained by the
multiplication of the age/sex specific death rate (mx) with the number of
persons of that particular sex and age in the population being investigated
(Px).
The expected number of deaths in the
entire population being investigated is obtained by adding the expected deaths
for each age/sex group. The observed number of deaths are then divided by the
expected deaths to give the standardized mortality ratio.
Standard
mortality Ration
= Observed deaths (d) x 100
Expected death (Px x Mx
1
|
Age
group
|
Death
rates per 100 in general population
|
Population
of study group (1000)(Px)
|
Expected
deaths Mx x Px
|
Observed
deaths
|
|
0-5
|
0.045
|
75
|
0.675
|
20
|
|
6-15
|
0.045
|
1.25
|
0.75
|
9
|
|
16=
|
0.03
|
3.0
|
0.60
|
1
|
|
Total
|
0.01
|
|
2025
|
30
|
Indirect
standardization uses the reference population to provide age-specific rates.
Within each age stratum, one multiplies the reference rate by the number of
people in the study population to people in the study population to determine
the number of cases that would have been expected if that were the rate in the
study group. Values greater than I indicate a higher mortality than expected,
standard mortality ratios can be compared for different outcomes within the
same study population.
The use of standard rates is
controversial. Any summary measure can hide patterns that might have important
public health implications in age standardization, one might fail to detect
age-specific difference in risk access time or place e.g people at younger ages
might have a higher risk in recent years compared to previous years, while
older people could have the opposite pattern. Despite these risk, standardized
rates have been found to provide useful summary measures, especially when
outcomes are rare and specific rates display wide random variability.
ABUSE OF STANDARDIZED RATE
One of the biggest potential abuses
of standardized rates is by health care planners who use the standardized rates
to estimate demand for services. This is an incorrect practice. The
standardized rates reflect the member of new cases that would arise. In a
hypothetical population, the actual number of cases expected is given by the
crude rate which should always be employed in health care planning analysis.
CONCLUSION
Adjusted
and Standardized rates are usually required for age because of its marked
effect on disease and death. Other variables that may be standardized include
sex, occupation, personal habits, ethnic groups etc.
Indirect standardization has also
played a major role in studies of occupational diseases. Indirect
standardization was a tool used before direct standardization. The standard
approach to explaining standardization involves the concept of expected and
observed counts.