Asst Prof Dr. Ahmed Sameer Al-Nuaimi - MBChB, MSc epi, PhD Dep .of Community Medicine-Baghdad College of medicine Email: info@topmedresearch.com Adjustment of Rates

Learning Objectives Define adjusted rate. Value the role of adjusted rates in comparison. Discuss the advantages and disadvantages of adjusted rate. Compare adjusted rate to crude and specific rates. List two methods of rate adjustment. Master the calculation of direct and indirect method of rate adjustment. Define the standard population. Interpret the value of SMR.

Definition Adjusted rates: are summary rates that have undergone statistical transformation to permit fair comparison between groups differing in some characteristic that may affect risk of disease. They are useful in comparison between two (or more) populations or the same population over time. Two types of commonly used adjusted rates are: Adjusted (standardized) death rate: takes into account the population structure (age, or gender) and its effect on overall death rate. Standardized fertility rate (age adjusted general fertility rate): The age structure of the female population affects the general fertility rate, therefore it is better to control for the confounding effect of age before making comparisons between populations.

Advantages / Disadvantages adjusted rates provide the advantage of crude rates in being summary measures (single figure), overcoming at the same time the crude rate inherent property of unfairness in comparison. It also provide the advantage of specific rates in permitting fair comparisons between 2 groups of homogenous structure, overcoming at the same time the difficulty in comparing two populations by using multiple specific rates for comparison. Disadvantages: adjusted rates are not real figures. In this context they are absolutely useless when used outside the comparison for which they were calculated.

Methods of rate adjustment There are essentially two methods for adjusting rates, the direct method and the indirect one. The age is the most common factor adjusted for, and it will be used in the coming examples. Other confounding factors may be used in the same context when needed. Direct method of rate adjustment: The actual age specific rates observed in two or more studied populations are applied to an arbitrarily chosen population of known age structure referred to as "standard population".

Choices for a standard population There are 3 options for selecting a standard population: Any of the two compared populations. The sum of the two compared populations. The US population for the year 2000 (the last decennial census). To simplify the calculations the total of the population is forced to a sum of one million and the resulting standard population is called the standard million. To calculate this standard million the relative frequency of each age group in the US population is multiplied by one million.

Calculation of million standard population

Hypothetical Example The population size by age groups and observed number of deaths in each age group were provided for a certain population at year 1990 and repeated at year 2000. Calculate the crude and age adjusted death rates and comment on comparing the death between the 2 points of time.

Answer First calculate the standard population

Answer Crude Death rate = 862 / 900,000 = 95.8 per 100,000 Age adjusted death rate at 1990 = 2,238 /1,800,000 = 124.3 per 100,000

Answer Crude Death rate = 1,130 / 900,000= 125.6 per 100,000 Age adjusted death rate = 1,830 /1,800,000 = 101.7 per 100,000

Interpretation / Conclusion The crude death rate in year 2000 is higher than that in the previous decade (year 1990), i.e. in general the risk of death has increased over time, but the question is how to interpret this finding. “Is there a deterioration in health status for the population over time which explains this increase in crude death rate?” A comparison of age adjusted death rates reflects a reduction in death rate over time (from 124.3 to 101.7 per 100,000 population), i.e. improvement in health status of the population. The change in population structure overtime towards an older population explain the increase in crude death rates over time.

Exercise: (home work) The population size by age groups and observed number of deaths in each age group are provided for the population of Georgia at year 1999 and 2000. Calculate the crude and age adjusted death rates and comment on comparing the death between the 2 points of time.

Indirect method of rate adjustment (SMR) Here one or more populations are compared to a reference population. The age specific rates of the studied populations are either unknown or are excessively variable (unstable) because of small number. The more stable rates of the reference population are applied to the studied population to calculate the expected number of deaths. The standardized mortality ratio (SMR) is a comparison of the actually observed to expected number of deaths. SMR = Observed deaths Expected deaths

Interpretation of SMR An SMR equal to 1 indicates that the risk of death in the studied population is equal to the reference population. This is the null value for SMR, which is similar to that of RR and OR. An SMR in excess of 1 indicates a higher risk of death in the studied population An SMR <1 indicates a lower risk of death in the studied population.

Exercise The table below contains data for deaths from TB in White Miners of US 1950. The age specific death rates from TB of males in the general US population are also provided. Calculate the SMR for White Miners compared to general male population.

Answer-calculations of SMR Interpretation: Being a miner increase the risk of death from TB by 2.4 times compared to general population of white males