Standardization of Rates Prof. Ashry Gad Mohamed Prof. of Epidemiology College of Medicine, K SU

Objectives: To understand the difference between crude and standardized rates. To calculate the direct standardized mortality or morbidity rates. To calculate the indirect standardized mortality or morbidity rates To understand the concept of standard population

Why we standardize rates?  Every large population is heterogeneous. Socio-demographic, genetics, occupation, diet & every health related aspect.  Then, any population is a composite of diverse subgroups.  So, We are always mindful of population diversity.

Crude Rate An overall measure that does not take explicit account of the composition of the population. It is an average of the values of individual subgroups weighted by their relative sizes (Democracy). The crude rates may be equal, but the strata specific rates may differ greatly.

AgeNo.DDRNo.DDR < Total If populations differ in composition then at least some of what we observe may be attributable the these differences

Adjustment & standardization Procedures for facilitating the comparison of summary measures across groups. Adjustment attempts to remove the effects of such extraneous factors that prevent a FAIR comparison. Standardization is one of the adjustment procedures. (restriction, matching, stratification & multivariate analysis)

Population and Deaths by Age in 1970 for White Females in Miami, Alaska and the U.S. AgeMiamiAlaskaU.S. PopDeathRatePopDeathRatePopDeathRate < Total CDR

Is life in Alaska more conductive to longevity than life in Miami? Crude Death Rate in Miami More than Alaska, but the strata specific death rates are nearly similar. Greater proportion of women in older age groups where mortality is high live in Miami.

Direct method of standardization Directly standardized rate = ∑ ( Stratum-specific rates x standard weights) ∑ (standard weights) = (r1N1 + r2N2 + r3N rnNn) (N1 +N2 +N Nn) = ∑ (rk x Nk ) ∑ (Nk)

= ∑ (rk x Nk ) = Nk ∑ (rk x ) ∑ (Nk) N rk Rate in k stratum of the study population. Nk = Number of persons in k stratum of the standard population. N = Total number in the standard population.

= Nk ∑ (rk x ) N Miami = 1.19 x X x x x = 6.92/thousand Alaska = 1.59 x X x x x = 6.71/thousand

Again, Summary indices from two or more populations are more easily compared than multiple strata specific rates. Small numbers in some strata may lead to unstable specific rates.

Standardized ratios & differences Rates that have been standardized by direct method using the same standard population may be compared in relative or absolute terms. Standardized Rate Ratio (SRR) = Directly standardized rate for Miami 6.92 = Directly standardized rate for Alaska 6.71 = 1.03

Indirect standardization Used when stratum numbers are small e.g. Small city. It used the stratum specific rates of the standard population of sufficient size & relevance. Direct Indirect Rate from study pop. Stratum from Study pop. Stratum from standard pop. Rate from standard pop.

Comparison of indirectly standardized rates can be problematic. To carry out indirect standardization we have to calculate the  Standardized Mortality Ratio (SMR) in case of death  Standardized Morbidity Ratio in case of disease

SMR = Observed deaths Expected deaths ED = ∑ (stratum specific rates from x Stratum sizes from) the standard pop. The study pop. = ∑ (Rk x nk ) Observed number of deaths = ∑ dk SMR = Observed deaths = ∑ dk Expected deaths ∑(Rk x nk) -dk = number of deaths in k stratum of the study population(observed deaths) -nk = size of the k stratum of the study population. -Rk = death rate in the k stratum of the standard population.

Death rates by age in two occupations and a standard population AgeOccupation AOccupation BStandard population No.DeathRateNo.DeathRateNo.DeathRate Total SMR = (0.001)(1000) +(0.003)(5000) (0.001)(5000) +(0.003)(1000)

Indirectly Standardized Rate = Crude Death rate SMR x in the standard population ISR for pop. A = 1.38 x = ISR for pop. B = 1.75 x = Directly SR for pop A=(0.002 x30000)+(0.004 x 40000) / = Directly SR for pop B =(0.002 x30000) + (0.004 x 40000) /70000 =

Miami Indirect SR = 5022 x 8.13 = 6.84 (1.34x )+(0.58x80259) (49.51x92168) Alaska Indirect SR = 285 x 8.13 = 7.32 (1.34 x 37164) +(0.58x 20036) (49.51x2077) AgeMiamiAlaskaU.S. PopDeathRatePopDeathRatePopDeathRate < Total CDR

Crude and Age Standardized 1970 Death Rates per 1000 for White Females in Alaska, Miami and the U.S. Alaska Miami U.S. Crude Direct Indirect Which Rates are closer?

Choice for standard population Standardized measures describe a hypothetical state of affairs, which is a function of the standard population chosen. The total country population from previous census is commonly used. The choice of standard should reflect the study goals.

A clean industry may be a good standard for an industrial population exposed to suspected occupational health hazards. If more standard is available choose the greater relevance and broader comparatibility.

References Victor J, Schoenbach Rothman, Kenneth. Chapter 5 In: Modern Epidemiology, pages Lilienfeld & lilienfeld. Measures of Mortality In: Foundations of epidemiology, pages Mausner & Kramer : Epidemiology: an introductory text, pages

Thank you