Case reports indicate that exposure to manganese among miners may lead to central nervous system damage, particularly a Parkinson-like syndrome. This syndrome, thought to be associated with increased mortality, is characterized by cogwheel rigidity, neuropsychiatric and cognitive manifestations, balance and speech changes, dystonia more pronounced than that seen with Parkinson disease, and resting tremors less pronounced than those seen with Parkinson disease. A group of investigators are further investigating the association between exposure to manganese and increased mortality among miners. As part of their statistical analysis, they calculate a standardized mortality ratio of 1.75 among manganese miners compared to the general population. Which of the following statements is most applicable to this study result?
A)Manganese miners are 75% more likely to develop Parkinson-like syndrome.
B)The crude rate of death is 75 times higher among miners compared to the general population.
C)The observed number of deaths among miners is 75% higher than expected.
D)The standardized mortality ratio is an unadjusted measure of overall mortality.
E)The standardized mortality ratio is not generally applicable to occupational studies.

Question

Case reports indicate that exposure to manganese among miners may lead to central nervous system damage, particularly a Parkinson-like syndrome.  This syndrome, thought to be associated with increased mortality, is characterized by cogwheel rigidity, neuropsychiatric and cognitive manifestations, balance and speech changes, dystonia more pronounced than that seen with Parkinson disease, and resting tremors less pronounced than those seen with Parkinson disease.  A group of investigators are further investigating the association between exposure to manganese and increased mortality among miners.  As part of their statistical analysis, they calculate a standardized mortality ratio of 1.75 among manganese miners compared to the general population.  Which of the following statements is most applicable to this study result?
A)Manganese miners are 75% more likely to develop Parkinson-like syndrome.
B)The crude rate of death is 75 times higher among miners compared to the general population.
C)The observed number of deaths among miners is 75% higher than expected.
D)The standardized mortality ratio is an unadjusted measure of overall mortality.
E)The standardized mortality ratio is not generally applicable to occupational studies.

Quizplus · Accepted Answer

Explanation: The standardized mortality ratio (SMR) is calculated using the following formula: SMR = observed number of deaths/expected number of deaths By definition, the expected number of deaths is calculated based on age-specific rates of death in a standard reference population (eg, total US population). The values obtained would represent the expected number of deaths among the population of interest (eg, miners) if the individuals in this population had the same death rates as the standard population. To obtain the SMR, the observed number of deaths in the population of interest is then divided by the expected number of deaths based on the standard population. An SMR of 1.75 indicates that the observed number of deaths among the miners in this study was 1.75 times (or 75%) higher than would be expected if these miners had the same death rate as the standard population. SMR represents an adjusted measure of the overall mortality (Choice D). It is typically used in occupational epidemiology (Choice E). Mortality is often adjusted for age (and less commonly for gender, race, or other factors). (Choice A) The SMR is specifically related to mortality rates. SMR is different from relative risk, which usually compares the risk of a certain outcome (eg, Parkinson-like syndrome) in exposed individuals (eg, miners with exposure to manganese) to that in nonexposed individuals (eg, miners without exposure to manganese). (Choice B) The crude (unadjusted) mortality rate of miners may be higher than that of the standard population. However, it may also be lower (eg, if there is a high proportion of younger people among miners). Educational objective: The standardized mortality ratio represents an adjusted measure of overall mortality and is calculated by dividing the observed number of deaths in the population of interest (eg, miners) by the expected number derived from the reference population ("standard").

Case Reports Indicate That Exposure to Manganese Among Miners May