Indirect Standardization and Surveillance

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Indirect StandardizationandSurveillance

• PH2610
• Introduction to Epidemiology
• Fall 2002

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Cautions

• Although adjusted rates can be very useful in
  making comparisons
• the first step should always be to carefully
  examine the strata-specific rates for anything
  interesting

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Standardized ratesIndirect method

• When strata-specific rates are unknown or
  unreliable (because of small samples size)
• Apply rates from a standard population
• to each age stratum in the study population
• to obtain expected number
• This adjusted rate interpreted as
• the rate that would have been experienced by
  the study population if their rates had been
  similar to the reference population

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Indirect standardization

• SMR Standardized mortality ratio
• Observed number of deaths per time
• Expected number of deaths per time
• The basic question are the observed deaths
  greater than expected

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SMR

• SMR 0
• indicates observed is not unusual
• SMR 1.0
• indicates morbidity (or mortality) exceeds
  expected
• SMR 2.0 indicates two-fold increase
• SMR
• indicates morbidity (or mortality) is less than
  expected

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• Example
• DWIs among students attending the University of
  Margaritaville
• student population 1000
• Observed DWIs 200
• DWIs among college students nation-wide reported
  to be 18
• Expected 18 1000 180
• SMR 200 / 180 1.11
• Conclude that Margaritaville students have an 11
  higher rate of DWIs compared to US

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Standardized Mortality Ratio

• What information is needed to calculate
  Standardized Mortality Ratio?
• SRM Sum of observed deaths
• sum of expected deaths

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Direct / Indirect

• Both methods consist of taking a weighted average
  of category-specific rates
• The difference lies in the source of the weights
  and rates

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Proportional Mortality Ratio

• With only the numbers and causes of death among
  the exposed group but not the structure of the
  population from which they arose
• PMR the proportion of deaths from a specified
  cause relative to all deaths among the cohort
  compared with the corresponding proportion in the
  non-exposed group or general population

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Proportional Mortality Ratio

• Number of deaths due to a specific disease
  proportionate to all deaths in a population
• Not a measure of the risk of dying from a
  particular disease
• Relative importance of a particular disease
• Useful for health care planning and indicating
  areas for further study

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PMR
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Years of potential life lost

• A measure that recognizes that death occurring at
  a young age represents a greater loss (in terms
  of future productive years) that death occurring
  at an older age

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Surveillance

• The government is very keen on amassing
  statistics. They collect these, add them, raise
  them to the nth power, take the cube root and
  prepare wonderful diagrams. But you must never
  forget that ever one of these figures comes in
  the first instance from the village watchmen, who
  just puts down what he damn well pleases.
• Sir Josiah Stamp (1880-1941)
• British Economist

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Surveillance (CDC definition)

• The ongoing systematic collection, analysis, and
  interpretation of health data essential to the
  planning, implementation, and evaluation of
  public health practice

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Purpose of Surveillance

• Evaluate prevention and control programs
• Assess burden of disease
• Detect perturbations in rates in time/space
• Identify risk factors for disease occurrence
• Stimulate research
• Set priorities

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Surveillance

• Active
• cases are sought - field visits to find cases
• congenital syphilis - sentinel health event
• Passive
• reporting is required
• other STDs
• Sentinel health event

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Active Surveillance

• Regular periodic collection of case reports from
  health care providers or facilities
• Data more accurate than other types of
  surveillance
• Expensive
• From Foundations of Epidemiology, Lilienfeld
  Stolley

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Active Surveillance

• Connecticut Tumor Registry actively reviews
  hospital records throughout the state for
  incident cases of cancer and benign tumors

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Passive Surveillance

• Reports of cases given by health care
  professionals at their discretion
• Inexpensive
• Data likely to underestimate the presence of
  disease in the population

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Passive Surveillance

• Reporting of toxic shock syndrome in Wisconsin in
  the early 1980s
• Reporting of acanthosis nigritans in the Lower
  Rio Grande Valley

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Active vs Passive

• Active requires more effort by the data
  collection center
• Active is more expensive
• Active results in more complete and accurate data
• Comparison of two types active results in more
  than twice as many cases identified

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Sentinel Surveillance

• Relies on reports of cases of disease whose
  occurrence suggest that the quality of preventive
  or therapeutic medical care needs to be improved
• Polio
• Malignant mesothelioma
• Congenital syphilis -

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Sentinel health event

• Case report indicates a failure of the health
  care system or indicates that special problems
  are emerging
• Very inexpensive
• Applicable only for select diseases

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Adjustment of rates (review)Cohort
EffectSurveillance
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Adjustment of Rates