Topic Introduction
- morbidity statistics
- relative incidence of sickness and injury occurring among given group of people.
- morbidity – percentage of people in a population who get sick from a particular disease.
Definitions
- risk à proportion
- rate à measure over time
o special type of ratio
o numerator included in denominator
o example
§ 5523 deaths , of which 221 due to coronary heart disease
§ proportion of total deaths due to coronary heart disease is 221 / 5523 or 4%
- population – any defined group of people
- morbidity – disease, illness, injury - non- death health outcome
- mortality – death
- risk % - number of events / number in pop
- rate – number of events – time measure
o another type of ratio
- ratio – number of cases / population
- mid year population – time measure
- person – years observation – time measure.
Some disadvantages of morbidity data
- issues
o not routinely collected (except for some infectious diseases)
o expensive to collect
o can go out of date as knowledge of disease increases rapidly.
o changes to criteria for including / excluding a case.
- surveillance
o continuous / systematic process
o collection / analysis / interpretation / dissemination
o descriptive information for monitoring health problem.
- surveillance systems
o passive
§ cases of reportable diseases reported as they are observed
o active
§ periodic (weekly) calls / visists to institutions etc to obtain required data
o sentinel
§ relies on reports of disease (which is a preventable event) that can be used to identify problem in system of prevention / detection or treatment.
[google trends for flu etc http://www.google.org/flutrends/
Morbidity Measures
- incidence
o number of new cases of disease within defined population during specified time period.
o incidence rate
§ number of newly reported cases of given disease
§ in a calendar year
§ divided by mid year population
§ quotient multiplied by convenient factor
o types
§ cumulative incidence
§ incidence density
- cumulative incidence
o total number of people who become diseased during specified period of time
o provides an estimate of probability / risk that individual will develop disease during specified period of time
o assumes entire population at rik has been followed up for entire specified period of time
- incidence density
o accounts for varying time periods of follow up, and thus maximizes whatever data is available
o = number of new cases during given time period / total person time of observation (disease free)
o denominator à sum of each individuals time at risk or sum of that time each person remained under observation and free from disease.
- prevalence
o number of affected persons in population at specific time / number of persons in population at that time
o types
§ point prevalence
· number of persons (old & new) in defined population who have specified outcome at single point in time
§ period prevalence
· combination of point prevalence and incidence
Relationship between Incidence and Prevalence
- incidence measures appearance of disease à new
- prevalence measures existence of disease à all
o depends on 2 factors : incidence and duration of disease
- assuming steady state situation
o prevalence = incidence * duration
- Four common measures of morbidity:
| Type | Disease | Measure | Definition |
| Risk | Existing | Point prevalnce | persons with disease at given time / total pop at that time |
| Risk | New | Cumulative incidence | number of new cases in time period / number of persons at risk at beginning of period |
| Risk | Existing and new | Period prevalence | total number of cases in time period / mid period pop |
| Rate | New | Incidence rate (or density) | number of new cases in time period / person years of observation |
Measures of Fertility
- fertility – occurrence of births , especially live births
- vs fecundity à capacity to bear children à upper bound for fertility
- Crude birth rate
o number of births in defined population during specified period / mid year pop
o quotient multiplied by 1000
- General fertility rate
o number of live births in calendar year / number of women ages 15 – 44 at mid year
o give heterogenous figure à overlooks age condition of female population in child bearing age
- Age Specific Birth rate (ASBR) or Age Specific Fertility rate (ASFR)
o number of resident live births to women in specific age group for specified geographic area / total pop of women in same age group for same time period / geographic region.
- Total Fertility rate (TFR)
o more practical measure of pop growth
o age specific fertility rates combined
o single rate à TFR
o TFR à age groups between 10 and 49
o TFR = (∑ ASBR) * 5
o rate estimates number of children a hypothetical cohort of 1,000 females in specified pop would bear if they all went thru their childbearing years experiencing the same age specific birth rates for a specified time period.
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