Post on 27-Mar-2015
transcript
Rates and measures(of disease occurrence)
Chris Williams (adapted from EPIET slides)
Statistics track (STA)
• Rates, ratios, proportions
• Graphs and maps; tables
• Sensitivity and specificity
• Hypothesis testing; confidence intervals
• Sampling techniques and sample size
Descriptive studies - context in epidemiology
Epidemiology studies the occurrence of illness:
the frequency and distribution of diseases in the population and their determinants
frequency and distribution: who, when, where
determinants: why
1 person in Lazareto was stung by a jellyfish- How to express this?
1 person in Lazareto was stung by a jellyfish- How to express this?
Ouch!
Who is in the denominator ????In what time period did they occur???
number of cases“we have 5 cases of tetanus”
On its own very little informative!!
counts
Descriptive studies – measuring disease occurrence
Week 1 Week 2 Week 3
Jellyfish sting counts
Proportion Ratio Rate
What, who is in the denominator ????In what time period did they occur???
Measuring disease occurrence
• The division of 2 numbers• Numerator INCLUDED in the denominator• In general, quantities are of same nature• In general, ranges between 0 and 1 • Percentage = proportion x 100
Proportion
malespopulation
= 400 / 1000 = 40%
Proportion of rotten apples
=2/4 = 50%
Oranges to apples- a proportion?
• The division of two numbers
• Numerator NOT INCLUDED in the denominator
• Allows to compare quantities of different nature
Ratio
bedsdoctors 850 / 10 = = 85 / 1
malesfemales
= 5 / 2 = 2.5 /1
3 / 1 =participantsfacilitators
• The division of 2 numbers• TIME INCLUDED in the denominator• Speed of occurrence of an event over time
Rate
Births in 2007
Population in 2007= 2000 / 15 000 000 =
= 0.00013 = 1.3 per 10,000 inhabitants per year
• Rate may be expressed in any power of 10:
100, 1000, 10000, 100 000…
Measuring disease occurrence
Number of cases of disease
Population
–Number of cases of a disease in a given population at a specific time
–Proportion of the population that had the disease at a given time
–Probability of having the disease
prevalence
Number of NEW cases of disease during a period
Population at the beginning of the period
Measuring disease occurrence
–Number of new cases of a disease in a in a given population at a specific time
–Proportion of the population that acquire or develop a disease in a period of time
–Probability of developing a disease
incidence(cumulative incidence)
Incidence Rate
Number of NEW cases of disease
Total person- time of observation
• Proportion of the population that acquire or develop a diseases in a period of time
• Speed of developing a disease
Denominator:- is a measure of time - the sum of each individual’s time at risk and free from disease
Measuring disease occurrence
Cumulative Incidence = 3 cases / 6 persons = 50%
Incidence Rate = 3 cases / 22 person-years = 0.14
= 14 cases / 100 person-years
2003 2004 2005 2006 2007 2008 2009
l
x
xll x
l
l
l
Person 1 3
4
6
3
1
5
22 p.y
Person 2
Person 3
Person 4
Person 5
Person 5
Time-person
Week 1 Week 2 Week 3
How many facilitator weeks?
Attack Rate
• Cumulative incidence during an outbreak
Expressed for the entire epidemic period, from the
first to the last case
• Not really a rate but a proportion!
Measuring disease occurrence
Outbreak of cholera in country X in March 1999
Number of cases 490
Population 18,600
Attack rate 2.6%
Measuring disease occurrence
Descriptive
Prevalence Incidence
Probability ofhaving the disease
Probability ofdeveloping the disease
RISKBurden
Measuring disease occurrence
Let’s talk about ODDS now
Probability that an event will happen
Probability that an event will not happen
Jellyfish odds today
Odds
Stung No sting
Total
Swimming off lazareto 3 5 8
3/8
Odds of sting = ------------- = 3:5 = 0.6 5/8
Probability that an event will happen
Probability that an event will not happen
Odds of a rare event equals the risk of rare event
,
Summary of disease occurence
• PREVALENCE• INCIDENCE
– Cumulative incidence– Incidence rate– Attack rate
• ODDS
Buffy Summers, “Class Protector”
• Host: ..Sunnydale High isn't really like other high schools. A lot of weird stuff happens here."Students:”Zombies! Hyena people!”Host:"But whenever there was a problem.. you seemed to show up and stop it. Most of the people here have been saved by you. We're proud to say that the class of '99 has the lowest mortality rate of any graduating class in Sunnydale history. And we know that at least part of that is because of you
For lunch time… what are these?
Number of deaths in 2007 in SpainSpanish population in 2007
Cases of diabetes in 2008 in Spain
Spanish population in 2008
16 malaria deaths 1000 deaths occured
30 guests with diarrhoea300 wedding guests
100 hours at the Lazareto bar150 hours in the lecture room
References
• Epidemiology, an introduction. Rothman KJ.
• Epidemiology in Medicine. Henneckens CH, Buring JE.
• Modern infectious disease epidemiology. Giesecke J.
• Dictionary of epidemiology. Last J.
Cases Non cases
Exposed (e) a b a+b
Non exposed (ne) c d c+d
a+c b+d
Risks, Odds and 2x2 tables
Risk of being a case in exposed = a / a+bRisk of being a case in non exposed = c / c+d
Odds of being exposed among cases= (a/a+c)/(c/a+c)= a / cOdds of being exposed among non cases = (b/b+d)/(d/b+d)= b/d
Odds of a rare event equal the risk of rare event
Cases Non-cases Total
Hepatitis A
30 49,970 50,000
The number of hepatitis cases during an outbreak
30 / 50,000
Odds of disease = ------------------------ = 0.006 49,970 / 50,000
Risk (CI) of disease = 30/50,000 = 0.006