Case and Measures of Disease Frequency Flashcards
In terms of frequency, what does epidemiology involve? (2)
- Estimating the frequency and distribution of diseases in populations
- Investigating the effect of suspected risk factors on the frequency of diseases
Which key variables are used to quantify how common an illness is in a population during or at a specific time? (3)
Cases
Size of population
Time
List 5 different outcomes of interest
Disease Infection Death Physiological measurement Usage of health services
what are some important things to consider when defining a case?
- How is the case being identified? - clinically or lab tests
- What are the boundaries for including cases? - all cases, or just severe cases?, what are the cut offs for measuring continuous indicators such as blood pressure?
- In incidences of multiple events, are we considering all cases or just the initial case?
Define prevalence simply
The frequency of existing cases
Define incidence simply
The frequency of new cases
What are prevalent cases?
Existing cases
What are incidence cases?
New cases
Define prevalence more in depth
The proportion of people within a defined population that has the outcome at a specific point in time
Give the formula for calculating prevalence
Number of existing cases at time T /study population at time T
Define incidence in more depth
The rate of new cases of an outcome occurring in a population over a period of time
Give the formula for incidence
Number of New cases during Time T / Population at Time T
What are the two measurements of incidence?
Risk and Rate
What is the epidemiological definition of Risk
Risk relates the number of new cases to the size of the population at risk at the start of the study period
What is the epidemiological definition of rate
what does it take into account?
Rate relates the number of new cases to the person time at risk, which is a measure that takes into account the changes to the size of the population at risk during the follow up
What is interesting about rates and risks of rare diseases?
The risk and rates will be numerically similar
Define incidence in terms of risk
The probability of occurrence of disease during a defined time period in a population which is initially free of disease
Define incidence mathematically in terms of risk
Risk = Number of new cases during Time T / Population initially at risk Risk = d/N
What is the risk of a student eating a big mac between October and next September if there are 800 students and big macs are freely available everyday in the canteen and over the time period 200 students reported eating a big mac?
Population initially at risk = 800 (N)
New cases = 200 (d)
Risk = d/N = 200/800 = 25%
How is risk always presented?
Risk is always a proportion, so it always varies between 0-1 or 0-100%
In terms of recurrent diseases, what does risk refer to?
Risk always refers to non recurrent disease OR the first incidence of a recurrent disease
What are some others names for risk?
Cumulative incidence
Incidence risk
Probability
What is a common use of risk in terms of epidemiological studies
we can calculate the risk of an outcome in certain strata of a population (for example in different age groups or in males and females) and then we can compare the risk between these strata to see how the risk changes
Define odds simply
Odds = the ratio of people who get a disease compared to people who do not get the disease
Define odds mathematically
Odds = The number of new cases within Time T / The number of non cases within Time T
Calculate both the risk and the odds of sneezing in this situation
- lecture theatre of 200 people for 1 hour, 50 people sneeze in this hour.
Risk = people who sneeze/people who are at risk of sneezing Risk = 50/200 = 25%
Odds = People who sneeze/People who do not sneeze Odds = 50/150 = 0.33
What is the term for the denominator for the rate formula?
the person-time at risk (Y)
What is the main advantage to using rates as a measure of disease frequency?
They allow us to take into account how a population may change over time, and this is usually what happens
What is the definition of rate?
Relating the number of new cases to the person time at risk
What is the numerical definition of Rate?
Rate = Number of new cases / Total person time at risk Rate = d/Y
What is the person time at risk?
The sum of the time each person in the study population was at risk during the study period
Which factors does the person time measure take into consideration?
Takes into account changes to the size of the population at risk during the follow up period
What is particular about using death as an outcome of interest when calculating the person time at risk in order to calculate rate in a study?
Once someone has died, they cannot die again, so after they have died they are no longer at risk.
This means that the observation time before their death will be the period of risk for this individual and after they die, the time period after death will not contribute to the denominator
Person A - observed for 6 years, alive, but moves away so can’t be studied.
Person B - Observed for 4 years then dies
Person C - Observed for 10 years, alive when the study ends
Outcome of interest in the study is death
What is the total person time at risk?
What is the rate?
- Total person time at risk (Y) = 20 years
- Number of new cases (d) = only 1 person died so = 1
- Rate = d/Y = 1/20 person years
For the example in Notes, calculate the rate of disease in the study.
Person time at risk = all those who never get the disease contribute the full 12 month study period, all those who get the disease will only contribute risk time until they contracted the disease
Y = 108 months (9years)
New cases = 4 people contracted the disease over the study time = 4
Rate = d/Y = 4/9 person years
For the example in Notes, calculate the RISK.
10 people initially at risk
4 new cases
Risk = 4/10 or 2/5
When will the denominators that we use to express our rates be important?
When we might want to compare rates between different areas, different populations ect
What is the common standard for denominators in epidemiology?
- we usually use a round number which is usually a multiple of 10
- we usually want the numerator to be greater than 1
How do we convert our rates to the standard denominator format?
- Convert the rate to 1 person year
2. Multiply this until you give per 100 person years
Convert a rate of 4 per 9 person years into the standard denominator format.
4 per 9 person year
4/9 = 0.444 per 1 person year
0.444 x 100 = 44.4 per 100 person year
What is the most significant difference between Rate and Risk?
Unlike Risk, Rates are not proportions
What are 3 alternative names for Rate in epidemiology?
Incidence Rate
Incidence density
Force of Morbidity
What can Rates be used to estimate?
The incidence of recurrent diseases
For the second example in Note, what is the time at risk?
84 months (7 years)
In example 2 in Note, what is the incidence of new cases?
There are 16 cases across the time period of the study
For the second example in Note, calculate the rate of the recurrent diarrhoea
express this in the standard format
Rate = d/Y Rate = 16/7 person years
(/7) = 2.29 per person year (x100) = 229/100 person years
Give the graphical presentation of risks and rates ( Notes)
and use this to state the formulas for these
What is the formula for odds over time?
Odds over time T = d/N-d
Graphically demonstrate how risk and rates are very similar for rare diseases (Notes)
Briefly, explain why risks and rates are similar for rare diseases.
Risk = number of people with the outcome/the number of people at risk at the start Rate = number of people that have the outcome/the amount of person time at risk
Rare means that out of lots of people, very few will contract the disease, so these means that throughout the study as not a lot of people contract the disease, the person time at risk is not effected a lot as most people remain at risk.
so for a 1 in 10,000 disease
risk would be 1/10,000
and rate would also be very close to 1/10,000 as the denominator hasn’t be changed a lot as not many people have contracted the disease
In which situation may the risks and rates of a rare disease be different?
why?
when the population is not stable and people are moving in and out of the population.
This is because rate takes into consideration any changes to the size of the population
what is significantly different about the risk and rates of a rare disease in an epidemiological study?
The confidence intervals for the risks and rates will be calculated differently
When a disease is common, what is significant about rates and risks?
You need to make a decision to determine which one to use in your particular scenario
What is prevalence of a disease dependent upon?
The incidence of the disease and the duration of the disease
Which are the two circumstances in which prevalent cases can leave the prevalent pool of cases?
The diseases person can either die or recover from that disease
when a disease is stable, what is the formula for it’s prevalence?
Prevalence = Incidence x Duration
What does it mean if a disease is stable?
The incidence of the disease is fairly constant and also the rate of death and recovery from the disease is also fairly constant
“study of 4000 older adults in UK measured the frequency of pneumonia, found 11 episodes of pneumonia per 1000 person years”
Which type of measure of disease frequency is this?
Rate - basing the context of the disease on the person time at risk
” 39 out of 1000 infants born in Kenya in 2014 died in the first year of life”
which measure of disease frequency is this?
Risk - placing the new cases into the context of the size of the population at the start of the study
” 1% of women having a baby in a hospital in Brazil in 1999 were found to be HIV positive when tested at delivery”
Which measure of disease frequency is this?
Prevalence
What is the first example of a common cause of error in measuring disease frequency?
- Errors in quantifying the number of cases, it can be quite hard to define a case in some instances and this will give us an error in the numerator when calculating
What is the second example of common errors that can occur when measuring disease frequency?
- errors can occur when we try to determine the population at risk, may be hard to define the population or there may not be extensive information on this particular population.
Define prevalence
Prevalence is the proportion of persons in a defined population that have the outcome under investigation at a defined instance in time
What is period prevalence?
The proportion of persons within a defined population that have the outcome under investigation during a specified time interval
What are the two measures of incidence?
Rate and Risk
What is the difference between prevalence and incidence?
Prevalence is concerned with measuring the frequency of existing cases whereas Incidence is concerned with measuring the frequency of new cases
What is the definition of infection?
Entry and multiplication of an infectious agent within a host body
What is the definition of clinical infection?
The presence of an infection with the addition of the clinical symptoms of the infection
What is the definition of a sub clinical or inapparent infection?
The presence of an infection within a host without the clinical symptoms to accompany it but where tests have confirmed the presence of the infection within the host.
What is the definition of Immune?
Resistant to an infection, can be acquired by a host after an infection through nature or through vaccine, or passive immunity can be delivered through maternal antibodies or blood products. Can be permanent or wane with time.
What is the definition of susceptible?
Not immune to a disease
What is the definition of intensity of an infection?
A feature of a parasitic infection
Usually a geometric measurement of the size of the load of the parasites within a host.
What is an age specific rate?
A rate for a specific age group - the numerator and denominator refer to the same age group
What is a case fatality rate?
The proportion of cases of a specific cause that die in a specific period of time, usually expressed as a %
What is a cause specific mortality rate
A rate estimated as the number of deaths from a specified cause in a defined population in a defined period of time
What is an infant mortality rate?
what is specific about this?
The number of deaths in children (under a certain age) divided by the number of live births in that same population in that same time period
- not really a rate but a ratio of two numbers
