Epidemiology Flashcards
How else can a discrete variable be categorised?
Count variable (i.e red cell count)
Which type of study would be the most appropriate to test the effect of a new statin on the incidence of coronary heart disease in patients with diabetes?
Randomised control trial
Which of the following methods cannot be used to compare the proportions in two sets of observations?
T test (difference between the two means)
What is prevalence?
How common a disease or condition occurs at a given time.
Prevalence calculation
(Number of diseased persons)/(Number of persons in the population) = Prevalence
What is relative risk?
The relative risk (RR) or risk ratio is the ratio of the probability of an outcome in an exposed group to the probability of an outcome in an unexposed group.
What is absolute risk?
Absolute risk of a disease is your risk of developing the disease over a time period.
What is incidence?
Probability to develop the disease in a very small time interval
What is the observation period?
Observation period is the time from the start of the observation until the moment he/she develops disease.
How are incidence and prevalence correlated?
Incidence is positively correlated with prevalence
Types of observational studies
Case report Case series Cross-sectional (prevalence) Case control (retrospective) Cohort (prospective)
Types of experimental studies
Interventional, RCTs
What is a case report?
Study of one individual with a disease, timely or rare information
What is a case series?
Study of several patients with similar symptoms. May lead to general hypothesis. No controls.
What is a cross sectional study?
Measures exposure and disease in study group at one time point (“snapshot”).
Frequently takes the form of survey, (e.g. questions about diagnosed coronary heart disease, family history, diet).
What is a cohort study?
Examines one or more health effects of exposure to a specific factor.
Subjects are defined according to their exposure status and followed over time to determine their health outcomes.
What is a case-control study?
Individuals who have the disease of interest are in a ‘case’ group, their exposure to specific factors are compared to a ‘control’ group from the same population sample
What is an ecological study?
Evaluates an association using the population rather than the individual as the unit of analysis. The rates of disease are examined in relation to factors described on the population level.
Two types of cohort studies?
Prospective, and retrospective
Prospective cohort study?
Exposure groups are followed into the future in order to observe the outcomes
Retrospective cohort study?
Both the exposures and outcomes have already occurred when the study begins.
What can RCTs calculate from results?
A risk
Can case control studies calculate risk?
No
What quantitative data can a case control study produce?
Odds ratio
What is an odds ratio?
Probability that an event will occur divided by the probability that it will not occur
Case control study table?
Cases and controls by exposed and unexposed
Odds ratio from case control table?
exposed cases/unexposed cases = x
exposed controls/ unexposed controls = y
odds ratio = x/y
Strengths of case control
Good for rare disease
Relatively fast
Relatively inexpensive
Can look at the association between the disease of interest and many kinds of exposures
Weaknesses of case control
Susceptible to bias
May be hard to find suitable controls
Time relations may not be clear
Associations that are revealed may not be causal
Relative risk calculation
Risk of developing a disease in the exposed group divided by the risk in the unexposed group.
Relative risk table
Present/ absent disease by exposed/not exposed
Relative risk calculation formula
exposed disease present / exposed present + absent
all over
not exposed disease present/ not exposed present + absent
If RR = 1
There is no association between the factor under consideration and the disease.
If RR<1
If RR < 1 there is a decreased risk of developing the disease if one is exposed to the factor under consideration.
If RR>1
If RR > 1 there is an increased risk of developing the disease if one is exposed to the factor under consideration.
Infection cycle needs?
An infectious source
Receptive susceptible host
Spread to further hosts - infection amplifies
Incubation period
Time between catching an infection and symptoms appearing
Latent period
Time between catching an infection and diagnostic signs (still asymptomatic)
Infectious period
The time period during which an infected person is able to transmit the disease to a susceptible host or vector.
Not necessarily associated with symptoms
What is basic reproduction rate (R0)?
R0 = the average number of persons infected by one disease source
If R0 < 1
The disease will eventually disappear
If R0 > 1
The disease will continue to spread
R0 formula?
R0 = C x P x D
C: the number of contacts an infected person makes per unit time
P: the probability of transmission per contact
D: the duration the infected person is infectious to others
Differences in transmission between enveloped and nonenveloped viruses
Non enveloped viruses are more virulent, because they don’t bud from the cell, they cause host cell lysis
Non enveloped vs enveloped virus mode of transmission
Enveloped - secretions
Non-enveloped - faecal oral
What are non-enveloped viruses resistant to and why?
Due to the protein shell, these viruses are resistant to heat, acids, and drying
How does vaccination help the population?
Preventing people joining the Infectious Pool.
What is pc?
The critical level of immunisation to eradicate the disease
How is pc found
Calculating how far the proportion of susceptible has to be reduced in the face of an agent of a given Reproductive Rate, R0
The higher R0, the higher pc needs to be.
pc calculation
pc=1−1/R0
What is the most likely cause of the drop in disease incidence in vaccinated groups?
Immune protection
What is the most likely cause of the drop in disease incidence in unvaccinated groups?
Herd immunity
What is the most likely reason for a similar carbohydrate conjugate vaccine strategy to fail with another serotype of meningococcus?
The antigen for another serotype (B) may be too similar to a human antigen
What does “a relative risk of 1.82” mean?
The risk for developing heart disease is 1.82 times greater (or 82% higher)
What do you understand by “95% confidence interval 1.25 to 2.64”?
We are 95% certain that the true relative risk lies within the range 1.25-2.64
When can we say risk is insignificant?
When confidence interval includes 1 (i.e no effect)
Investigate whether a person’s sex was a risk factor for malaria. In their village, there
were exactly 400 adults.
120 men had experienced at least one episode of malaria, whereas eighty men had remained malaria-free. In contrast, 120 women were malaria-free and 80 had experienced malaria.
What was the malaria incidence rate in men?
120/200=0.6
Investigate whether a person’s sex was a risk factor for malaria. In their village, there
were exactly 400 adults.
120 men had experienced at least one episode of malaria, whereas eighty men had remained malaria-free. In contrast, 120 women were malaria-free and 80 had experienced malaria.
What was the malaria incidence rate in women?
80/200 = 0.4
Investigate whether a person’s sex was a risk factor for malaria. In their village, there
were exactly 400 adults.
120 men had experienced at least one episode of malaria, whereas eighty men had remained malaria-free. In contrast, 120 women were malaria-free and 80 had experienced malaria
What is the relative risk for men of malaria?
0.6/0.4
