2.1 epidemiology Flashcards
how can variables be divided ?
- discrete (or categorical)
- continuous
List categorical or discrete variables & what they are ?
- nominal - categories into which individuals classified, and have no numerical relationship e.g. sex
- ordered (ordinal) - ranking of categories e.g mild, moderate, severe
- interval (categorical) - distance between measures on a scale that has meaning e.g. one, two people in a household
list continuous variables & what they are ?
- Interval (continuous) - distance between measures on a scale has meaning e.g. temperature, but ratio between measurement don’t have meaning
- ratio - distance and ratio between measurements are defined
What’s difference between histogram and bar chart ?
- histogram - no gaps, present continuous variables
- bar chart - has gaps, used for discrete or categorical data
Histogram with no gaps is used to present ?
continuous variables
To summarise properties of frequency distribution what is described ?
- shape
- location
- spread
give 3 questions that can be asked with how can shape be used to describe frequency distribution ?
- Is it unimodal or bimodal?
- Is it symmetric or skewed?
- Are there any large gaps or outliers?
How is location described with frequency distribution ?
- mode - value which occur most often
- median - middle value
- mean (average) - arithmetric average
measures of spread describe …?
how variable or spread out the data are
IQR useful for …. distribution as it is resitant to ….
skewed, outliers
- What does standard deviation measure ?
- when is it only used ?
- spread in data around the mean
- when you have described the location using mean
workbook pg 24 Q
why is correlation coefficient used ?
to summarise the strength of a linear correlation
what’s difference between ratio and proportion ?
- ratio - division of 2 unrelated numbers
- proportion - division of 2 related numbers
What can rates be used to do ?
compare information on health and disease in different populations
Strictly what does a rate always imply ?
‘per unit time’
What do rates require ?
numerator (cases) and denominator (population) each relating to same specified time period
Without using rates, what may be misleading ?
comparisons of numbers of cases in different populations
what 2 concepts are included in the ‘extent od disease’ in a population ?
incidence & prevalence
relating to incidence:
1. it focuses on …?
2. numerator = ?
3. denominator = ?
- events
- number of new cases
- should only include population at risk of the disease or event
How may incidence be expressed as ?
incidence rate (a rate) or a cumulative incidence (a proportion or risk)
what is cumulative incidence ?
a proportion or risk
- cumulative incidence often defined as ?
- can be expressed as ?
- proportion of ‘at risk’ population that becomes diseased over a specified time period
- %
What is calculation for incidence ?
number new cases in defined population over a given time / number in defined at risk population over same period of time
Using the example of X-linked Duchenne muscular dystrophy which only affects males.
who would not be included in the ‘at risk’ population ?
- females
- people who already have the condition should also be excluded
When reporting incidence rate what should be thought of ?
i.e. the incidence rate of meningococcal meningitis in Westminster, London (place) is 10 per 100,000 population (person) per year (time).
time, place, person
what is incidence density ?
a specific method of calculating Incidence rate if people are followed up for different amounts of time
which study can incidence density often occur ?
cohort , where particpants have differing lengths of follow-up
calculation for incidence density ?
number of new cases in defined population in a specified period / total at-risk person-time of observation
Give 4 examples of reasons why calculating the incidence is useful
- measure new cases of disease when disease is of very short duration e.g. massive stroke
- see if new cases of infectious disease are getting more frequent to see if there is an epidemic in progress
- monitor the effect of prevention programmes
- compare people exposed to a potential hazard with those not exposed to help determine if exposure is a real danger to health
Calculate the total ‘at risk’ person-time of observation for the following ,
If 10 ‘at risk’ people are followed up for 5 years, 20 people are followed up for 7 years and 25 people are followed up for 10 years
(10 x 5) + (20 x 7) + (25 x 10) = 440 person years.
Whilst incidence focuses on …. or …. , what does prevalence focus on ?
new events or cases
health status
What is prevalence ?
the proportion or percentage of a population in a particular health state
calculation for prevalence ?
number of cases at a given time / number of population at that time
Why is point prevalence the most used term for prevalence ?
it refers to the prevalence (existing cases) at a snapshot in time
In a population of 100,000 people there are 20,000 smokers. What’s the prevalence of smoking ?
20 000/ 100 000 = 20%
or 20 in 1000
what is prevalence not ?
a rate
Calculating a prevalence is useful for when ?
1. provide idea about the ….. of disease i.e extent to which disease is a …. in the community
2. know the need for particular …… which is important for planning for ….. to know how many people need …. for
3. know how many people with a ….. such as smoking is in each population
- burden, problem
- health services, long-standing disease, caring
- characteristic
Why are prevalence and incidence related ?
all the prevalent cases must at some time have been incident cases,
when other things equal higher incidence ill imply higher prevalence
Why is the relationship between incidence and prevalence not quite as simple as when other things being equal, higher incidence will imply higher prevalence ?
number of prevalent cases is constantly being added to by new (incident) cases, and constantly being depleted by patients dying or recovering
What is prevalence influenced by ?
death rate, cure rate as well as the incidence rate
will prevalence increase or decrease if …
new treatment is found which keeps people with the disease alive longer
increase
will prevalence increase or decrease if …
more patients are cured or die
decrease
Calculation that represents when the incidence rate and the rates of recovery and death are constant ?
P ~ (I x D)
P = prevalence
I = incidence rate
D = duration of disease
Often morbiditiy is not as accurately recorded as … ? so…?
deatjs so many illness episodes are not brought to the attention of the health-care system
morbidity rate is what ?
number of existing or new cases of a particular disease per 100 population (not technically always a rate)
what’s a attack rate ?
number of new cases of disease that develop during a defined, short period of time per number of healthy populations at risk at the start of the period
what’s mortality rate ?
a special case of an incidence rate where the event is death, rather than the onset of disease
life expectancy = average number of …
years that a newborn is expected to live if current mortality rates apply
infant mortality rate calculation ?
number of deaths per year occuring within first year of life x 1000 / total number of live births in the year
