week 2 - measures of frequency

Question 1

Define Epidemiology

Answer 1

Epidemiology is the science of measuring disease or other aspects of health, identfying the causes of ill health and intervening to improve health

Question 2

how are ‘attack rates’ calculated

Answer 2

amount of people who became ill divided by the amount of people exposed

Question 3

what does relative risk describe and how is it calculated

Answer 3

Relative risk describes how many times more likely a person is to become ill if they are exposed to something, compared to if they are not exposed. It is calculated by the percentage of people who became ill after exposure divided by the percentage not ill after exposure.

Question 4

Name some different subdisciplines of epidemiology.

Answer 4

Public Health or infectious disease epidemiology. Nutritional epidemiology. Social epidemiology. Environmental epidemiology. Eco-epidemiology. Cancer epidemiology. Injury epidemiology. Perinatal epidemiology. Occupational epidemiology. Molecular epidemiology. Clinical epidemiology. Life course epidemiology.

Question 5

Epidemiology aims to answer…

Answer 5

What, who, where, when and why

Question 6

Define prevalence and discuss how it is calculated

Answer 6

Prevalence describes the percentage of people in an area/ country living with a disease/ illness.
It is calculated by (number of people living with the illness/ disease divided by the total number of people in the population ) x 100

Question 7

Define incidence

Answer 7

Incidence describes the number of new cases of a disease/illness in an area/ country.

Question 8

Define incidence proportion (or cumulative incidence)

Answer 8

Describes the proportion people who were at risk of developing a disease or illness compared to the proportion of people who did develop it in a specific time period. This is sometimes referred to as attack rate.

Question 9

What is the difference between incidence proportion (IP) or cumulative incidence and incidence rate (IR) or incidence density

Answer 9

IP is the proportion of people who develop disease/ illness in a specific period. IP = the number of people who develop disease in a period divided by the number of people at risk of getting the disease at the start of the period. it is a proportion/ percentage and not a rate.
IR or incidence density also describes how quickly people are developing disease/ illness. however is more accurate if person/time is avaialble. IR = number of people who develop disease in a specific period divided by the totally person-time when people were at risk of getting the disease.

Question 10

What is incidence proportion usually referred to as in clinical trials?

Answer 10

Experimental event rate (EER) or control event rate (CER)

Question 11

What is incidence rate sometimes called?

Answer 11

Incidence density

Question 12

When is a person considered ‘at risk’ of developing a disease/ illness?

Answer 12

An individual is considered ‘at risk’ until the actual moment they do develop the disease, until death or until they are lost to follow up.

Question 13

What are crude rates referring to and what are the disadvantages of looking at crude rates.

Answer 13

Crude rates refer to the basic rates that describe the overall incidence of disease in a population without taking in to account any other characteristics or features of that population. For example the incidence of cancer or heart disease in Australia. Crude rates make overall comparisons between two different populations (for example heart disease in Australia vs heart disease in Ethiopia) unreliable because they have not considered the different characteristics (e.g. age) that would impact the rates. For example a country in the western world where people live longer will have higher crude rates of heart disease compared to a developing country where people don’t live as long. This can lead to poor assumptions of health in those countries.

Question 14

What is standardisation?

Answer 14

Standardisation calculates the overall incidence or mortality rate that you would expect to find in a standard population if it had the same age specific rates as your study population.

Question 15

How is direct standardisation calculated using this example…

a) Age group – 85+
b) Number of deaths in Germany – 16213
c) Number of people in population – 376000
d) Mortality rate (per 100,000) – 4311.97
e) World standard population – 500
f) Cases expected in standard population – 21.56

Answer 15

multiply the age specific rate (d) by the number of people in the same age group in the standard population (e) to calculate the number of events you would expect to see if it had the same rates as your study population (f). do this for each age specific age group and then divide the total number of events expected in all age groups of the standard population (f) by the total number of people in the standard population (e) to get the standardised rate.

Question 16

What do you need to be cautious about when using standardisation?

Answer 16

There are many different ‘standard’ populations and in practice you can age-standardise to any population. If you standardise to two very different populations you will get two very different standardised rates and the relationship between different populations may change.

Question 17

Define rates, ratios and proportions

Answer 17

Rates include some measure of time for example 60km/hr or 17/100000 per year. Unlike a proportion it has no upper limit.
A ratio is simply one number divided by another number. Eg number of beers drunk in 1 year divided by the number of people in the population.
A proportion is a special type of ratio in which everything or everyone in the numberator is also counted in the denominator. Eg the number of people who develop disease divided by the total number of people in population (those with and without disease). It can never be less than 0 or greater than 1. It can be presented as a percentage between 0-100%. All proportions are ratios but not all ratios are proportions.

Question 18

What is indirect standardisation?

Answer 18

Indirect standardisation calculates the number of cases you would expect to see in a study population if it had the same age specific rates as the standard population. This rate is then compared to the actual or ‘observed’ number of cases to calculate a standardised incidence (or mortality). Can be used for age, sex, sex race and any other factors.

Question 19

How is indirect standardisation calculated in the below example…

a) Age group – 80+
b) Male population brazil (x1000)- 80706
c) Mortality rate for males germany (per 100000) – 211.47
d) Expected deaths in Brazil – 70978.43
e) Observed number of deaths in Brazil – 39437

Answer 19

Multiply each age specific rate in the standard population (column c) by the number of people in the study (column b) to calculate the total number of events you would expect to see in the study populationif it had the same rates as the standard population (column d). divide the total number of events actually observed (e) by the number of events expected (d) to get the standardised mortalise ratio (SMR) or standardised incident ration (SIR).
Answer above = 0.56 which suggests the mortality rate in brazil is almost half that of Germany.

Question 20

What does PMR stand for and what does it measure?

Answer 20

Proportional (or proportionate) mortality rate. This measures the relative importance of a particular cause of death in a given population. It is used when there is insufficient information to calculate an SMR (standardised mortality rate). Most commonly used in occupational studies. It is calculated by dividing the proportion of deaths due to a specific cause in a group of interest by the proportion of deaths due to the same cause in a comparison group.

Question 21

What is the case – fatality ratio (CFR or case fatality rate) and how is it calculated?

Answer 21

Described the proportion of people with a given disease or condition who die from it in a given time. Common measure of short-term severity of an acute disease and allows direct assessment of effectiveness of an intervention. Eg CFR for myocardial infarction is usually measured over a period of 28 days. Eg if 100 people were diagnosed with MI and 9 people died than the CFR for MI would be 9 divided by 100 x 100 = 9% CFR.

Question 22

When is case – fatality ratio an appropriate measure and when is it not.

Answer 22

CFR is appropriate for short term mortality (a month or so) but is less useful in conditions in which death may occur further down the track.

Question 23

How is survival rate/ relative survival rate calculated?

Answer 23

Survival rate is calculated by the proportion of patients who are still alive a specified number of years after diagnosis. This is often adjusted to allow for the fact that, depending on the age group being considered, some people would have been expected to die anyway from causes other than their cancer – this is knowns as relative survival rate.

Question 24

What are the millennium development goals?

Answer 24

1- Eradicate extreme poverty and hunger
2- Achieve universal primary education
3- Promote gender equality and empower women
4- Reduce child mortality
5- Improve maternal health
6- Combat HIV/ AIDS, malaria and other diseases
7- Ensure environmental sustainability
8- Develop a global partnership for development

Question 25

Identify some mortality indicators

Answer 25

Maternal mortality ratio, stillbirth or fetal death rate, neonatal mortality rate, infant mortality rate, child or under-five mortality rate, adult mortality rate.

Question 26

What are the advantages and disadvantages of using the disability- free life expectancy?

Answer 26

``` Advantages = relatively simple to calculate Disadvantages = an arbitrary decision has to be made on what level of disability will lead to someone being classified as disabled and years of life lived with a disability are not counted at all and thus are effectively considered as bad as being dead. ```

Question 27

What does YLL refer to and how is it measured/ what are the benefits.

Answer 27

YLL = years of life lost and refers to the number of potential years a person has lost if they die due to a disease. For example if a person was expected to live for 80 years but died for a cardiac arrest at 60 years old than that equals 20 YLL. Therefor conditions that lead to the death of younger people, while less common, contribute to a higher number of YLL.

Question 28

Describe QALY’s and when they are most beneficial.

Answer 28

QALY’s = Quality- adjusted life years. QALY’s weight each year of life by the perceived quality of life from a value of one for perfect health down to zero for death. 1 QALY = 1 year of life in perfect health. 0.5 QALY = 1 year of life with 50% disability or ill – health or 6 months of perfect health. QALY’s are commonly used when comparing two clinical treatments and thus a new radical treatment which increases life expectancy by 10 years but leads to severe side effects/ disability made gain lower QALY’s than a drug that has less side effects but only increases life expectancy by 8 years.

Question 29

What is Health Adjusted Life Expectancy (HALE)

Answer 29

combines QALY’s with measures of life expectancy to calculate the equivalent number of years a person can expect to live in full health.

Question 30

Define Daly’s and give an example of how DALY’s are calculated.

Answer 30

Daly’s are disability adjusted life years. Like YLL, they estimate loss of life but they have the major advantage that they not only count years of life lost to premature death but also years of life lost through disability. The extent of disability is weighed from zero to one, from zero being a year spent in perfect health to one for a year lost to death. Therefore if a person lives with a moderate disability for 10 years , this might equate to a loss of 5 years of healthy life or 5 DALY’s.