Module 3: Measures

Question 1

What is prevalence? What is it also known as?

Answer 1

Presence of disease in a group.

“Burden” of disease.

Question 2

Prevalence has no explicit consideration of time except for what?

Answer 2

Time over which prevalence calculated.

Question 3

What is incidence? What is it also known as?

Answer 3

New occurrence of disease.

“Risk” of disease.

Question 4

Incidence refers to what transition?

Answer 4

Non-diseased to diseased state.

Question 5

The COVID-19 dashboard we viewed refers to what three measures?

Answer 5

Active cases = prevalence

Total cases reported daily chart = incidence

Histogram = “epidemiological curve” - visual representation of disease risk over time.

Question 6

There are 10 students in a tutorial. 2 have a cold. Report the prevalence in proportion and in odds.

Answer 6

Proportion: # of colds / # of ppl. in tutorial = 0.2 = 20% (20 out of 100 ppl. in tutorial).

Odds: # of colds / # of ppl. without colds = 0.25

Question 7

What are units used when reporting proportion?

Answer 7

Units are people, no time element.

Question 8

When thinking about proportion and odds, what happens when prevalence is low?

Answer 8

The proportion and odds are similar.

Question 9

What is point prevalence? Provide an example.

Answer 9

The amount of disease in a population at a point in time, snapshot; the most common option, assumed if not specifically stated otherwise.

“Do you currently feel depressed?”

Question 10

What is period prevalence? Provide an example.

Answer 10

The amount of disease in a population during a time period/frame.

“In the past year, have you felt depressed?”

Question 11

What is lifetime prevalence? Provide an example.

Answer 11

The amount of disease in a population at any point during the lifetime of those alive in the population.

“Have you ever felt depressed?”

Question 12

If a professor keeps track of all colds in their tutorial for one year, they count 5 colds. What is the incidence PROPORTION of colds in their tutorial during the academic year?

Answer 12

Number of new colds / # of people at risk = 0.5 = 50% (50% risk of getting a cold over one year).

Question 13

What is the incidence rate? What does Patten liken it to?

Answer 13

The rate at which people become ill; time explicitly included.

Patten likens to velocity, km/h.

Question 14

What is “person-time”? What does it mean when somebody refers to 1 or 5 person-years?

Answer 14

The sum of the time periods of observation of each person who has been observed for all or some of that time period.

1 person-year = one person at risk who is observed for one year.
5 person-years = one person at risk who is observed for five years OR 5 people each at risk for 1 year.

Question 15

If a professor keeps track of all colds in their tutorial for one year, they count 5 colds. What is the incidence RATE of colds in their tutorial during the academic year?

Answer 15

Number of new colds / total person-time at risk = 5 per 10 person-years, expressed as 0.5 person-year−1.

Question 16

What is the difference between incidence proportion and rate?

Answer 16

Proportion: New events / Those at risk in the relevant period. No explicit time in proportion.

Rate: New events / Total person-time at risk. Time explicitly included.

Question 17

What are attack rates? When are they used?

Answer 17

Attack rates are types of incidence proportions (never referred to as “attack proportions”).

When a particular disease corresponds with a particular risk interval, like food poisoning or flu season, attack rates used to describe and compare different outbreak scenarios.

Question 18

What is cumulative incidence? Why is it used?

Answer 18

Proportion of cases that accumulate over an interval of observation in an at-risk cohort.

People who are ill in earlier time periods are not at risk of illness in later time periods, so we use an exponential function to calculate the true proportion of individuals who will be sick after multiple time periods.

Question 19

How is cumulative incidence calculated?

Answer 19

Cumulative incidence, n years = 1 − (1 − Annual Incidence Proportion),n

Question 20

What is the relationship between prevalence and incidence?

Answer 20

Prevalence = Incidence Rate × Mean duration of disease

Prevalence Odds = Incidence Rate,t−1 × Mean duration of disease,t

Question 21

How is mortality rate calculated and expressed?

Answer 21

Number of deaths per year / The mid-year population in the same year

Usually expressed as “per 100,000” by multiplying the above by
100,000.

Question 22

For mortality rates, the mid-year population is meant to reflect what?

Answer 22

Person-time at risk that year.

Question 23

How are specific mortality rates calculated?

Answer 23

Cause-specific mortality rates: # of deaths due to disease X per year / The mid-year population in the same year

Sex-specific mortality rates: # of deaths among females per year / The mid-year female population in the same year

Age-specific mortality rates: # of deaths in age group X per year / The mid-year population of age-group X in the same year

Question 24

Common age-specific mortality rates refer to babies. What are these rates?

Answer 24

Perinatal mortality: # of deaths age 0-6 days or still birth of 28 weeks + gestation / # of total births

Infant mortality: # of deaths less than 1 year / # of live births

Neonatal (first 28 days of life); Post-neonatal (>28 days, <1 year); Under 5; all use live births as denominator.

Question 25

What is case-fatality rate (or case-fatality proportion)? How is it calculated?

Answer 25

Percentage of people who die when they get a disease.

Number died while having disease / # of people with disease x 100

Question 26

With case-fatality rates, “rate” is used but with what caveat?

Answer 26

Risk interval characterized by illness rather than specific time interval.

Question 27

“Potential Years of Life Lost” (PYLL) refers to what? Provide an example.

Answer 27

A measure of premature mortality or early death, referring to a standard reference age (e.g., 75).

Death at age 20 = 75 - 20 = 55 PYLL

Question 28

Mortality rates/causes and PYLL will give different estimates of the importance of different causes of death. Provide an example of this.

Answer 28

Suicide - low cause-specific mortality rate, ranks highly in causes of PYLL because people who commit suicide are typically young.

Question 29

What is life expectancy? What does it use?

Answer 29

An estimate of future survival experience of contemporary cohort, a projection.

Uses current age-specific mortality rates to guess how many years, on average, a baby born this year can be expected to live.

Question 30

Life expectancy reacts to changes in _____ _____.

Answer 30

Current mortality.

Question 31

What are the 5 traditional measures of mortality and morbidity?

Answer 31

Life expectancy.

Mortality rate.

Number of chronic conditions.

Prevalence of a disease.

Incidence of a disease.

Question 32

Newer measures of health try to get at quality of life. Take health status like health-related quality of life - what does that mean?

Answer 32

Asking somebody how they would rate aspect of health.

Question 33

Newer measures of health include health-adjusted life years (HALYs) and health-adjusted life expectancy
(HALEs). What are these?

Answer 33

Numbers that account for length of life and health-related quality of life summarized for a population.

Question 34

The basic idea of HALYs and HALEs is what? Explain.

Answer 34

Not all time alive is of equal value - life years can be adjusted by health (life years x health-related quality of life).

Question 35

If someone’s health-adjusted calculation was 80 x 0.75, what does that mean?

Answer 35

They lived until 80 years, their health-related quality of life was 0.75, therefore they had 60 perfect-equivalent life years.

Question 36

The Health Utility Index (HUI) is commonly used in population health surveys. Each respondent answers questions regarding function in the HUI’s 8 dimensions. The answers are applied to the “standard Canadian
preferences” for that particular health status. What does that mean?

Answer 36

Researchers ask people what trade-off they would be willing to accept if moved to a lower stage of a health dimension. Implicit trade-offs that people feel represent equal trade-offs between different states of health.

Question 37

A HUI = 0.75 for one individual means what?

Answer 37

Their scores are representative of a person with a health utility score of 0.75, not that the respondent preference indicates a weight of 0.75.

Question 38

Explain how Quality Adjusted Life Years (QALYs) are calculated and what weights are used? What does 1 QALY mean?

Answer 38

Life years × health-adjusted quality of life per year

Health-related Quality of Life: A weight [0,1] where health utility derived from evaluating different health states.

1 QALY = 1 year full health

Question 39

Explain how Disability Adjusted Life Years (DALYs) are calculated and what weights are used?

Answer 39

Years of life lost + years lived with a disability

“Disability weight” [0,1]

Question 40

If someone’s living 10 years with a disability weight = 0.5, what does that mean?

Answer 40

5 years lost.

Question 41

What is the measure of health for the Global Burden of Disease?

Answer 41

DALYs

Question 42

With regards to disease and exposure, what question does strength of association ask vs. attributable risk?

Answer 42

Strength of association: To what extent is the disease associated with the exposure?

Attributable risk: How much of the disease can be attributed to the exposure?

Question 43

Incidence of disease is a building block for other measures, it’s the _____ _____ of something occurring.

Answer 43

Absolute risk.

Question 44

We often compare the absolute risk between what two groups?

Answer 44

Those with the exposure versus those without.

Those with the disease versus those without.

Question 45

How is Relative Risk (also known as risk ratio) calculated?

Answer 45

Risk (incidence) in the exposed / Risk (incidence) in the unexposed

Question 46

Interpret an RR = 1; RR > 1; RR < 1.

Answer 46

RR = 1: No association (risk in exposed = risk in unexposed)

RR > 1: exposure associated with increased risk (risk in the exposed > risk in the unexposed)

RR < 1: Exposure associated with decreased risk (risk in the exposed < risk in the unexposed)

Question 47

We’re examining the risk of osteoporosis among those who are long-term users of corticosteroids as opposed to non-users. We get an RR = 2. Interpret this.

Answer 47

The risk of osteoporosis among those who are long-term users of corticosteroids is twice the risk of osteoporosis in non-users.

Question 48

How is an Odds Ratio (also known as relative odds) calculated?

Answer 48

Odds in one group / Odds in another group

Question 49

What two groups do Odds Ratios pertain to?

Answer 49

Exposed versus the unexposed - “What are the odds that the disease will develop in an exposed person?”

Diseased versus not diseased - “What are the odds that a person with the disease was exposed?”

Question 50

Interpret an OR = 1; OR > 1; OR < 1.

Answer 50

OR = 1 : No association.

OR > 1: Exposure positively associated with disease.

OR < 1: Exposure negatively associated with disease.

Question 51

We’re examining the risk of osteoporosis among those who are long-term users of corticosteroids as opposed to non-users. We get an OR = 2.02. Interpret this.

Answer 51

The odds of osteoporosis among those with long-term use of corticosteroids is approximately double the odds of non-users.

Question 52

What two reasons make Odds Ratios particularly useful?

Answer 52

The exposure OR and the disease OR are equal.

The OR can approximate the RR when the disease is rare (i.e. numbers of exposed with disease and unexposed with disease are small).

Question 53

How is the Risk Difference calculated?

Answer 53

Risk (incidence) in exposed − Risk (incidence) in unexposed

Question 54

We’re examining the risk of osteoporosis among those who are long-term users of corticosteroids as opposed to non-users. We get a RD = .01. Interpret this (hint: can’t use %. Why is this?).

Answer 54

The risk of osteoporosis is 0.01 higher among long-term users of corticosteroids compared to non-users.

Do not use % in risk differences because we are talking about absolute magnitudes of probability units.

Question 55

Numbers needed to treat/harm assume causality. The number needed to treat and the number needed to harm pertain to what outcomes?

Answer 55

NNT: positive outcome (cure)

NNH: negative outcome (adverse event)

Question 56

Numbers needed to treat/harm use what as part of their calculation?

Answer 56

Risk differences.

Question 57

In a clinical trial, cure for the treatment group = 66% and cure for the control group = 33%. What is the risk difference and what is the implication?

Answer 57

Risk Difference = 0.66 − 0.33 = 0.33

The treatment increases the probability of being cured by 0.33.

Question 58

In a clinical trial with a risk difference of 0.33, what is the number needed to treat?

Answer 58

NNT: 1 / 0.33 = 3

3 people must be treated for 1 of them to have been cured by the treatment.

Question 59

We’re examining a rare medical event - in the general population: 1 in 100,000 in one month;
in people using a new drug: 5 in 100,000 in one month. What is the risk difference and what is the implication?

Answer 59

Risk Difference = 5 − 1 = 4

The new drug increases the risk of an adverse event by 4 in 100,000.

Question 60

We’re examining a rare medical event, possibly related to a new drug, with a risk difference of 4 (i.e., 4 in 100,000). What is the number needed to harm?

Answer 60

NNH: 1 / 4/100,000 = 25,000

5,000 people need to be treated with this drug to create 1 adverse event beyond the usual rate.