Thinking about Populations over time: Incidence and Cohort Studies

Question 1

Prevalence

• Prevalence measures the frequency of “…” of a disease in a given … at a designated … (the numerator)
  
  • E.g?

Answer 1

• Prevalence measures the frequency of “cases” of a disease in a given population at a designated time (the numerator)
• E.g. diagnosed asthma in children aged 5-11 years

Question 2

Calculation of prevalence requires a suitable … (E.g. GP registered patients, schoolchildren) - the number of people who are ‘.. …’ of the disease

Answer 2

Calculation of prevalence requires a suitable denominator (E.g. GP registered patients, schoolchildren) - the number of people who are ‘at risk’ of the disease

Question 3

Prevalence = number of people …. …. / number of people …

Answer 3

Prevalence = number of people with disease/ number of people who could have disease

Question 4

Prevalence is expressed as either … (3)

Answer 4

Prevalence is expressed as a percentage (e.g. 70%), a proportion of 1 (0.7 is equivalent), or a proportion per unit of population (700 of every 1000 people)

Question 5

What is incidence?

• A measure of the number of … - in a month or a year, for example - expressed as a … of a population which is at risk
  
  • Often expressed as per …, per … or even per … people

Answer 5

A measure of the number of new cases of a condition in some given time period - in a month or a year, for example - expressed as a proportion of a population which is at risk

Often expressed as per 1000, per 10,000 or even per 1,000,000 people

Question 6

To establish incidence, a group of people is followed through … and the onset of a … is measured

Answer 6

To establish incidence, a group of people is followed through time and the onset of a disease/health event is measured

Question 7

Relationship between Prevalence and Incidence

• Prevalence depends on:
  
  • The … of a disease and
  • The … between … and recovery (or death)
• Prevalence = … x … ….

Answer 7

• Prevalence depends on:
  
  • The incidence of a disease and
  • The time between onset and recovery (or death)
• Prevalence = incidence x disease duration

Question 8

Prevalence = … x disease …

Answer 8

Prevalence = Incidence x disease duration

Question 9

Why might we want to know about incidence - Diabetes?

• Understanding diabetes and its risk factors (exposures) and outcomes
  
  • Accurate knowledge of disease, trends, geographical differences -> health care providers, researchers and policy makers
• Implications for
  
  • …- health, happiness
  • … – current and future economy, labour workforce
• Informing prevention and public health …
• Service planning and c…
  
  • S… and assessment
  • Staffing, training, resources, specialisms
• Identifying and prescribing targeted and indicated interventions
• Evaluating … of interventions

Answer 9

• Understanding diabetes and its risk factors (exposures) and outcomes
  
  • Accurate knowledge of disease, trends, geographical differences -> health care providers, researchers and policy makers
• Implications for
  
  • Individuals - health, happiness
  • Society – current and future economy, labour workforce
• Informing prevention and public health interventions
• Service planning and commissioning
  
  • Screening and assessment
  • Staffing, training, resources, specialisms
• Identifying and prescribing targeted and indicated interventions
• Evaluating effectiveness of interventions

Question 10

Measuring Incidence

• Incidence is the number of instances of illness/disease case onset, in a given … in a defined …
  
  • The numerator is the …
  • The denominator is the …
• This type of incidence is also known as …

Answer 10

• Incidence is the number of instances of illness/disease case onset, in a given period in a defined population
  
  • The numerator is the number of new events in a population
  • The denominator is the average number of persons at risk during this period
• This type of incidence AKA incidence risk, cumulative/ crude incidence

Question 11

Incidence = Number of … cases in a … … period / total population at …

Answer 11

Incidence = Number of new cases in a given time period / total population at risk

Question 12

Measuring Incidence - The Numerator

• As per prevalence = … for the disease
  
  • PLUS - clear and consistent definition as to what counts as a … …

Answer 12

• As per prevalence = caseness for the disease
  
  • PLUS - clear and consistent definition as to what counts as a new case

Question 13

Measuring Incidence - The Denominator

• … population at risk
• Must be the population truly at … of developing the disease/ condition
• As applies to a … of time, typically take the …-point value, e.g. population at …-point in a year

Answer 13

• Total population at risk
• Must be the population truly at risk of developing the disease/ condition
• As applies to a period of time, typically take the mid-point value, e.g. population at mid-point in a year

Question 14

Calculating Incidence

• We can use a … table
• DISEASE INCIDENCE
  
  • Incidence of disease in … = A / (A + B)
  • Incidence of disease in … = C / (C + D)

Answer 14

• We can use a Contingency table
• DISEASE INCIDENCE
  
  • Incidence of disease in exposed = A / (A + B)
  • Incidence of disease in unexposed = C / (C + D)

Question 15

Calculating Incidence - All-cause mortality in patients newly diagnosed with T2 Diabetes, with and without CVS disease

• What is the incidence of death amongst people newly diagnosed with T2 diabetes with CVS disease?
• Use Calculation A or B?

Answer 15

• Calculation A / (A + B)
• Exposed group
• What is the incidence?
• 3535 / (3535 + 8844) = 0.29
  
  • Incidence of diease in exposed - 290 per 1000 or 0.29

Question 16

Diabetes: Numerator & Denominator

Changing/varied/complex diagnostic criteria, e.g.

• different … / changing … within tests / no … i.e. - self-reported doctor diagnosis
• …-… diabetes medication usage – e.g. metformin – prevention and treatment (may result in false positives)
• Type 2 - … onset + usual presentation without acute … disturbance in type 1 complicates identifying … of …

Answer 16

• different tests / changing thresholds within tests / no test i.e. - self-reported doctor diagnosis
• self-reported diabetes medication usage – e.g. metformin – prevention and treatment (may result in false positives)
• Type 2 - slow onset + usual presentation without acute metabolic disturbance in type 1 complicates identifying time of onset

Question 17

Diabetes: Numerator & Denominator

• Changing s…, m… and …-keeping practices
  
  • Quality and Outcomes Framework (2004) - improved screening and management record-keeping
  • e.g. Primary Care Trusts screen for cardiovascular risk in 40 -75 years often with glucose test
• Greater NHS … choice = greater … between / out of NHS Trusts makes it harder to follow patients

Answer 17

• Changing screening, management and record-keeping practices
  
  • Quality and Outcomes Framework (2004) - improved screening and management record-keeping
  • e.g. Primary Care Trusts screen for cardiovascular risk in 40 -75 years often with glucose test
• Greater NHS patient choice = greater movement between / out of NHS Trusts makes it harder to follow patients

Question 18

Why does it matter? - Changes in Diabetes - Numerator & Denominator

• Affects … estimates
  
  • … positives and negatives
  • Increased/decreased numerator and denominator
• Affects … of findings
  
  • Exclude sub-populations of interest from numerator/denominator
• Affects … of causal factors and outcomes
  
  • Obscure …/ascertaining impact of exposures, and of temporal or geographical trends
• Affects service … and …
  
  • Unable to meet demand/wasted resources
  • Inappropriate targeting of …
• Affects …
  
  • Under or over-…/over-… of individuals

Answer 18

• Affects incidence estimates
  
  • False positives and negatives
  • Increased/decreased numerator and denominator
• Affects coverage of findings
  
  • Exclude sub-populations of interest from numerator/denominator
• Affects identification of causal factors and outcomes
  
  • Obscure identification/ascertaining impact of exposures, and of temporal or geographical trends
• Affects service planning and commissioning
  
  • Unable to meet demand/wasted resources
  • Inappropriate targeting of resources
• Affects treatment
  
  • Under or over-investigation/over-treatment of individuals

Question 19

How might we find data to measure incidence?

• The … study
• Focus on … of … relevant to particular outcome
• Group of individuals … from disease selected, usually at …
• Participants selected into … and …-… group for …/s of interest
• Occurrences of disease onset (…) are recorded
• … are made between groups with respect to … rates

Answer 19

• The cohort study
• Focus on identification of exposures relevant to particular outcome
• Group of individuals free from disease selected, usually at random
• Participants selected into exposed and non-exposed group for exposure/s of interest
• Occurrences of disease onset (incidence) are recorded
• Comparisons are made between groups with respect to incidence rates

Question 20

Types of cohort study

• A … cohort is preferred, following a population forward over time
• However a … cohort can be used, especially with high quality routine data such as electronic health records
• Other types of cohort study could include:
  
  • Studying the … of a population
  • Studying the … of a population with a … (e.g…)
• Studying individuals with exposure/disease to identify risk of …

Answer 20

• A prospective cohort is preferred, following a population forward over time
• However a retrospective cohort can be used, especially with high quality routine data such as electronic health records
• Other types of cohort study could include:
  
  • Studying the entirety of a population
  • Studying the entirety of a population with a specific period (e.g. consecutive babies born in 2010)
  • Studying individuals with exposure/disease to identify risk of poor outcome/ death

Question 21

Strengths of the Cohort Study

• More than … disease related to … exposure
• Can offer some evidence of … – … relationship
• Good when exposure is …
• Can calculate … and … risk

Answer 21

• More than one disease related to single exposure
• Can offer some evidence of cause – effect relationship
• Good when exposure is rare
• Can calculate incidence and relative risk

Question 22

Disadvantages of the Cohort Study

• Potential for … to follow-up
• Often requires … sample
• Less suitable for … diseases
• If prospective, long time to … and it is …
• If retrospective, data availability and quality may be …
• Vulnerable to …

Answer 22

• Potential for losses to follow-up
• Often requires large sample
• Less suitable for rare diseases
• If prospective, long time to complete, expensive
• If retrospective, data availability and quality may be poor
• Vulnerable to confounding

Question 23

Confounding and the Cohort Study

• A confounder is a variable that influences both the … and … causing a spurious association
• A cohort study offers … protection against confounding than a cross-sectional study because it establishes temporal …
  
  • But it does not avoid the problem of both exposure and disease being … by other …
• Selection bias: systematic differences … groups (outset and/or during)
• Information bias: systematic differences in … … groups

Answer 23

• A confounder is a variable that influences both the exposure and disease causing a spurious association
• A cohort study offers more protection against confounding than a cross-sectional study because it establishes temporal precedence
  
  • But it does not avoid the problem of both exposure and disease being influenced by other variables
• Selection bias: systematic differences between groups (outset and/or during)
• Information bias: systematic differences in measurement between groups

Question 24

Sources of cohort study data?

• ​Secondary sources include … (3)
• Primary source … (1)

Answer 24

• ​Secondary sources include
  
  • Mortality registers
  • Hospital/Medical records
  • Census data
• Primary source
  
  • Survey data

Question 25

Strengths and Weaknesses of Secondary Data:​

• Cost?
• If anonymous, … ethical/governance approval needed
• … by what data already gathered
• Poor … and … data

Answer 25

• • Cheap
• • If anonymous, minimal ethical/governance approval needed
• -Limited by what data already gathered
• –Poor accuracy and missing data

Question 26

Strengths and Weaknesses of Primary Data:

• • Gather … data
• -Difficult to achieve … sample
• More or less expensive than secondary?

Answer 26

• • Gather additional data
• -Difficult to achieve representative sample
• -More expensive

Question 27

Measuring Incidence - Relative Risk

• Clear advantage of the cohort study is that we don’t just have to measure … – we can use the cohort study to …
• … … or … … (RR) is the … of developing a disease in … group compared to developing a disease in … group

Answer 27

• Clear advantage of the cohort study is that we don’t just have to measure (crude) incidence – we can use the cohort study to compare incidence between the two groups
• Relative Risk or Risk Ratio (RR) is the risk of developing a disease in exposed group compared to developing a disease in unexposed group

Question 28

Cohort Study Data Analysis - Relative Risk:

Strength of association:

• RR = <1.0
  
  • Risk in exposed group … than the risk in non-exposed group.
  • The exposure may be … against the disease (… association).
• RR=1.0
  
  • Risk in exposed group … to the risk in non-exposed group.
  • The exposure is … associated with the disease (… association)
• RR = >1.0
  
  • Risk in exposed group … than the risk in non-exposed group.
  • The exposure may be a … … for the disease (… association).
• RR of 1.5 -> risk of outcome …% higher in exposed than unexposed group
• RR of 3.0 -> risk in exposed group is … times as high as unexposed
• RR of 0.8-> risk of outcome …% lower in exposed than unexposed group

Answer 28

• RR = <1.0
  
  • Risk in exposed group less than the risk in non-exposed group.
  • The exposure may be protective against the disease (negative association).
• RR=1.0
  
  • Risk in exposed group equal to the risk in non-exposed group.
  • The exposure is not associated with the disease (no association)
• RR = >1.0
  
  • Risk in exposed group greater than the risk in non-exposed group.
  • The exposure may be a risk factor for the disease (positive association).
• RR of 1.5 -> risk of outcome 50% higher in exposed than unexposed group
• RR of 3.0 -> risk in exposed group is three times as high as unexposed
• RR of 0.8-> risk of outcome 20% lower in exposed than unexposed group

Question 29

Calculating Relative Risk

• All‐cause mortality in patients newly diagnosed with type 2 diabetes, with and without established cardiovascular disease
  
  • WHAT IS

Relative risk of death amongst people newly diagnosed with type 2 diabetes with cardiovascular disease compared to without?

Answer 29

• RR = Incidence of disease among exposed (A/A+B) / Incidence of disease among non-exposed (C/C+D)
• Answer = 3535/3535 + 8844 / 21,960/21,960 + 91,753 (0.29 / 0.19 = 1.53 in 1990-2005 (1.53 RR in 15 year study period)
• Over 1 = risk in exposed group is greater than risk in non-exposed group - exposure may be a risk factor for outcome (Risk in exposed group 53% greater than the risk in non-exposed group)

Question 30

Issues with Relative Risk

• RR of 3.0 could mean 0.003/0.001 i.e. 3 vs 1 person in 1000 (absolute risk difference of … people)
• OR
• RR of 3.0 could mean 0.90/0.30 i.e. 900 vs 300 people in every 1000 (absolute risk difference of … people)
• Just RR = know what difference proportionally your exposure makes in having the outcome but not difference it makes in terms of actual number that would have the disease
  
  • What should researches provide to combat this issue?

Answer 30

• RR of 3.0 could mean 0.003/0.001 i.e. 3 vs 1 person in 1000 (absolute risk difference of 2 people)
• OR
• RR of 3.0 could mean 0.90/0.30 i.e. 900 vs 300 people in every 1000 (absolute risk difference of 600 people)
  
  • What should researches provide to combat this issue? - researches should provide absolute difference in risk in addition to relative risk

Question 31

Summary of Thinking about Populations over time: Incidence and Cohort Studies

Answer 31

• Incidence is a measure of the number of new cases of a condition in some given time period – in a month or a year, for example – expressed as a proportion of a population which is at risk
• Prevalence depends on the incidence of a disease and the time between onset and recovery (or death)
  
  • Prevalence = incidence x disease duration
• Incidence = number new cases in a given time period / total population at risk
• Cohort studies are used to measure incidence
  
  • Group free from disease identified at baseline and followed up to record incidence
  • Often selected at baseline into a group with exposure of interest and a group without
  • The two groups are then compared with respect to incidence – to test association between exposure and outcome
• Relative risk is a way of quantifying the exposure-outcome association – typically used in cohort studies
  
  • Ratio of incidence in exposed compared to non-exposed group
  • Calculated as incidence in exposed group / incidence in non-exposed group
  • Provides both strength and direction of association
• Cohort studies offer some protection against confounding as exposures measured before outcomes – but are still vulnerable to effects of other variables confounding exposure-outcome association