Thinking about Populations over time: Incidence and Cohort Studies Flashcards

1
Q

Prevalence

  • Prevalence measures the frequency of “…” of a disease in a given … at a designated … (the numerator)
    • E.g?
A
  • 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
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2
Q

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

A

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

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3
Q

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

A

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

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4
Q

Prevalence is expressed as either … (3)

A

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)

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5
Q

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
A

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

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6
Q

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

A

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

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7
Q

Relationship between Prevalence and Incidence

  • Prevalence depends on:
    • The … of a disease and
    • The … between … and recovery (or death)
  • Prevalence = … x … ….
A
  • Prevalence depends on:
    • The incidence of a disease and
    • The time between onset and recovery (or death)
  • Prevalence = incidence x disease duration
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8
Q

Prevalence = … x disease …

A

Prevalence = Incidence x disease duration

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9
Q

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
A
  • 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
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10
Q

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 …
A
  • 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
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11
Q

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

A

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

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12
Q

Measuring Incidence - The Numerator

  • As per prevalence = … for the disease
    • PLUS - clear and consistent definition as to what counts as a … …
A
  • As per prevalence = caseness for the disease
    • PLUS - clear and consistent definition as to what counts as a new case
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13
Q

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
A
  • 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
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14
Q

Calculating Incidence

  • We can use a … table
  • DISEASE INCIDENCE
    • Incidence of disease in … = A / (A + B)
    • Incidence of disease in … = C / (C + D)
A
  • We can use a Contingency table
  • DISEASE INCIDENCE
    • Incidence of disease in exposed = A / (A + B)
    • Incidence of disease in unexposed = C / (C + D)
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15
Q

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?
A
  • 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
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16
Q

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 …
A
  • 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
17
Q

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
A
  • 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
18
Q

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
A
  • 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
19
Q

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
A
  • 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
20
Q

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 …
A
  • 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
21
Q

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
A
  • 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
22
Q

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 …
A
  • 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
23
Q

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
A
  • 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
24
Q

Sources of cohort study data?

  • ​Secondary sources include … (3)
  • Primary source … (1)
A
  • ​Secondary sources include
    • Mortality registers
    • Hospital/Medical records
    • Census data
  • Primary source
    • Survey data
25
_Strengths and Weaknesses of Secondary Data:_​ * Cost? * If anonymous, ... ethical/governance approval needed * ... by what data already gathered * Poor ... and ... data
* + **Cheap** * + If anonymous, **minimal** ethical/governance approval needed * -**Limited** by what data already gathered * --**Poor** **accuracy** and **missing data**
26
_Strengths and Weaknesses of Primary Data:_ * + Gather ... data * -Difficult to achieve ... sample * More or less expensive than secondary?
* + Gather **additional** data * -Difficult to achieve **representative** sample * -**More** **expensive**
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
* 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
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
* 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
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?
* 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)
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?
* 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**
31
_Summary of Thinking about Populations over time: Incidence and Cohort Studies_
* 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