Week 5

Question 1

What are types of Observational studies?

Answer 1

• Case reports and case series
• Ecologic studies
• Cross-sectional studies
• Case-control studies
• Cohort studies

Question 2

What is an observational study?

Answer 2

Epidemiologic studies where the investigators collect, record and analyze data on subjects without controlling exposure status or conditions of the study

Question 3

What is a Case reports and case series study?

Answer 3

An unusual health event or series of events can identify a new problem and – sometimes – suggest a potential cause. (ex: covid) (people with similar symptoms)

Case report is a detailed description about an individual patient (specific to 1 person - 1 case)

Case series is an extension of the case report that describes the characteristics of a group or cluster of individuals with the same condition. (a few dozens of cases)

Question 4

What are Ecologic studies?

Answer 4

• Studying a group’s characteristics to determine an association (association only on the characteristics of the group)
• Overall or summary measure of exposure and outcome
• Inexpensive, relatively quick, establish preliminary hypothesis at the group level, likely based on existing data resource

Question 5

What are Cross-sectional studies?

Answer 5

examines the relationship between exposure and disease prevalence at a single point in time (ex: For diabetes, cancer, etc or positive mental health outcomes)

descriptively and analytically
- descriptively: characterize the prevalence of a health problem in a specified population
- analytically: compare the proportion of exposed persons who are diseased with the proportion of unexposed persons who are diseased.

Question 6

How does sampling work for cross-sectional studies?

Answer 6

Target population: people you want your results to apply to (ex: population of Ottawa, you can gather from data base)
Source population or sampling frame work: people from whom the population is selected (random sampling of stratification from Ottawa citizens)
Sample: people approached to take part in the survey
Study population: people who actually took part in the survey

Question 7

When are cross-sectional studies used?

Answer 7

• Estimate the occurrence of risk factors in segments of the population characterized by social determinants
• Use for planning or administering preventive or health care services, surveillance programs, surveys and polls
• Establish preliminary evidence for a causal relationship between an exposure and an outcome

Question 8

What is a cohort study?

Answer 8

Study population
Exposure -> Outcome
No exposure -> Outcome

Question 9

What are types of cohort studies?

Answer 9

• Prospective/longitudinal cohort studies (Study in the present and outcome in the future)
• Retrospective/historical cohort studies (Study in the past and outcome is in the present)
• Mixed cohort studies (ambidirectional cohort studies) (study from the past and now and then look at outcome in the future)

Question 10

What can you calculate with an open cohort?

Answer 10

Open = dynamic - different people in the city, people die everyday so what you observe is different everyday

Incidence rate

Question 11

What can you calculate with a closed cohort?

Answer 11

(assumption of no loss to follow-up and no losses due to competing risks - no changes)
Cumulative incidence or incidence rate

Question 12

What can you calculate with a fixed cohort?

Answer 12

(when loss to follow-up occurs for closed cohort)
Incidence rate

Question 13

What can you calculate in a contingency table?

Answer 13

Total of exposed
Total of non-exposed
Total of cases
Total of non-cases

Question 14

What are Indicators for cohort study?

Answer 14

Incidence rate (Absolute risk)
- Exposed group: A/Texposed
- Nonexposed group: C/Tnonexposed

Risk ratios (when exposed how likely will you get the disease)
- RR=(A/Texposed)/(C/Tnonexposed)

Risk difference
- RD=(A/Texposed)-(C/Tnonexposed)

Attributable risk
- AR=[(A/TExposed)- (C/Tnonexposed)]/ A/TExposed

Population - attributable risk (Incidence of disease in a population, associated with the prevalence of a risk factor)
- PAR=AR*P
- P=prevalence of exposure to a risk factor=(A+B)/Total

Population-attributable fraction (fraction of disease in a population is attributable to exposure to a risk factor)
- PAR%= PAR/ [(A+C)/Total]*100

Question 15

How to interprate the risk ratio (or relative risk)?

Answer 15

(a/a+b) / (c/c+d)

Ratios > 1.0 indicate risk is higher among exposed than unexposed (risque)

Ratios = 1.0 indicate no association

Ratios < 1.0 indicate risk is lower among exposed than unexposed (protecteur)

Question 16

how to calculate the rate ratio?

Answer 16

Only when in an open or fixed cohort:
a/Person years exposed / c/person years unexposed

Question 17

What is the Population attributable risk?

Answer 17

• Measure the excess incidence of disease in a community that is associated with a risk factor;
  Identify the relative important risk factors;
• Direct health policy into the more important or the relative weak but prevalent common risk factors.

Question 18

Choose one you think has a higher risk for stroke.
A 35- year man who smokes
A 60- year man who does not smoke

Answer 18

See slide 27

The relative risk decreases with age. However, absolute risk increases with age, mainly because of stroke is more common in elderly regardless the status of smoking.

Age contributes more than smoking in the occurrence of stroke.

Question 19

What is the difference between Cohort study and case-control study?

Answer 19

Cohort study
- Exposure at the beginning of study
- Disease after follow-up of all individuals

Case-control study
- Case status from registry
- Exposure on cases and controls
- First identification of persons with the disease and a suitable control of persons without the disease
- Key: comparison between disease + vs. disease -

Question 20

How does a case-control study work?

Answer 20

Study population
Outcome + -> Exposure?
Outcome - -> Exposure?

Question 21

How to get suitable controls?

Answer 21

Make sure you cases and controls are comparable

Would your cases have been selected to be controls had they not gotten the disease?

Would your controls have been selected to be cases had they not gotten the disease?

(we will see in another class)

Question 22

Can we determine the incidence rate from a case-control study?

Answer 22

Why can’t calculate incidence rate for cases and non-cases groups : cuz we only looked at the people who had the disease – we don’t have the total number of people (denominator)

Question 23

What can we calculate instead?

Answer 23

Odds ratio = (A/C)/(B/D)

If bigger than 1 = exposure is a danger
If smaller than 1 = its a protector factor

Question 24

What is something we do with our controls sometimes during our case-control studies?

Answer 24

Matching
Try to match a very similar control to a case

Question 25

See slide 36

Question 26

What tests can we use for our case-control studies when we have unmatched cases and controls?

Answer 26

- Chi-square 2 × 2 analysis; - Mantel-Hanszel statistic; - Fisher’s Exact test; - Unconditional logistic regression.

Question 27

What tests can we use for our case-control studies when we have matched cases and controls?

Answer 27

- Chi-square 2 × 2 analysis; - Mantel-Hanszel statistic*; - Fisher’s Exact test; - Conditional logistic regression

Question 28

Can you think of more than one reason why a matched case-control study could take longer to complete than an unmatched study?

Answer 28

Find matched controls, sometimes more than one per case; (harder to get the samples or get people involved) Since only the discordant pairs contribute to the statistical analysis, achieving a desired statistical power depends on obtaining a particular number of discordant pairs.

Question 29

Why bother with matching if it means a longer case-control study?

Answer 29

- We match to eliminate the possibility of the relationship being confounded by the matching variable because both the case and the control are similar for that variable. - We don't want to match on too many variables because it will cause an extreme delay in the completion of the study.

Question 30

What are the advantages for Case control study?

Answer 30

- Quick and cheap; - Only feasible method for very rare disorders or those with long lag between exposure and outcome; - Fewer subjects are needed than cross-sectional studies.

Question 31

What are the advantages for Cohort study?

Answer 31

- Ethically safe; (cuz no intervention) - Subjects can be matched; - Can establish timing and directionality of events; - Eligibility criteria and outcome assessments can be standardized; - Administratively easier and cheaper than RCT.

Question 32

What are the disadvantages for Case control study?

Answer 32

- Reliance on recall or records to determine exposure status; - Confounders; - Selection of control groups is difficult; - Potential bias: recall, selection.

Question 33

What are the disadvantages for Cohort study?

Answer 33

- Controls may be difficult to identify; - Exposure may be linked to a hidden confounder; - Blinding is difficult; - Randomization is not present; - For rare diseases, large sample sizes or long follow-up are necessary.

Question 34

What is a type of Advanced case-control designs?

Answer 34

Nested case-control study This is a case-control study within a cohort study. At the beginning of the cohort study (t0 or base line), members of the cohort are assessed for risk factors. Cases and controls are identified subsequently at time t1. The control group is selected from the risk set (cohort members who do not meet the case definition at t1.) Typically, the nested case-control study is less than 20% of the parent cohort. See slide 43

Question 35

What are the advantages for a Nested case control study?

Answer 35

Efficient – not all members of parent cohort require diagnostic testing Flexible – allows testing of hypotheses not anticipated when the cohort was drawn (at t0) Reduces selection bias – cases and controls sampled from the same population Reduces information bias – risk factor exposure can be assessed with investigator blind to case status

Question 36

What are the disadvantages for a Nested case control study?

Answer 36

Reduces power (from parent cohort) because of reduced sample size by: 1/(c+1), where c = number of controls per case

Question 37

What is a Case-cohort study?

Answer 37

In April of 1986, Ross Prentice published a paper in Biometrika introducing the case-cohort design. This innovative design uses a sub-sampling technique in survival data for estimating the relative risk of disease in a cohort study without collecting data from the entire cohort. See slide 47

Question 38

What are the advantages of a Case cohort study compared to nested case-control study?

Answer 38

Efficient – not all members of parent cohort require diagnostic testing Flexible – allows testing of hypotheses not anticipated when the cohort was drawn (at t0) Reduces selection bias – cases and controls sampled from same population Reduces information bias – risk factor exposure can be assessed with investigator blind t0 cases

Question 39

What are the disadvantages of a Case cohort study compared to nested case-control study?

Answer 39

Increased potential for information bias - subcohort may have been established after t0 - exposure information collected at different times (e.g. potential for sample deterioration)

Question 40

How to communicate risk?

Answer 40

Relative risk> absolute risk, e.g. 20 fold vs. 0.00001; (harder to get the absolute risk and people dont get the same feeling hearing that your risk is 20x higher than definitely 0.00001) “Loss” > “Gain” framing, e.g. risk is more attractive; (prefer to talk about the outcome) “Positive” > ”Negative” framing, e.g. persuading patients to take health services; (instead of saying you should quit smoking, we say that the nonsmokers will have a bgetter health outcome so we recommend to do the same. Give a recommendation or changes instead of saying to quit) More information needed;

Question 41

Where can we find Vital statistics and mortality data for cohort or case-control studies?

Answer 41

Census data (Canadian survey every 5 years - long form) Civil registration systems - Registries of births, deaths and marriages - National death registers Public Health Surveillance Systems - i.e., Canadian Chronic Disease Surveillance system - Stats Canada??

Question 42

In most countries, it is a legal requirement that:

Answer 42

- vital events, such as births and deaths, are registered - a medical practitioner completes a death certificate whenever anybody dies

Question 43

Deaths are classified according to standardized rules:

Answer 43

- International Classification of Diseases (ICD) (up to version 11) - coding rules on underlying cause(s) from causes listed on death certificates a. ensure comparable data over time, between countries, and b. aid storage, retrieval, statistical manipulation and analysis

Question 44

In what ways we can use existing data?

Answer 44

Simple description – patterns by person, place and time - e.g. prevalence of potential causes versus incidence of disease Migrant studies - Comparing people who move from a low-incidence area to a high-incidence area (or vice versa) Ecological or correlation studies - Comparing the prevalence of exposure and incidence of disease at a population level - Note: ecological fallacy – patterns seen at a group level may not hold at the individual level Big data

Question 45

What is big data?

Answer 45

Volume - Many variables per record and/or many records Velocity - Rapidly available (especially with smart technology) Variety - Many different types of data e.g. sensors in smartphones can measure physical activity, population movements, social networks …