Nov 27

Question 1

What is a case crossover study?

Answer 1

• observational study design when an exposure is transient
• ex: cell phone use, exercise, sleep disturbances
• for a fraction of follow up time, they are going to be a case
• for a fraction of follow up time, they are going to be a control
• look for exposure preceding case and control windows

Question 2

What are advantages of case crossover?

Answer 2

• self-matching (best we can do to control confounding)
• efficient (select only cases)
• multiple control windows possible (day before, year before event, etc.)

Question 3

What are disadvantages of case crossover?

Answer 3

• information bias: inaccurate recall of exposure during the control window
• selecting an appropriate control period- need similar circumstances as a case period
• choosing the appropriate window

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Question 4

How is a case crossover study analyzed?

Answer 4

• same as matched case-control study
• look at discordant pairs

Question 5

What is the measure of association for a case crossover study?

Answer 5

• ratio of observed frequency of physical exertion during the hazard period to the expected frequency (ie. control period)
• found expected frequency by looking at usual activity levels, frequency of heavy exertion in the control window (day before symptoms), frequency of heavy exertion in neighbourhood controls
• compare relative risks using standard methods for matched case-control study
• instead of concordant and discordant pairs, use hazard period and control period classified as concordant or discordant for heavy exertion

Question 6

What is a nested case-control study?

Answer 6

• case control study conducted with participants enrolled in a prospective cohort study
• cases and controls are drawn from the population in a fully enumerated cohort

Question 7

What are advantages of nested case-control?

Answer 7

• efficient design
• similar estimate of risk obtained after analyzing blood samples from only a small sample of the entire population
• fraction of the cost
• overcomes 2 major problems with case control design: recall bias, clearly defines source population

Question 8

What are issues with case control design?

Answer 8

• only estimate HR, RR
• can only be done once

Question 9

What is a case-cohort study?

Answer 9

• at start of follow up, select a subcohort
• at the time, we know their exposure status but not who is going to develop the event
• sub cohort will have same pattern of distribution as the full cohort
• take all of the cases that occur outside of subcohort and the cases that end up being in the subcohort
• hazard ratios and hazard rates can be calculated
• same sub cohort can be used for many things

Question 10

What is an N-of-1 trial?

Answer 10

• ultimate level of evidence in clinical medicine
• does a particular treatment work for that patient?
• in the literature, we look at average responses
• crossover RCT with 1 participant

Question 11

How is N-of-1 trial conducted?

Answer 11

• patients receive active and inactive treatment in randomly allocated sequence
• patient and physician are blinded to treatment sequence
• at the end, results during active period and placebo phases are compared
• if it helped the patient, we treat them with this
• if it didn’t help, we don’t treat them with it

Question 12

What are limitations of N-of-1 trial?

Answer 12

• results apply to one patient
• researchers cannot use results to suggest the treatment should be employed to treat all patients with the disease or even people who share similar characteristics with the person in the trial
• would need full RCT to see if this was applicable in others