DATA COLLECTION PRINCIPLES

By Farrokh Alemi, Ph.D.

Examine a subset of cases

Sampling

Examine a subset of cases

Randomization

Angry patients do not take satisfaction surveys

Non-response Bias

Convenience Bias

Confounding VariableReal Cause

Presumed Cause

Effect

Confounding VariableFire

Firemen on scene

Burning house

Blaming HeroesSeverity of Illness

Medication Treatment

Patient Outcomes

Supposed 10% of patients report their attitudes on the web and 40% of patients who write a comment on web are negative, what percent of our patients are dissatisfied?

Think it Through

Think it Through

All patient

s

Report on web

Satisfied

Not satisfie

d

Do not report

Satisfied

Not satisfie

d

10%

60% 0.06%

0.04%

0.54%

0.46%

60%

40%

40%90%

Think it Through10

%

60%

40%

90%

Oops

Observational Studies

Cases Exposed

Controls not

Exposed

Outcomes

Outcomes

FuturePastPresentTime

Investigator

Retrospective Studies

Retrospective Studies

Cases Exposed

Controls not

Exposed

Outcomes

Outcomes

FuturePastPresentTime

Investigator

Implied Equivalence

FuturePast

InvestigatorCases

Exposed

Controls not

Exposed

Outcomes

Outcomes

PresentTime

Mat

ched

O

ther

wis

e

Vioxx Study

In observational studies, controls are matched to cases in all relevant variables except for exposure to treatment

Take Home Lesson

Identify cases. Select two controls that match on age & have lowest random number. Calculate the proportion of success for case and matched controls

Do One:

Patient ID Treated Age Outcome Random 1 Yes Young Positive 0.242 No Young Positive 0.853 Yes Old Negative 0.644 No Old Negative 0.705 No Old Negative 0.876 No Old Negative 0.727 No Old Negative 0.868 No Young Negative 0.169 No Young Positive 0.17