Use of Whole Population Registers: Advantages and Disadvantages

Problems in Observational Studies Who gets included? Who gets lost? How to ensure completeness of recruitment? How to standardise assessment? How to collect routinely gathered data fit for purpose? Source of control group(s)?

Threats Internal validity External validity

Problem with: ‘my clinical series’ Who are your patients? Which patients could you have included? Which patients have you lost And why?

Issue of Catchment population Your area Your Clinic

Issue of Catchment population Other clinics Your area Your Clinic

Issue of Catchment population Your area Your Clinic Other Clinic

Issue of Catchment population Your area Your Clinic Other Clinic ???

Issue of Catchment population Your area Your Clinic ??? Other Clinic

Does it matter May or may not? Selection factors related to: Disease severity Access Costs Education Co-morbidity Waiting time etc

Who gets lost: The issue of left censorship

Recruiting patients from clinic 2007 2001

Recruiting patients from clinic: Attenders between January 2003 and December 2004 2007 2001

Recruiting patients from clinic: Attenders between January 2003 and December 2004 2007 2001

Recruiting patients from clinic: Attenders between January 2003 and December 2004 2007 2001

Recruiting patients from clinic: Attenders between January 2003 and December 2004 2007 2001

Recruiting patients from clinic: Attenders between January 2003 and December 2004 ? Died ? Remitted ? Lost hope 2007 2001

What is the message? Recruiting current attenders is biassed towards: Survivors Continuing problems Specific socio-economic groups Treatment responders/non-responders People who like you!

Ideal Whole population Captured at time of onset (inception cohort)

How to ascertain cases from whole population? Fix population: Health plan coverage Other special group (eg Nurses) Geographical (beware of selection factors for 1 and 2)

Tertiary Care Secondary Care Primary Care Self Care

Threshold vary: Disease severity Socio-economic/education Availability of care Psychological factors

Minimum entry severity point has to be primary care BUT Still legitimate to use other cut offs if external validity

Choices for ascertainment Detect diagnosed cases based on database search and chart review Administrative database (eg Pharmex, GPRD) Institutional database (eg Mayo Clinic) Set up prospective system

Use of diagnosed cases Cheap Quick Will allow retrospective recruitment Not relying on compliance

Prospective system Accuracy of data Reliability of data Timeliness of data Build in appropriate follow up ?consent/ethics

Attrition: Losses from cohort Why: Die Get better Deteriorate (DNA) Lose interest Change doctor Move

In practice: Losses from cohort are greater threat to validity that failure to recruit

Minimise attrition Engaging subjects with research Frequent contact Feedback Consent Baseline data on key informants Consent to access medical and other records Linkage to other datasets Subjects do opt in