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

Your area

Your Clinic

Other clinics

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

2001 2007

Recruiting patients from clinic

2001 2007

Recruiting patients from clinic:

Attenders between January 2003 and December 2004

2001 2007

Recruiting patients from clinic:

Attenders between January 2003 and December 2004

2001 2007

Recruiting patients from clinic:

Attenders between January 2003 and December 2004

2001 2007

Recruiting patients from clinic:

Attenders between January 2003 and December 2004

2001 2007

Recruiting patients from clinic:

Attenders between January 2003 and December 2004

? Died

? Remitted

? Lost hope

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:

1. Health plan coverage

2. Other special group (eg Nurses)

3. Geographical

(beware of selection factors for 1 and 2)

Self Care

Primary Care

Secondary

Care

Tertiary 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