Briefs about Epidemiological Study Designs

Briefs about Epidemiological Study Designs

Study design	Population	Exposure	Health Effect	Confounders	Problems	Advantages
Case reports, case series and other descriptive studies	Community or various sub-populations	Records of past measurements	Mortality and morbidity statistics; case registers; other reports	Difficult to sort out	Hard to establish exposure-effect relationships	Cheap, useful to formulate hypothesis
Cross-sectional study	Communities or special groups; exposed vs non-exposed	Current	Current	Usually	Current exposure may be irrelevant to current disease	Can be done quickly; can use large populations; can estimate prevalence
Case-control study	Disease (cases) vs non-diseased (controls)	Records or interview	Known at start of study	If confounders can be identified and measured they may be addressed	Difficult to generalise; may incorporate biases; cannot derive rates	Relatively cheap and quick; particularly useful for studying rare diseases
Time-series study	Large community (several million people); susceptible groups such as asthmatics	Current e.g. daily changes in exposure	Current e.g. daily variations in mortality	Often difficult to sort out, e.g. effects of influenza	Many confounding factors, often difficult to measure	Useful for studies on acute effects
Historical (retrospective cohort study	Special groups e.g. workers, patients, injured persons	Records of past measurement	Records of past or current diagnosis	Often difficult because of retrospective nature; depends on availability of previously obtained data	Need to rely on records that may not be accurate	Less expensive and quicker than prospective study; can be used to study exposures that no longer exist
Prospective cohort study	Community or special groups; exposed vs non-exposed	Defined at start of study (can change during study)	To be determined during study	Usually easy to measure	Expensive, time consuming; exposure categories can change; high dropout rate possible	Can estimate incidence and relative risk; can study many diseases in one study; can describe associations that suggest cause effect relationships
Experimental (invention study)	Community or special groups	Controlled/ known already	To be measured during study	Can be controlled by randomisation of subject	Expensive ethical considerations; study subjects compliance required	Well accepted results; strong evidence for causality or efficacy of intervention

(adapted from WHO, 1991)

Applications of different observational study designs

	Ecological	Cross-sectional	Case-control	Cohort
Investigation of rare disease	+ + + +	-	+ + + + +	-
Investigation of rare cause	+ +	-	-	+ + + + +
Testing multiple effects of cause	+	+ +	-	+ + + + +
Study of multiple exposures and determinants	+ +	+ +	+ + + +	+ + +
Measurement of time relationship	+ +	-	+a	+ + + + +
Direct measurement of incidence	-	-	+b	+ + + + +
Investigation of long latent periods	-	-	+ + +	+c/-

Key: +…+ + + + + indicates the degree of suitability
-not suitable
a If prospective
b If population-based
c If retrospective or historical cohort study
(adapted from Beaglehole et al, 1993)


Advantages and disadvantages of different observational study designs

	Ecological	Cross-sectional	Case-control	Cohort
Probability of: selection bias recall bias loss to follow-up	N/A N/A N/A	Medium High N/A	High High Low	Low Low High
Confounding	High	Medium	Medium	Low
Time required	Low	Medium	Medium	High
Cost	Low	Medium	Medium	High

(from Beaglehole et al, 1993)

Dear and Respected Dr Arshad Mehmood!
Please! Download and Save the above Table it is very much usefull for you to understand,which type of study is used for which pupose. It will widen your understandabilty about the various types of studies.
By the way sir,since some mishappenings with your account you dont appear on the Forum.
I cannot come to your home,as I am getting training in MCH in the Hayat Abad Medical Complex from 24 March to 2nd April (11 days). The Daily TA,DA and Lodging is around 3300/Day.From Pabbi Hospital only one DR from Paeds Samiullah is attending,the other Doctor I think Nazir did not come today if Dr Nazir is not Comming Please Tell Dr Taqveem to come I will talk to the EDO about this change.
There are Drs from all over the province,The majority have done MPH From various Universities,Today was the first day,I went there well prepared,there was good discussion about the Subject matter,I represented Abasyn University superbly, all the facilitators appreciated me too much and Ienjoyed it. The others were all "PAIDAL" (By Foot)
The timings are from 08:30 am to 6:00 pm.I requested them to let me go to attend the MPH Class on 3 scheduled days,They are considering it but have not repllied yet.
Have a Nice Night,
Yours
Dr Abu Zar

Aziz, I dont know if any of the cretins reads the stuff regularly posted by you and me, because in exam, most of them write that the best method to purify water is to mix Potassium Cyanide in it.

I have been in teaching stuff right from the time when I was a student. I have seen many difficult students with varying abilities, but they somehow succeeded because "THERE WAS A WILL TO LEARN", there was a sincerity with the profession.
The current lot has been experimented with a variety of ways, but to no avail, because there is just one single reason responsible and that is, that they simply dont want to learn and just want a degree, as most of institutions have made it mandatory to have a higher qualification in health.
So what we have, plenty of Afghani stuff, some and few of them are hard working and intelligent, others are plain idiots. Similarly our Paki lot is mostly nurses, Lhv's, homeopaths, and quacks.
Anyhow, I have lost all hopes in these idiots. I concentrate on a few good people now rather than wasting my energies on these rough nuts