Berksonian bias, Pygmalion effect and Hawthorne effect

As these are difficult to understand and find examples of, i have compiled some info in easiest way i could. I hope it helps you all :)

 Hawthorne effect: the alteration of behaviour by the subjects of a study due to their awareness of being observed.

 Pygmalion effect: Investigator inadvertently conveys his high expectations to subjects, who then produce the expected result. A "self-fulfilling prophecy".

 Berkson bias: usually occurs when cases and controls are selected from hospital inpatients.More specifically, when both the exposure and outcome affect the selection and leads to a false negative association.It looks confusing but just look at this example:


Consider an investigator studying a relation between diabetes and CHD.He goes to a hospital and gets a list of people admitted with CHD and he selects equal number of controls(inpatients not having CHD).


Let us create a 2x2 table here
CHD +
CHD -
Exposure (DM) +
a
b
Exposure (DM) -
c
d

Both diabetes and CHD are causes of hospitalisation.And we select our cases and controls from these hospitalised patients.The problem of choosing the control group(those who don't have CHD) from inpatients is that when compared to general public,the number of diabetics among the controls are much more(diabetes is a cause of hospitalisation). So that increases the value of b (exposed controls) with a corresponding decrease in d (not exposed controls) when compared to general population.


Odds ratio= ad/bc


So decrease in d and increase in b will cause a decreased odds ratio(decreased association) when compared to the value that would have been obtained if the study has been conducted in general population.Thus,here when exposure and result both affects selection,we get a decreased association.

Comments

Post a Comment

Popular posts from this blog

Chronic Mercury Poisoning Mnemonic

Image
  " TABLE " T - Tremor (Intentional, coarse, affecting arms, hands, tongue and later legs, also called hatter's shake/glass blower's shake/danbury tremors) A - Acrodynia (Pink disease, characterised by pain, paraesthesia, pinkish discolouration of hands and feets) B - Blue-Black line on gums L - Lentis (Mercuria lentis, deposition of brown pigment on anterior lens capsule without affecting visual acuity) E - Erethism (Psychological disturbancecharacterised by shyness, timidness, loss of memory,depression, insomnia)
Read more

Custom Medical Dictionary for MS Office for Free

Frustrated that Microsoft Word marks difficult medical terms as mispelled? I was, but this is a simple solution I created that works well.         You get frustrated adding such underlined words every time to the inbuilt custom dictionary of Microsoft Word by right clicking and adding. To avoid such repeated annoyance, the more permanent solution would be installing a medical spell checker itself which works in the background. So here is the Solution:  "MedSpellChecker"  - a custom dictionary for medical transcription with a collection of medical words that could work in tandem with Microsoft Word. Download  "MedSpellChecker" Medical Dictionary for Microsoft Word  and follow instructions to get it working :) Windows To Add a Dictionary: 1.      Download and save the Medical Dictionary File to your desktop. o    In IE, right click on the link and click “ S...
Read more

Differential Diagnosis of Chronic Watery Diarrhea

Chronic Watery Diarrhea can be subdivided in: osmotic, secretory and functional. Secretory Diarrhea:: bowel secretes too much fluid  VERY HIGH volume stools (>1L) Stool Osmolar Gap less than 50. No changes on Fasting. Caused by enterotoxins, bile acids(when ileal resection), carcinoid sx, VIPoma. Osmotic Diarrhea::   Non absorbable solutes that stay in bowel and drag water in its SMALLER volume stools. (<1L) Stool Osmolar Gap more than 50. Stops on fasting. ex. lactose intolerance and celiac disease Functional Diarrhea::   IBS, where hallmark is hypermotility,  has smaller volume and symptoms disappear at night.
Read more