Incidence and Prevalence in Population at Risk! - USMLE Forums
USMLE Forums Logo
USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
Home
USMLE Articles
USMLE News
USMLE Polls
USMLE Books
USMLE Apps
Go Back   USMLE Forums > USMLE Step 1 Forum

USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam


Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 01-15-2011
USMLE Forums Addict
 
Steps History: 1+CK+CS
Posts: 112
Threads: 20
Thanked 128 Times in 39 Posts
Reputation: 143
Stats Incidence and Prevalence in Population at Risk!

I am confused about how to calculate these 2 parameters, for example if we have 100 new cases, total population 1000. Some books say incidence should be number of the new cases divided by the number of the total population AT RISK. It means 100/ 1000-100 which 100/900 because they consider only 900 are at risk as the 100 new cases already have the disease. Other books just divide 100 by 1000. In first aid they put that comment about incidence only (When calculating incidence, don't forget that people currently with the disease, or those previously positive for it, are not considered at risk).

So which method should we use for the exam and does it apply for incidence and not for prevalence ?
Reply With Quote Quick reply to this message



  #2  
Old 01-15-2011
S.Martin's Avatar
USMLE Forums Scout
 
Steps History: Not yet
Posts: 34
Threads: 3
Thanked 20 Times in 11 Posts
Reputation: 30
Arrow It's the time you look at the population that decides

It depends on when you look at the population.

There are two types of Incidence Rates:

Incidence Rate: New cases (events) / Population at risk over a period of time, in your example 100/900

Cumulative Incidence Rate: New cases (events) / Population at risk at the beginning of the study, in your example 100/1000

The same applies to prevalence, always the denominator is the population at risk, it's just the time you look at them.
Reply With Quote Quick reply to this message
The above post was thanked by:
dr z (01-16-2011), drmuh (01-17-2011), noothan (04-19-2011)
  #3  
Old 04-19-2011
USMLE Forums Scout
 
Steps History: Not yet
Posts: 40
Threads: 14
Thanked 12 Times in 7 Posts
Reputation: 22
Default

Would it be possible to clarify on this question, in regard to prevalence?

First Aid refers to both denominators (of incidence and prevalence) as the population "at risk." Because of this, in a population of 100,000, of which 1,000 are HIV positive, is the point prevalence of HIV(+) individuals:

(1,000 / 100,000), or
(1,000 / 99,000)?


In a population of 100,000 (without death / migration / etc), of which 1,000 subjects are already HIV(+), and each year an additional 100 patients become infected with HIV, is the period prevalence over 5 years equal to:

(1,500 / 100,000),
(1,500 / 99,000), or
(1,500 / 98,500)?


Thanks for your time and help,
Tig
Reply With Quote Quick reply to this message
  #4  
Old 04-19-2011
bebix's Avatar
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 1,357
Threads: 194
Thanked 3,259 Times in 881 Posts
Reputation: 3269
Default

Edit:
Cumulative Incidence is the same as just Incidence = at this time, how many cases we have = 100/1000...numerator is just a subgroup of the denominator.

An incidence rate, or simply rate, is defined in terms of number of new cases per observed person-time (rate)
Reply With Quote Quick reply to this message
The above post was thanked by:
Claus_CU (04-20-2011)
  #5  
Old 04-19-2011
USMLE Forums Scout
 
Steps History: Not yet
Posts: 40
Threads: 14
Thanked 12 Times in 7 Posts
Reputation: 22
Default

Quote:
Originally Posted by bebix View Post
IR = period time in this case 1 year = 100/1000 (entire population was at risk in that particular period of time)

Cumlative Incidence is the same as just Incidence = at this time, how many cases we have = 100/1000...in this case both are the same
Could you define the acronyms you use (IR)?

Also, my question was regarding point-prevalence and period-prevalence, not incidence.

Thanks,
Tig


(edit: that is, unless you're responding to the OP instead of me)
Reply With Quote Quick reply to this message
  #6  
Old 04-19-2011
bebix's Avatar
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 1,357
Threads: 194
Thanked 3,259 Times in 881 Posts
Reputation: 3269
Default

Quote:
Originally Posted by Tig2575 View Post
Could you define the acronyms you use (IR)?

Also, my question was regarding point-prevalence and period-prevalence, not incidence.

Thanks,
Tig


(edit: that is, unless you're responding to the OP instead of me)
The point prevalence (or prevalence proportion) is the proportion of people who have the disease at the time of the study and
The period prevalence is the proportion who have had the disease over some past time interval (such as the past 2 years)

and btw, if they ask for IR...we should use: new cases per observed person-time (rate)...
Reply With Quote Quick reply to this message
  #7  
Old 04-20-2011
Claus_CU's Avatar
USMLE Forums Addict
 
Steps History: 1 + CS
Posts: 113
Threads: 8
Thanked 88 Times in 61 Posts
Reputation: 98
Default

Quote:
Originally Posted by S.Martin View Post
It depends on when you look at the population.

There are two types of Incidence Rates:

Incidence Rate: New cases (events) / Population at risk over a period of time, in your example 100/900

Cumulative Incidence Rate: New cases (events) / Population at risk at the beginning of the study, in your example 100/1000

The same applies to prevalence, always the denominator is the population at risk, it's just the time you look at them.
There is only ONE type of Incidence "Rate"…Cumulative Incidence is NOT a RATE!…itīs just a proportion or a ratio.

Incidence (aka Cumulative Incidence) = # NEW cases in specified period / # people at risk AT the BEGINNING of the specified period

You MUST include the NEW cases in the DENOMINATOR!!!

In INCIDENCE Rate (a "REAL" RATE!), the DENOMINATOR is expressed in "person-time" units (person months, years, etc.)

Go to this web site (tufts university) for more examples
http://ocw.tufts.edu/Content/1/lectu.../194069/194120
Reply With Quote Quick reply to this message
The above post was thanked by:
bebix (04-20-2011), noothan (04-20-2011)



Reply

Tags
Biostatistics-Epidemiology

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
USMLE Steps History
What steps finished! Example: 1+CK+CS+3 = Passed Step 1, Step 2 CK, Step 2 CS, and Step 3.

Choose "---" if you don't want to tell.

Favorite USMLE Books
What USMLE books you really think are useful. Leave blank if you don't want to tell.
Location
Where you live. Leave blank if you don't want to tell.

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes


Similar Threads
Thread Thread Starter Forum Replies Last Post
Suicidal Risk Assessment? Yallah USMLE Step 1 Forum 10 04-21-2012 10:20 PM
True Positives and False Positives in this population achistikbenny USMLE Step 1 Forum 4 08-30-2011 09:35 AM
Risk factors for Osteoporosis adityaseju USMLE Step 1 Mnemonics 2 12-23-2010 02:13 AM
effect of physical activity on the incidence of osteoporosis khushboo USMLE Step 1 Forum 3 09-14-2010 02:45 AM
Attributable Risk Percent Ayshee USMLE Step 2 CK Forum 1 07-06-2010 09:38 AM

RSS Feed
Find Us on Facebook
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

USMLE® & other trade marks belong to their respective owners, read full disclaimer
USMLE Forums created under Creative Commons 3.0 License. (2009-2014)