FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups 
 ProfileProfile   PreferencesPreferences   Log in to check your private messagesLog in to check your private messages   Log inLog in 
Forum index » Science and Technology » Chem » Analytical
Population SD or sample SD with many replicates?
Post new topic   Reply to topic Page 1 of 1 [6 Posts] View previous topic :: View next topic
Author Message
Peter Frank
science forum beginner


Joined: 12 Apr 2006
Posts: 2

PostPosted: Wed Apr 12, 2006 4:09 pm    Post subject: Population SD or sample SD with many replicates? Reply with quote

Hi,

The standard deviation is supposed to be calculated with N-1 for
samples and N for the population.

However, in analytical chemistry the population is practically always
indefinite because there is an indefinite number of possible
measurements. So, to be exact the standard deviation would always have
to be calculated with N-1.

Nonetheless, on http://science.widener.edu/svb/stats/descript.html I
read that the population standard deviation can be calculated for large
sample sets (usually more than 20 measurements). Is it an alright
practice to use the population SD formula for measurements with many
replicates or do you always use the sample SD formula?

Regards,
Peter
Back to top
Dr. Dickie
science forum beginner


Joined: 14 Jul 2005
Posts: 23

PostPosted: Wed Apr 12, 2006 4:21 pm    Post subject: Re: Population SD or sample SD with many replicates? Reply with quote

"Peter Frank" <peter_frankde@yahoo.de> wrote in message
news:1144858178.299230.264760@v46g2000cwv.googlegroups.com...
Quote:
Hi,

The standard deviation is supposed to be calculated with N-1 for
samples and N for the population.

However, in analytical chemistry the population is practically always
indefinite because there is an indefinite number of possible
measurements. So, to be exact the standard deviation would always have
to be calculated with N-1.

Nonetheless, on http://science.widener.edu/svb/stats/descript.html I
read that the population standard deviation can be calculated for large
sample sets (usually more than 20 measurements). Is it an alright
practice to use the population SD formula for measurements with many
replicates or do you always use the sample SD formula?

Regards,
Peter


I would say, if the sample SD is different from the population SD, then you
should use the sample SD--since this is what you measured. If it is not
different, what difference does it make?


--

Dr. Dickie
Skepticult member in good standing #394-00596-438
Poking kooks with a pointy stick
Repeal the 17th amendment; let's reinstate the proper checks and balances
and end mob rule in my lifetime!
Back to top
Peter Frank
science forum beginner


Joined: 12 Apr 2006
Posts: 2

PostPosted: Wed Apr 12, 2006 4:36 pm    Post subject: Re: Population SD or sample SD with many replicates? Reply with quote

Dr. Dickie wrote:

Quote:
"Peter Frank" <peter_frankde@yahoo.de> wrote in message
news:1144858178.299230.264760@v46g2000cwv.googlegroups.com...
Hi,

The standard deviation is supposed to be calculated with N-1 for
samples and N for the population.

However, in analytical chemistry the population is practically always
indefinite because there is an indefinite number of possible
measurements. So, to be exact the standard deviation would always have
to be calculated with N-1.

Nonetheless, on http://science.widener.edu/svb/stats/descript.html I
read that the population standard deviation can be calculated for large
sample sets (usually more than 20 measurements). Is it an alright
practice to use the population SD formula for measurements with many
replicates or do you always use the sample SD formula?

Regards,
Peter


I would say, if the sample SD is different from the population SD, then you
should use the sample SD--since this is what you measured. If it is not
different, what difference does it make?

Well, I guess, this is what the above rule of thumb is based on. With
larger numbers of replicates, the differences between sample SD and
population SD become smaller and smaller since N-1 and N get closer and
closer to each other (relatively speaking).

So, I conclude from your statement that for smaller numbers of
replicates, the standard deviation is calculated using N-1 anyway, and
for larger numbers of replicates it doesn't really matter.

Regards,
Peter
Back to top
David Stone
science forum beginner


Joined: 27 Mar 2006
Posts: 14

PostPosted: Wed Apr 12, 2006 7:29 pm    Post subject: Re: Population SD or sample SD with many replicates? Reply with quote

In article <1144859782.869766.157900@e56g2000cwe.googlegroups.com>,
"Peter Frank" <peter_frankde@yahoo.de> wrote:

Quote:
Dr. Dickie wrote:

"Peter Frank" <peter_frankde@yahoo.de> wrote in message
news:1144858178.299230.264760@v46g2000cwv.googlegroups.com...
Hi,

The standard deviation is supposed to be calculated with N-1 for
samples and N for the population.

However, in analytical chemistry the population is practically always
indefinite because there is an indefinite number of possible
measurements. So, to be exact the standard deviation would always have
to be calculated with N-1.

s/indefinite/infinite/

[snip]
Quote:

I would say, if the sample SD is different from the population SD, then you
should use the sample SD--since this is what you measured. If it is not
different, what difference does it make?

Well, I guess, this is what the above rule of thumb is based on. With
larger numbers of replicates, the differences between sample SD and
population SD become smaller and smaller since N-1 and N get closer and
closer to each other (relatively speaking).

So, I conclude from your statement that for smaller numbers of
replicates, the standard deviation is calculated using N-1 anyway, and
for larger numbers of replicates it doesn't really matter.

If you use the (N-1) formula, you will never be wrong within the
limits of your calculation since, as already noted, (N-1) -> N for
large N. That way, you also eliminate the risk of accidentally using
the (N) instead of (N-1) formula for small numbers of replicates.

N >= 20 is the usual rule of thumb for biased vs. unbiased standard
deviations as estimates for sigma
Back to top
Dr. Dickie
science forum beginner


Joined: 14 Jul 2005
Posts: 23

PostPosted: Thu Apr 13, 2006 10:43 am    Post subject: Re: Population SD or sample SD with many replicates? Reply with quote

"Peter Frank" <peter_frankde@yahoo.de> wrote in message
news:1144859782.869766.157900@e56g2000cwe.googlegroups.com...
Quote:
Dr. Dickie wrote:

"Peter Frank" <peter_frankde@yahoo.de> wrote in message
news:1144858178.299230.264760@v46g2000cwv.googlegroups.com...
Hi,

The standard deviation is supposed to be calculated with N-1 for
samples and N for the population.

However, in analytical chemistry the population is practically always
indefinite because there is an indefinite number of possible
measurements. So, to be exact the standard deviation would always have
to be calculated with N-1.

Nonetheless, on http://science.widener.edu/svb/stats/descript.html I
read that the population standard deviation can be calculated for large
sample sets (usually more than 20 measurements). Is it an alright
practice to use the population SD formula for measurements with many
replicates or do you always use the sample SD formula?

Regards,
Peter


I would say, if the sample SD is different from the population SD, then
you
should use the sample SD--since this is what you measured. If it is not
different, what difference does it make?

Well, I guess, this is what the above rule of thumb is based on. With
larger numbers of replicates, the differences between sample SD and
population SD become smaller and smaller since N-1 and N get closer and
closer to each other (relatively speaking).

So, I conclude from your statement that for smaller numbers of
replicates, the standard deviation is calculated using N-1 anyway, and
for larger numbers of replicates it doesn't really matter.

Regards,
Peter

That about sums it up (except that since it "doesn't matter," you use the

sample SD in every case--that way, you do not have to worry whether or not
it is different from the population SD).

--
------
Dr. Dickie
"Let be be finale of seem.
The only emperor is the emperor of ice-cream."
-- Wallace Stevens
Back to top
Marvin
science forum Guru Wannabe


Joined: 24 Mar 2005
Posts: 224

PostPosted: Thu Apr 13, 2006 5:38 pm    Post subject: Re: Population SD or sample SD with many replicates? Reply with quote

Peter Frank wrote:
Quote:
Hi,

The standard deviation is supposed to be calculated with N-1 for
samples and N for the population.

However, in analytical chemistry the population is practically always
indefinite because there is an indefinite number of possible
measurements. So, to be exact the standard deviation would always have
to be calculated with N-1.

Nonetheless, on http://science.widener.edu/svb/stats/descript.html I
read that the population standard deviation can be calculated for large
sample sets (usually more than 20 measurements). Is it an alright
practice to use the population SD formula for measurements with many
replicates or do you always use the sample SD formula?

Regards,
Peter

In either case, the data give an estimate of SD, with an

uncertainty.
Back to top
Google

Back to top
Display posts from previous:   
Post new topic   Reply to topic Page 1 of 1 [6 Posts] View previous topic :: View next topic
The time now is Wed Oct 18, 2017 4:53 pm | All times are GMT
Forum index » Science and Technology » Chem » Analytical
Jump to:  

Similar Topics
Topic Author Forum Replies Last Post
No new posts Alternatives to Valco sample stream selection valves ? MaP Analytical 0 Wed Jul 12, 2006 12:33 pm
No new posts Distribution of slope in a terrain sample DGoncz@aol.com Math 1 Sun Jul 02, 2006 9:45 am
No new posts mathematics of population dynamics johnwl4@aol.com Math 1 Sun Jun 25, 2006 2:56 pm
No new posts Population dynamics johnwl4@aol.com Recreational 0 Sat Jun 24, 2006 6:01 pm
No new posts how to measure tolal carbon in water sample naguibwaguihnaguib@gmail. Chem 3 Mon Jun 19, 2006 4:31 am

Copyright © 2004-2005 DeniX Solutions SRL
Other DeniX Solutions sites: Electronics forum |  Medicine forum |  Unix/Linux blog |  Unix/Linux documentation |  Unix/Linux forums  |  send newsletters
 


Powered by phpBB © 2001, 2005 phpBB Group
[ Time: 0.1625s ][ Queries: 16 (0.1438s) ][ GZIP on - Debug on ]