JiscMail Logo
Email discussion lists for the UK Education and Research communities

Help for SPM Archives


SPM Archives

SPM Archives


SPM@JISCMAIL.AC.UK


View:

Message:

[

First

|

Previous

|

Next

|

Last

]

By Topic:

[

First

|

Previous

|

Next

|

Last

]

By Author:

[

First

|

Previous

|

Next

|

Last

]

Font:

Proportional Font

LISTSERV Archives

LISTSERV Archives

SPM Home

SPM Home

SPM  1999

SPM 1999

Options

Subscribe or Unsubscribe

Subscribe or Unsubscribe

Log In

Log In

Get Password

Get Password

Subject:

Re: "omnibus"

From:

Ian Nimmo-Smith <[log in to unmask]>

Reply-To:

Ian Nimmo-Smith <[log in to unmask]>

Date:

Fri, 01 Oct 1999 11:49:05 +0100

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (119 lines)

Steve Fromm <[log in to unmask]> said:

> There is frequent use of the term "omnibus" (as in "omnibus
> statistic") in the SPM literature.

> Does this term have a precise definition? 

My copy of Chamber's Twentieth Century Dictionary (of the english language) 
includes a definition of "omnibus" as an adjective meaning 'widely 
comprehensive; of miscellaneous contents'. In this case there is an omnibus 
meaning of 'omnibus'!

In SPM it is used in two broad senses:

[1] The first, more classical sense, originates in the Analysis of Variance 
(ANOVA) of a designed experiment, where the F-test for the effect of a factor 
A with more than 2 levels (say k) is often described as an "omnibus" F-test. I 
think the usage of this term can probably be traced back to R A Fisher in the 
1930s.

The reason for this name is that the test includes all the possible contrasts 
and comparisons involving the different levels of the factor A. The numerator 
sum-of-squares of the F statistic is the total of the sums-of-squares for any 
complete set of (k-1) orthogonal contrasts between the k levels of A.

More generally the term 'omnibus F test' gets applied to any situation where 
the numerator degrees of freedom of the F statistic are greater than 1 (e.g. 
effects of interest including conditions and covariates).

Here is an example from [log in to unmask] where it is used in this sense:

(From http://www.mailbase.ac.uk/lists/spm/1998-07/0118.html )

| I'm not sure about the meaning of the SPMF map page after the Design 
| Matrix prompting in SPM Statistic. Does it represent the result of the 
| test of significance of the equality of variances between pixels or 
| something else? 

It's an "extra-sum-of-squares" F-test, assessing the additional 
variance accounted for by the effects of interest (condition effects, 
covariates of interest) after accounting for the effects of no interest 
(nuisance covariates, subject block effects, global effects). As such 
it's an omnibus test for all possible contrasts of the effects of 
interest, answering the question "do the effects of interest model 
anything?" - i.e. "is there any evidence of effects of interest". 

(end of example)

The book 'Human Brain Function' (Frakowiak et al. 1997) also uses the word in 
this sense in the phrase 'omnibus hypothesis' (p. 81).

[2] However the term gets used in a broader way in the SPM discussion list and 
in SPM-related publications. Here it can mean 'a statistic/test for a null 
hypothesis of no effect anywhere in the whole brain vs. some effect 
somewhere', by contrast with a more regionally specified null hypothesis.

This is of course the heart of the 'statistical parametric map' philosophy: to 
take the voxel based statistic (F, t, Z, extra sum-of squares, chi-squared or 
whatever) and consider the resulting 'field' (statistical parametric map). 
Typically one of two things is than done. Either [2.1] seeing whether there 
are any voxels for which the height of the field is significant at a level set 
so the probability that there is one or more such voxels is less than a chosen 
alpha (op. cit. p. 89); or [2.2] applying a more globally based appraisal of 
the field (e.g. spatial extent combined with height) (pp. 89-97).

I append three examples from [log in to unmask]:

(Example from http://www.mailbase.ac.uk/lists/spm/1999-07/0118.html)

> I believe that the set-level analysis implemented in SPM96 is similar 
> to, but not the same as, the MSOS [mean sum of squares] test described 
> by Worsley. This appears to be particularly true when k=0. As I am 
> using SPECT data, where f<1, this is the prefered level of k for 
> set-level inferences, if I understand Friston et al. [Neuroimage 4, 
> 223-35, 1996]. 

Yes indeed. The set-level inference (with k = 0) is a nice omnibus test 
that pertains explicitly to the set of suprathreshold clusters (that 
can then be tabulated). The MSOS is an alternative omnibus test that 
was motivated by the detection of diffuse (less regionally-specific 
effects). These effects cannot be described very concisely because 
they are distributed throughout the brain. 

(Example from http://www.mailbase.ac.uk/lists/spm/1999-02/0017.html)

In another study with apriori anatomical hypotheses, I have used SPM 
without correction for multiple comparisons, and used permutations for 
the omnibus test of significance (based on the number of suprathreshold 
clusters). Region of interest tests were then used for localization. And 
all was fine! 

(Example from http://www.mailbase.ac.uk/lists/spm/1999-01/0017.html)

1) Many people reference Bailey et al (1991) in setting a height threshold 
of p < .001, uncorrected, to reduce risk of false positives with an omnibus 
test. Should one assume that this is adequate and that, having rejected 
correction, one should use the SPM96 default extent threshold of p = 0.5? 

Conclusion: 'omnibus' is used by people discussing SPM to mean 'with respect 
to multiple parameters' and/or 'with respect to the whole brain'. This is 
confusing. I suggest it is restricted to the former (classical) meaning as 
used more widely in statistics, and that an alternative term (e.g. 'global') 
is used for the latter sense. Then we can speak unambiguously of 'global tests 
of contrast activation represented in a t-field' and 'global test of the 
effects of interest represented in an omnibus F-field'. Better than an 
'omnibus omnibus test'!

Ian
---
Ian Nimmo-Smith
MRC Cognition and Brain Sciences Unit
15 Chaucer Road
Cambridge CB2 2EF




%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Top of Message | Previous Page | Permalink

JiscMail Tools


RSS Feeds and Sharing


Advanced Options


Archives

November 2019
October 2019
September 2019
August 2019
July 2019
June 2019
May 2019
April 2019
March 2019
February 2019
January 2019
December 2018
November 2018
October 2018
September 2018
August 2018
July 2018
June 2018
May 2018
April 2018
March 2018
February 2018
January 2018
December 2017
November 2017
October 2017
September 2017
August 2017
July 2017
June 2017
May 2017
April 2017
March 2017
February 2017
January 2017
December 2016
November 2016
October 2016
September 2016
August 2016
July 2016
June 2016
May 2016
April 2016
March 2016
February 2016
January 2016
December 2015
November 2015
October 2015
September 2015
August 2015
July 2015
June 2015
May 2015
April 2015
March 2015
February 2015
January 2015
December 2014
November 2014
October 2014
September 2014
August 2014
July 2014
June 2014
May 2014
April 2014
March 2014
February 2014
January 2014
December 2013
November 2013
October 2013
September 2013
August 2013
July 2013
June 2013
May 2013
April 2013
March 2013
February 2013
January 2013
December 2012
November 2012
October 2012
September 2012
August 2012
July 2012
June 2012
May 2012
April 2012
March 2012
February 2012
January 2012
December 2011
November 2011
October 2011
September 2011
August 2011
July 2011
June 2011
May 2011
April 2011
March 2011
February 2011
January 2011
December 2010
November 2010
October 2010
September 2010
August 2010
July 2010
June 2010
May 2010
April 2010
March 2010
February 2010
January 2010
December 2009
November 2009
October 2009
September 2009
August 2009
July 2009
June 2009
May 2009
April 2009
March 2009
February 2009
January 2009
December 2008
November 2008
October 2008
September 2008
August 2008
July 2008
June 2008
May 2008
April 2008
March 2008
February 2008
January 2008
December 2007
November 2007
October 2007
September 2007
August 2007
July 2007
June 2007
May 2007
April 2007
March 2007
February 2007
January 2007
2006
2005
2004
2003
2002
2001
2000
1999
1998


JiscMail is a Jisc service.

View our service policies at https://www.jiscmail.ac.uk/policyandsecurity/ and Jisc's privacy policy at https://www.jisc.ac.uk/website/privacy-notice

Secured by F-Secure Anti-Virus CataList Email List Search Powered by the LISTSERV Email List Manager