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

Help for FSL Archives


FSL Archives

FSL Archives


FSL@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

FSL Home

FSL Home

FSL  June 2010

FSL June 2010

Options

Subscribe or Unsubscribe

Subscribe or Unsubscribe

Log In

Log In

Get Password

Get Password

Subject:

Re: Factorial design at first level

From:

SUBSCRIBE FSL Esther <[log in to unmask]>

Reply-To:

FSL - FMRIB's Software Library <[log in to unmask]>

Date:

Tue, 29 Jun 2010 14:19:51 +0100

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (104 lines)

sorry about the earlier post. It helps, if I include the original message. So here again:

Hi all,

we came across this thread, because we have very similar problems. It seems that the factor P is coded as a dummy variable. Why is P coded using the values -1 and 1 ? Would it be also possible to use two separate EV's (one for P and one for G) with the values 0 and 1? What would be the difference and how would the interactions look like?

Many thanks for any comment,

Esther


Dear Rajevandra,

> I have a couple questions about setting up first level factorial  
> designs and
> testing for interactions.
>
> I have factor P (perspective taking) which has two levels (other,  
> self).
>
> I also have factor G (guilt) which has four levels (none, low, med,  
> high).
>
> I thought of doing this with two EVs , one for each factor. The  
> factor P can
> be specified with  -1 and 1 for 'other' or 'self' in the three  
> column timing
> file (-1 and 1). For factor G, I can weight the events as -3, -1, 1,  
> 3 as
> weights in the 3 column timing file. Then in FEAT I can click on the
> interaction of EV for factor P and the EV for factor G. Is this a  
> valid set
> up for this design to assess main effects and interactions ?

in general it is a slightly less flexible design to embody ones  
assumption about linearity (i.e. a linearly graded response to non,  
low, med high) already at the design matrix stage. I would instead  
choose to model each cell of your 2x4 design separately and then use  
contrasts to look at main effects, interactions etc.

That means you would have 8 conditions, other-none, other-low etc etc.

If you now want to look at an interaction between P and a linear  
response to G (which you can think of as a "difference in linear slope  
between other and self) you can put in the contrast

[-3 -1 1 3 3 1 -1 -3]

which specifically tests for a "more positive linear relationship with  
G in condition "other" than in condition "self" ". This is assuming  
the order of conditions is as I indicated above. If instead you want  
to look for "more positive linear relationship with G in condition  
"selfr" than in condition "other" " you would use

[3 1 -1 -3 -3 -1 1 3]

You also need to know that there is an ambiguity between

"more positive linear relationship with G in condition "other" than in  
condition "self" " and "more negative linear relationship with G in  
condition "self" than in condition "other" " so when you see a "blob"  
in a contrast like this you need to go in and look at the simple main  
effects to see which of the cases you have.

> Also is it valid to weight factor G with 1, 2, 3, 4 (instead of -3,  
> -1, 1,
> -3)?

If I were to put the linearity directly into the design I would have  
two regressors, 1 1 1 1 to look for "any effect of G" and -3 -1 1 3 to  
look for "linear effects of G".

> Alternatively if I collapse the G levels into none+low and med+high
> resulting in two levels instead of 4, then I could use -1 and 1 in  
> the three
> column file. In this case what is the difference between using the
> interaction button in FEAT versus a double contrast (timing file to  
> specfify
> -1 and 1 at for  the two P levels;  -1 and 1 for the simplified G  
> levels;
> and a contrast of the EVs for P and G) ?

I believe they would be equivalent. Try it and see what you get.

> Finally, as a variant to above, how about creating 4 EVs, guilt,  
> neutral,
> other, self, and doing the double contrasts at the second level. Is  
> there a
> difference between this approach and using the interaction approach?

In general you want to perform the fancy modeling at the first level  
so you only have to take one cope per subject to the second level.  
This is so as not to inflate the degrees of freedom at the 2nd level.

> In general is there a recommended approach for doing more complicated
> factorial design interactions of first level inputs ?

My recommendation is to think of them in terms of the cells of a  
factorial design (a 2x4 in your case) and to model each of these cells  
independently. I think it is very much a personal preference what you  
think is easiest though.

Good luck Jesper

Top of Message | Previous Page | Permalink

JiscMail Tools


RSS Feeds and Sharing


Advanced Options


Archives

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


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