You can use partial eta squared as the measure of size effect. SPSS will create this output when you run ANOVA. You can also use generalized eta squared or omega squared.

http://www.sciencedirect.com/science/article/pii/S1747938X11000029

ThaNk you very much Ismail Nizam.

So we will do this manually or SPSS will give this output?

Partial Eta squared can be requested within SPSS under "options". The other ones have to be computed manually or with a excel spread sheet, for example by Daniel Lakens, who wrote excellent papers about effect sizes, just check him out.

Hello

Rainer Duesing

Thank you very much. Can Partial Eta squared represent effect size of Repeated measure ANOVA? 

In book entitled Discovering Statistics using SPSS by Andy Field Omega Squared is to be used for estimating effect size for Repeated measure ANOVA. They have recommended to do it manually. Whereas some other documents are stating that partial eta squared can also be used for overall effect size. 

So I am confused that which measure should I use? Partial Eta or Omega Squared?

And ya I am going through Daniel Laken's paper indeed its very beneficial. Thanx.:)

Sarah, you asked which measure should you use, partial eta2 or omega2.  I think that in order to help you figure that out, we need to know why you want a standardized measure of effect size in the first place.  If you have been asked by a reviewer (or supervisor) to provide a standardized measure of effect size because it is what is done in your discipline, then I would go with partial eta2 (because it is much easier to obtain using SPSS), and see if that satisfies whoever is asking.  As you can probably tell from my suggestion, I am not a big fan of standardized effect size measures of this type.  I think that much of the time, the observed mean differences more than adequate as measures of effect size.  See Baguley (2009), for example.  HTH.

EDITED:  The wiley.com link I gave for Baguley (2009) is not working for everyone.  I've added a link to the Pubmed abstract.  The "Linkout" links on that page might work better. 

http://onlinelibrary.wiley.com/doi/10.1348/000712608X377117/full

https://www.ncbi.nlm.nih.gov/pubmed/19017432

Dear Bruce,

the link you provided does not work (at least for me...), but I would agree with you partially and want to add something.

My two cents on this topic:

Partial Eta squared should not only be used for the sake of easy access, but because it is very commonly used and therefore easy to compare between studies. On the other hand, omega can tell you something about the effect size in the population. So, if you are interested in generalizing your findings you should prefer this effect size (but what would be pointless if noone would report it, unless you calculate it from their data). Thirdly, a non-standardized effect size seems to be the best way to understand your data and the impact of your effect.

In my opinion, there is no overall valid effect size. You have to ask yourself what you want to know, understand, or communicate.

Thank you so much Bruce and Rainer.