Formula for Cohen's d effect size

#1
Help!! I am struggling to figure out the effect size formula ( the spooled part) to get the effect size calculation. I have two groups (55 and 33) both with a SD of 12. I am particularly not understanding the s 2 over 1 part.

i have the main formula: u1-u2/Spooled, but the spooled formula is quite confusing.

(I am a PhD Psych major and this is an adv. stats. class)

I appreciate any help, I am completely stuck until I can get this effect size calculated!

Val
 

Dragan

Super Moderator
#2
Help!! I am struggling to figure out the effect size formula ( the spooled part) to get the effect size calculation. I have two groups (55 and 33) both with a SD of 12. I am particularly not understanding the s 2 over 1 part.

i have the main formula: u1-u2/Spooled, but the spooled formula is quite confusing.

(I am a PhD Psych major and this is an adv. stats. class)

I appreciate any help, I am completely stuck until I can get this effect size calculated!

Val
Can't you just use this formula (and plug in):

\( d=\frac{\bar{x}_{1}-\bar{x}_{2}}{s} \)

where

\( s=\sqrt{\frac{\left ( n_{1}-1 \right )s_{1}^{2}+\left ( n_{2}-1 \right )s_{2}^{2}}{n_{1}+n_{2}-2}} \).
 

Dragan

Super Moderator
#4
Thanks for your reply, but isn't the "s" what I have to calculate to finish the 1st formula?

Yes, you calculate the "s" using the second formula by using the two different samples sizes (55 and 33) and the variances 12^2 = 144 for both groups.

Once you have "s" from the second formula you plug it into the first formula and solve for Cohen's d.
 
#12
I see. It looks like the formula in my textbook and yours are different there. But I know how to get there now. I can't thank you enough for your time and patience-you have been a Godsend!
 

Dragan

Super Moderator
#13
I see. It looks like the formula in my textbook and yours are different there. But I know how to get there now. I can't thank you enough for your time and patience-you have been a Godsend!

It appears that some authors use the N - 2 instead of N in the denominator where n1 + n2 = N. Either way, the effect sizes will be close for large N sizes, however.