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<font face="Dialog" size="2">Dear Nina,</font> </p>
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<font face="Dialog" size="2">dependend samples t values can easily be transformed into Cohen's d as d = t / sqrt(df) [taken from Rosnow & Rosenthal, Effect sizes for Experimenting Psychologists, Canadian Journal of Experimental Psychology, 2003, 57:3, 221-237; you should find a pdf copy with a search on Google scholar]. For example, a t value of 5.15 with 19 degrees of freedom corresponds to a d of 5.15/sqrt(19) = 1.18. So just compute depsamplesT and divide the resulting vector or matrix by the number of dfs - no further FT functions are needed.</font> </p>
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<font face="Dialog" size="2">My two cents are that effect sizes are especially useful for integrating results across studies where the very same design is applied to different samples, e.g., in clinical trials on pharmaceutical products. In EEG/MEG studies, even when they are on the same topic, there will be very divergent experimenting protocols (timing, task, etc), and also the dependent variable will differ from study to study, depending on the chosen time points / frequencies / channels of interest. A common effect size metric like Cohen's d does not improve the comparibility much in this case. I would therefore simply report the t </font> </p>
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<font face="Dialog" size="2">statistic along with the corresponding df and p, and leave it for the interested reader to compute the corresponding d if there should be a need for that. </font> </p>
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<font face="Dialog" size="2">Best regards,</font> </p>
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<font face="Dialog" size="2">Gregor</font> </p>
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<font face="Dialog" STYLE="font-size: 10pt"><br>--
<BR>Dr. rer. nat. Gregor Volberg <gregor.volberg@psychologie.uni-regensburg.de> ( mailto:gregor.volberg@psychologie.uni-regensburg.de )
<BR>University of Regensburg
<BR>Institute for Experimental Psychology
<BR>93040 Regensburg, Germany
<BR>Tel: +49 941 943 3862
<BR>Fax: +49 941 943 3233
<BR><a href="http://www.psychologie.uni-regensburg.de/Greenlee/team/volberg/volberg.html
">http://www.psychologie.uni-regensburg.de/Greenlee/team/volberg/volberg.html
</a><BR><br><br></font>>>> Nina Kahlbrock <Nina.Kahlbrock@uni-duesseldorf.de> 9/5/2011 2:45 PM >>><br> </p>
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<span style="font-size: 10.0pt; font-family: Arial" lang="EN-US"><font face="Arial" size="2">Dear statistic experts,<o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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<span style="font-family: Arial; font-size: 10.0pt" lang="EN-US"><font face="Arial" size="2">I have a rather general question concerning effect sizes. I understand that t-values give me an approximation of the effect size, as they state how big the difference between two conditions is by taking into account the variance and the number of observations. However, as I recall, effect sizes (like Cohen’s d) are computed in order to find out how important the effect is, independent of the number of observations. However, if I understand the code correctly, dependent samples t-values are computed in a very similar way as Cohen’s d (which is “the” value for effect sizes), both including the variance and thus the number of observations in their functions.<o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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<span style="font-family: Arial; font-size: 10.0pt" lang="EN-US"><font face="Arial" size="2">In order to draw conclusions about the importance of the observed effect, is it necessary to compute effect sizes like Cohen’s d or is it sufficient to compute t-values as effect sizes for M/EEG data? If important to compute effect sizes in a way different from t-values, does anyone know of a function that computes effect sizes like that in FT?<o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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<span style="font-family: Arial; font-size: 10.0pt" lang="EN-US"><font face="Arial" size="2">Thank you in advance for your answer.<o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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<span style="font-family: Arial; font-size: 10.0pt" lang="EN-US"><font face="Arial" size="2">Best Regards,<o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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<span style="font-family: Arial; font-size: 10.0pt" lang="EN-US"><font face="Arial" size="2">Nina<o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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<span style="font-size: 10.0pt" lang="EN-GB"><font face="Arial" size="2">Nina Kahlbrock<o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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<span style="font-size: 10.0pt" lang="EN-GB"><font face="Arial" size="2">Institute of Clinical Neuroscience and Medical Psychology <o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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<span style="font-size: 10.0pt"><font face="Arial" size="2">Heinrich Heine University Duesseldorf<o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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<span style="font-size: 10.0pt"><font face="Arial" size="2">Universitaetsstr. 1<o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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<span style="font-size: 10.0pt"><font face="Arial" size="2">40225 Düsseldorf<o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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<span style="font-size: 10.0pt"><font face="Arial" size="2">Tel.: +49 211 81 18075<o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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<span style="font-size: 10.0pt"><font face="Arial" size="2">Fax. .: +49 211 81 19916<o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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<span style="font-size: 10.0pt"><font face="Arial" size="2">Mail: </font></span><span style="color: black; text-decoration: none"><font color="black" size="2" face="Arial"><a href="mailto:Nina.Kahlbrock@med.uni-duesseldorf.de">Nina.Kahlbrock@med.uni-duesseldorf.de</a></font></span><o p="#DEFAULT"></o> </p>
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<span style="color: black; text-decoration: none"><SPAN STYLE='COLOR:"black" ;FONT-SIZE:"10.0pt" ;FONT-FAMILY:"Arial"'><a href="http://www.uniklinik-duesseldorf.de/medpsychologie">http://www.uniklinik-duesseldorf.de/medpsychologie</a></SPAN></span><span style="font-size: 10.0pt"><font face="Arial" size="2"><o style="font-size: 10.0pt; font-family: Arial" p="#DEFAULT"></o style="font-size: 10.0pt; font-family: Arial"></font></span> </p>
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