There is software to determine this. One excellent and free app is G*Power.
http://www.psycho.uni-duesseldorf.de/abteilungen/aap/gpower3/ I would use the correlational study to give me an estimate of effect size. As you describe, I would use that in the software to estimate my number of participants to attain the desired power. Practicality constraints on number of available participants usually limits things. I did such an estimate using G*Power a few weeks ago for a study we are planning. We will need to collect data over two semesters because the anticipated number of participants available from one semester's worth of students would only give us power of about .66, whereas two semester's worth would bump us up over .90. Paul On Aug 27, 2013, at 8:18 AM, Michael Britt wrote: I'm reading an interesting piece of research on anthropomorphism which essentially states after a natural disaster if we use the term "mother nature" when describing it, people will be less willing to contribute to relief efforts ("Humanizing nature could help the perceiver to conceive natural events as imbued with intentionality and significance rather than considering them merely random and meaningless phenomena"). They did two studies. Here's the issue/question: * Study 1 was correlational and involved 96 students. The results were supportive at <.001 * Study 2 was an experiment (no need to go into the details) involving 56 students. The results were, in the authors words, "tangentially" supportive with p<.06 I think the study was well conducted so I don't mean to slight the researchers. My guess is that if they used more subjects they probably would have reached p<.05 - but would that have been an example of "selective stopping"? I assume it would be. So how exactly does a researcher determine beforehand - as we are suggesting they do - the number of subjects they ought to try to get for the study? I'm just not familiar with the process. Does one look at the effect sizes of previous related studies to determine if the effect is large or small and then make a decision? But let's say the effect is assumed to be small, so do you use 100 subjects? 500? How is this number determined? Appreciate the insight in this. Michael Michael A. Britt, Ph.D. mich...@thepsychfiles.com<mailto:mich...@thepsychfiles.com> http://www.ThePsychFiles.com Twitter: @mbritt --- You are currently subscribed to tips as: pcbernha...@frostburg.edu<mailto:pcbernha...@frostburg.edu>. To unsubscribe click here: http://fsulist.frostburg.edu/u?id=13441.4e79e96ebb5671bdb50111f18f263003&n=T&l=tips&o=27372 (It may be necessary to cut and paste the above URL if the line is broken) or send a blank email to leave-27372-13441.4e79e96ebb5671bdb50111f18f263...@fsulist.frostburg.edu<mailto:leave-27372-13441.4e79e96ebb5671bdb50111f18f263...@fsulist.frostburg.edu> --- You are currently subscribed to tips as: arch...@jab.org. To unsubscribe click here: http://fsulist.frostburg.edu/u?id=13090.68da6e6e5325aa33287ff385b70df5d5&n=T&l=tips&o=27373 or send a blank email to leave-27373-13090.68da6e6e5325aa33287ff385b70df...@fsulist.frostburg.edu
