I am currently arguing with someoe on how to correctly treat data with multiple measurements for each subject. In this case data was gathered for each subject within a short time for different conditions within each subject. All measurements gather exactely the same variable, just multiple.
One option now is to just group the data by conditions and not care that multiple data points come from one subject. However the data points from each subject are probably not completely independent.
The other alternative is, to first take the mean of all measurements for each condition from each subject and then compare the means. However this will probably impact the significance, since in the final analysis it is not taken into account, that the means have less error.
How can you correctly analyse such data? Is this somehow taken care of in SPSS? In principle it should be possible to calculate the error margin when calculating a mean and than considering this in the final analysis, but I do not guess that SPSS is somehow doing this calculation behind my back.