I have a data set of repeated observations and I am trying to determine if any of the observations are outliers. The research I've done has only shown methods that would determine if one value (maximum, minimum, or one questioned value) is an outlier, or if both the highest and lowest value are outliers. What I would like to be able show is if multiple values throughout the data set are outliers, as I suspect, without knowing exactly how many outliers are present. Any help or direction you could give me would be much appreciated.
You can draw a boxplot of the main outcome by occasion; the STATA code for doing so is: graph box "outcome", over("time/occasion") showyvars marker(1, msize(vsmall)) mark(1, mlab( "participantID" ))
Please, replace the variables in quotation with your variables. You have to ask the statistical software to label the markers/outliers.
You need to define what is an outlier. The data points coming from your repeated measures have a mean and standard deviation.
I would consider a point as outlier, if it is located more than 3 or 4 times the standard deviation away from the mean of the distribution.
You could therefore remove all the data points that fulfills this criterion...
You could remove one outlier at a time, and repeat the outlier test, as described in the Wikipedia entry for Grubbs' test.
If your data set is very small, you may end up removing all points though.