Beginner's question—Dealing with ordinal data (scoring of a medical effect) I'm a biology PhD student dealing with ordinal data (scoring of a medical effect: 0–3 points) for the first time and I'm looking for information regarding differences in treating ordinal numbers in


*

*calculating Mean, Median, SD, SEM etc.

*performing significance tests between two ordinal datasets

*calculating correlations


For the last point, I stumbled across the Spearman's coefficient. So far so good, but can I treat ordinal datasets the same way as cardinal datasets in terms of Mean, Median, SD, SEM and significance tests?
Any comments appreciated, I'm also happy with good literature, if it's not too hard to understand.
 A: In short, use Spearman's rho when calculating a correlation coefficient for ordinal data.  As to statistical tests, use the Mann-Whitney U test to compare two independent groups when you have ordinal data.  Means do not make sense for ordinal data (although occasionally you will find people reporting means for ordinal data), but medians make sense, as do distributions and cross tabulations and their associated Chi Square tests.  Most any introductory statistics text will go over the above-mentioned statistical techniques.  Social science textbooks tend to be more applied in orientation than mathematical statistics textbooks.  One of the more rigorous social science textbooks is "Statistics for the Social Sciences" by Rand Wilcox (1996).  Since you are a student, it probably makes sense to talk to a faculty member who teaches statistics, either in your department or some other department because doing a statistical analysis can get complicated for all sorts of reasons.  Also, if you collect data such as these in the future, you might consider a finer scale, if feasible.  Good luck!
