Really basic stats: what can we do with these variables?

Here is a quick guide for statistics that are ok to compute with kind of variable:

OK to compute....	Nominal	Ordinal	Interval	Ratio
frequency distribution.	Yes	Yes	Yes	Yes
median and percentiles.	No	Yes	Yes	Yes
add or subtract.	No	No	Yes	Yes
mean, standard deviation, standard error of the mean.	No	No	Yes	Yes
ratio, or coefficient of variation.	No	No	No	Yes

Source: http://www.graphpad.com/faq/viewfaq.cfm?faq=1089