Is my data ordinal or interval? This may seem like a really simple question but I think my data straddles the line between interval and ordinal and I'm not sure quite how to treat it. 
Simply, participants have to estimate the time between two events by selecting 1 of 4 intervals which they have been previously trained to identify. The 4 options are 200ms, 400ms, 600ms, and 800ms. 
At face value the data seems to be interval; the options are ordered, and each one separated from its neighbour by the same meaningful interval. 
However, does the fact that there are only 4 possible options mean that the data is more ordinal than interval? 
Each participant is exposed to 8 presentations of each delay during the experimental phase, I don't know whether I can reasonably average their responses (which would be possible if the data was interval) or whether I should look at the number of times they selected each interval (like I would if it was ordinal).
 A: Well, this may be a different way of thinking about your problem, but what if you exploit the fact that:
200 = fx200 where f = 1
400 = fx200 where f = 2 
600 = fx200 where f = 3 
800 = fx200 where f = 4? 

If I understand your problem correctly, each of your subjects will have to choose 8 different times among the set of 4 options {1x200, 2x200, 3x200, 4x200} and you are interested in summarizing the resulting information across these 8 times. Each time a subject chooses an option, all the information you need to fully specify that option is the multiplicative factor f. 
As an example, subject #1 might make the following choices of f: 
1st time:  f = 1
2nd time:  f = 3
3rd time:  f = 1
4th time:  f = 2
5th time:  f = 2 
6th time:  f = 4 
7th time:  f = 1
8th time:  f = 3 

So if you wanted to summarize the information corresponding to this subject, you could do it in a variety of ways, including: 


*

*Typical value of f across the 8 repetitions (i.e., the average of the 8 values of f provided by the subject) - this gives you information about the typical estimate of time chosen by the subject; 

*Number of times subject chooses a specific f value across the 8 repetitions (e.g., number of time subject chooses f = 4 - that is, the highest estimate of time - across the 8 repetitions);

*Number of times subject chooses a specific set of f values across the 8 repetitions (e.g., number of times subject chooses f = 3 or f = 4 - that is, the higher estimates of time - across the 8 repetitions). 
I hope someone else on this forum will read my answer and confirm whether what I propose makes sense. Again, this answer assumes you are interested in summarizing information for each subject across 8 repetitions.
A: That there are only a few legal options doesn't seem to invalidate the interval scale of the variable. In particular, the mean of this variable still makes sense to compute, because e.g. 612 ms is an understandable value.
Still, be sure not to take Stephens's levels of measurement to be stricter than they are. You can still compute modes and medians with interval-scaled data and these can be more appropriate than the mean depending on the distribution of the variable, your analytic goals, etc.
