What measurement scale is the angle measurement assigned to (interval or ratio)? I am a physiotherapist and am going to do clinical research based on RCT model. I am going to use a test based on degrees for ANGLES measurements. I am going to use a goniometer for such measurements. My question is in the title.
 A: You would — or should — regard an angle of 10 degrees as twice 5 degrees, which hangs with


*

*zero degrees is not an arbitrary zero

*ratios make sense.
Hence angle is ratio scale. But what hinges on the difference between that and interval scale?
Note: Although angles may be measured in degrees (or radians) sometimes that is only a measurement convention. Sometimes a trigonometric function of angle is closer to the real problem. I can think of fields in which sine, cosine, tangent are each better measures mathematically, statistically or physically. 
A: If the angles are all positive and not very large (all of them below $180^{\circ}$), then I agree that a ratio scale makes sense.
However, if you are adding angles, in particular to values above $360^{\circ}$, note that you should take care when computing quantities like the mean and the standard deviation. In that case, we should consider the measures to be a cyclical ratio, according to Chrisman's typology.
A: Indeed, there might be some problems with a ratio scale, but I tend to agree with Nick. Ratios make sense and there is a meaningful 0.
Regarding negative angles, maybe this is similar to speed and velocity. Velocity is a vector, so strictly speaking it is not on a ratio scale. But practically direction and amount can be treated separately. Speed is obviously on a ratio scale.
Another perspective is considering the allowed transformations. For angles only $a\cdot X$ is allowed. For instance from degrees to radians: $π/180 \cdot X$
Stevens wrote (1946, p. 679):

Ratio scales are those most commonly encountered in physics and are
possible only when there exist operations for determining all four
relations: equality, rank-order, equality of intervals, and equality
of ratios. Once such a scale is erected, its numerical values can be
transformed (as from inches to feet) only by multiplying each value by
a constant.

Seems fine to me with a goniometer. All four relations can be easily established.
There is also the perspective of measurement theory, which, for quantity would require: transitivity, antisymmetry, strong connexity, associativity, commutativity, monotonicity, solvability, positivity and the Archimedian condition. I guess, these indeed create problems when the interval is bounded. Consider monotonicity:
$X\geq Y$ if and only if $X+Z \geq Y+Z$
If X=360° and we add something to it, what is the result? If we say this is not allowed then we need to include another restriction, right? Similar problems occur for positivity and the Archimedian condition. So there is a point to whuber's "it's something altogether new". But the way angles are used still very much feels ratio-scale to me. I guess there are also natural bounds for some physical attributes, but they are also treated as ratio-scales.
