Updating my Letterboxd Rating Scale

Rating movies is fundamentally a problem of compression: you have a complex mix of feelings and emotions generated in your mind by a movie but, if you want to use a rating scale, you have to somehow map that complexity onto a single dimension of 5 or 10 arbitrary points.

Nonetheless, I, and it seems many movie enjoyers, find this compression exercise useful. Since I created my Letterboxd page, I've been using a 5-point rating scale (1-5 stars, no half stars). I did this because, when I started rating 10 years ago, I didn't feel like I had the sensitivity to use a 10-point scale. However, now that I've rated over 900 movies, I feel like I could use the extra resolution in rating.

In this post, I'll work on defining the scale I'll be using and some other options I considered. In a follow-up post, I'll actually describe how I went about changing the ratings on Letterboxd.


Starting Definitions

I think it helps to connect the ratings to a more familiar, more objective scale: letter grades.

If we're using a 5-point scale, letter grades map very nicely:

And, we can further assign a descriptor to each grade:

If I compare this to the descriptors I was using for the 5-point scale, it doesn't line up exactly:

Seems my scale was expanded at the top (great split into great/amazing) and compressed at the bottom (bad/terrible combined to just bad). Let's resolve this discrepancy before we get into the shift from 5-point to 10-point scale.


Mapping Letter Grades

We know that letter grades correspond with a continuous scale of 0-100, based on some kind of criteria. I'm not here to tell you what that criteria should be, only to say that, whatever criteria you use, it will correspond with a continuous variable (usually some aspect of quality or, in the case of movie ratings, maybe how much you liked it). Thus, with only 5-points of cateogrization, this continuous variable will need to be compressed in some way.

Expanding out the grade continuum a bit with pluses/minuses, we would get:

I know it's not traditional to do +/- for the "F" range, but I do it here for symmetry. Working from this set of grades, how would we map these 15 categories onto the 5-point scales shown above?


5-Point, Standard Mapping

For the first mapping (the one most people would default to, I think), it seems straightforward:


5-Point, Prioritizing the Positive End

As for the 5-point scale I was using, it would look slightly different:

This reflects my priority of having more resolution around the positive end of the quality spectrum.


10-Point, Prioritizing the Positive End

Given this, I think a clean way to add-in the extra 5-points for the half-stars would be to keep the ends (0.5/5.0) where they are and map the remaining 8 grades (from C- up to A) in the middle one-to-one with the star ratings:

Here, all grades C- up to A+ get their own ratings, but everything D+ and below gets a 0.5. This is what I settled on, but there are other options.


10-Point, Symmetrical

If you wanted a symmetrical scale--which would make sense if you're a critic and see pretty much all movies, good or bad--I think the following makes sense:

We see anything A- or above gets a 5.0 and anything D- or below gets a 0.5.


10-Point, Polarized

Or, maybe you are someone who tends to feel strongly one way or the other and would rarely feel a film was "ok". You could push the middle ranges into the ends and retain resolution at both ends: