Is Dexter getting better?

My latest entries have mostly been basketball-focused but the highly anticipated playoff matchups are to be blamed for that!

So after my post on the Johnny Depp - Tim Burton collaboration, I would like to take a stab at tracking the evolution of TV series. I think there are a broad type of questions that can be considered, such as:

How does the rating of individual episodes evolve throughout the course of TV series lifetime? Are there really "good" and "bad" seasons? Do TV series get cancel when the ratings go down by too much? Is there a common threshold? Do all seasons have high-rated cliffhangers at the end of the season?

Data

Similarly to the Johnny Depp analysis, I will be extracting my data from IMDB. To start off, I will focus on one particular TV series (and personal favorites): Dexter.

Plot

Let us plot the evolution of the individual episode ratings by "time":

Two main insights stand out:

• there appears to be a "seasonal" pattern within each season:
  - the ratings either stay flat or go down in the first few episodes
  - the ratings then shoot upwards during the second half of the season
• after 5 seasons of overall similar quality, it appears that the last season has not performed as well. For the first time in over 5 years ratings dropped before 8.0, and even the strong season finale was the lowest-rated finales.

If we compare the distribution of season 6's ratings with those of all prior seasons, the difference jumps out:

The best season 6 episode has a rating barely greater than the median rating of all past seasons!

I will start looking at other TV series and see if an overall low-rated season is the beginning of the end (hopefully not!). Do networks quickly panic and cancel shows as soon as they start dropping in overall quality?