Im currently working on Unsupervised K Means Clustering with the OKCupid dataset. After 5 hours of waiting, I have produced a k clusters vs inertia graph for the interval k = [0, 200].

**OKCupid Unannotated graph:**

**OKCupid Annotated with line:**

From Codecademy lessons, I know that I am supposed to choose the elbow point, which is the point where the graph starts to become linear. If I follow it strictly, that point seems to be around k = 100.

However, for k means clustering with the iris flower dataset, the known correct number of clusters is 3, corresponding to the three subspecies, which is the point just before the graph becomes linear.

**Iris K means evaluation annotated with line:**

So, Im wondering, should I choose k = 75, k = 100, or somewhere around or between those values?

P.S. Also, I just realized that the elbow point is actually just the inflection point, which we learn in calculus is the root of the second derivative of a function, fâ€™â€™(0)