Factors included in the average density of planets When the average density for a planet is stated does it incorporate the mass of the atmosphere, when a planet has an atmosphere, or is the mass of the atmosphere ignored?
Whether of not you include the mass of the atmosphere doesn't matter much. For instance, the mass of Earth's atmosphere is roughly $10^{-6}$ of Earth's mass, corresponding to the uncertainty in the gravitational constant $G$ used to calculate the mass in the first place.

But the volume that you use for calculating the average density could potentially matter, depending on your definition of the height of the atmosphere. For Earth, the lowest-orbiting satellites are at a height of $\sim100\,\mathrm{km}$, so if you include that, your volume will be 5% larger. This number can be much lower if you define the height of the atmosphere to be the scale height (0.3%), or much higher if you include the thermosphere (1600%).

Because the height of the atmosphere is quite arbitrary, this is rarely if ever included in the average density of a (rocky) planet.