Wha, Wha, Wha, What??? That's right ladies and gents - compensation isn't even necessary (in some cases). And, I'm not just referring to the instances where you're using two colors that don't even overlap, I'm talking about straight-up FITC and PE off a 488nm laser. Now, before you stop reading and jump over to your Facebook feed let me just assure you that you first learned of the superfluous nature of compensation when you were about 5 years old. You see, analyzing flow cytometry data with or without compensation is nothing more than a simple "spot the difference" game you use to find in the back of the Highlights magazine while waiting to get your annual immunizations from the pediatrician. If you take a look at the figure below you may be able to recognize the left panel as the FMO (Fluorescence Minus One) control and the right panel as the sample. Spot the difference? Instead of seeing the sun missing on the left and then appearing on the right, let's just substitute a CD8-PE positive population for the sun. It doesn't really matter if the image is compensated, you're just comparing the differences between the two.
As you can imagine, this is greatly simplifying the situation, and when you start adding more and more colors, you simply cannot create an n-dimensional plot that can easily be displayed on a two-dimensional screen. This could easily work for 2-color experiments - it could even work for 3-color experiments (maybe using a 3-D plot), but beyond that, you're going to have to do one of two things. 1. Bite the bullet and get on the compensation train, or 2. Abandon visual, subjective data display altogether and move to completely objective machine-driven data analysis. Compensation, much like display transformation is a visual aid used to help us make sense of our data, two parameters at a time. In our example above, we don't magically create more separation between the CD3+ CD8- and CD3+ CD8+ populations. The separation between them is the same, we're just visualizing that separation on the higher end of the log scale (when uncompensated) where things are compressed in one case, and on the lower end of the log scale (when compensated) where things spread. You didn't gain a thing.