For those who know Pandemic but don't know the On the Brink Expansion, the Virulent Strain challenge replaces the Epidemic cards with Virulent Epidemics that each make one of the diseases progressively worse. For example, the Complex Molecular Structure card has the effects of a normal Epidemic, but adds the additional effect that it requires one more card than usual to discover a cure for the Virulent Strain disease.
It is obvious that this makes the game more challenging. When I started experimenting with this challenge, I wasn't trying to determine whether a Virulent game was harder than a non-Virulent game; I was only trying to determine how much harder it would be. I expected that adding Virulent would add about as much difficulty as adding an extra Epidemic, and I thought that would be a more interesting comparison.
My results surprised me. My win rate for four players, five Epidemics, without Virulent Strain (4P5E) was 84.2% +/- 3.0% (p <.05, n = 582). My win rate for six Epidemics (4P6E) was 58.1% +/- 17.4% (n = 31). My win rate for five Epidemics with Virulent Strain (4P5EV) has been 84.0 +/- 7.2% (n = 100). This is a statistically significant difference between 4P6E and 4P5EV, but so far, 4P5E and 4P5EV look practically the same.
These results are consistent with some of my other measures. The percentage of easy wins is nearly the same, and the average number of Eradications is actually higher with Virulent Strain (though not to a statistically significant degree).
What's going on here? I know there have been games that I lost due to Virulent Strain. I see three non-exclusive possibilities:
1. I've been having a lucky streak, and am likely to get slapped down soon. This is the most likely possibility.
2. Over the thousand-plus games I've played since my 4P5E games, I've honed my Pandemic skills a bit.
3. The actual increase in difficulty from Virulent Strain is much smaller than it seems.