Comment Re:What Does This Mean (Score 1) 413
It's based on the weaker assumption that the number within each ward who change won't be significant. Or more accurately the net number.
Well, yes, obviously that's a better way of saying it (and more accurate). The effect is somewhat similar, though. The model basically ignores the fact that differences between individual candidates might matter (or candidate's actions, or campaigning, or whatever). While it may not be strictly equivalent to "straight ticket" voting, it assumes voters behave in similar ways, i.e., their party choices would never change (collectively) no matter which candidates were running or how those candidates acted.
While such an assumption may be true for many and likely a majority of voters in many areas, many elections are also won on much thinner margins. If even 10-20% of the electorate might actually vote for a different party if the candidate changes, it could sway this model significantly in many races.