A vote split is not built on the premise that all voters of one party would vote for another similar but that some would. The actual number or percentage of voters will vary however we can see how many it would take after an election by the results. These two links above will bring you to a page where you can adjust the percentage of Green Party votes over to the NDP and see what the overall election result could have been and the percentage needed to shift the balance of power. Reversing this process by moving NDP votes to the Green Party would not give the same result and that is why it can be said the Green Party splits the vote and not the other way around. One could also say that the Green Party splits the vote of the Liberals but although some voters would choose Liberals as their second choice, the platforms and guiding principals of Greens and NDP are too similar to ignore a general consensus of political choices.