Electoral threshold is where a paty needs above a certain number of valid votes (or percentage of valid votes) to win a seat.
The effective threshold is where the working of the election system dictates how many votes a party needs to take one seat.
The effective threshold is usually taken to be the number of valid votes divided by number of seats to be filled. But if there are wasted votes, then the effetive threshold does not apply strictly.
And the electoral threshold is quite strict -- some votes are not used to elect anyone if they are cast for parties who do not have threshold.
Therefore, parties with quota take more seats than just their vote tally divided by the quota.
And parties with less than quota do not elect anyone.
An electoral threshold of say 3 percent can mean perhaps as many as 13 percent of votes or more are not used to anyone.
that is the case if three parties have about 2 percent each, and six parties have 1 percent each, and 4 parties have about .25 percent of the votes.
In which case the parties with vote tallies over the electoral threshold would have about ten percent more seats than their vote share.
if they receive 30 percent of the votes, the party would take about 33 percent of the seats. (still much more fair and scientific than FPP but not exactly the proportional result that some promise will come with PR)
Where District Magnitude is large, a country might worry about a party with very small amount of support taking a seat. Even one seat can give some power such as holding power in a situation where something needs unanimous support.
If a country has this worry, its election system might include an electoral threshold.
This is an expression of preference for a limit on proportionality, instead of democratic open-ness.
Some countries have the electoral threshold set relatively high at say 5 percent; others set it at 2 or three percent.
Netherlands is exceptional in that its electoral threshold is set at below 1 percent, and is set at about the effective threshold, which is itself unusually low being .67 percent.
Wiki "2021 Dutch general election" at one time stated: "There is an official threshold of electoral threshold of 1/150th (0.67%) of votes to secure a seat (if it did not exist, the effective threshold would be roughly 0.4%)."
But that math looks wrong to me, unless they are somewhat arbitrarily saying 6 percent of votes will be cast for parties who don't get threshold.
150 seats each getting .4 percent = 60 percent of votes
so effective threshold is more than .4 percent (unless I am wrong somewhere)
150 seats each getting .67 -= 100.5 percent.
so looks like correct numbers are :
"There is an official threshold of electoral threshold of 1/150th (0.67%) of votes to secure a seat (if it did not exist, the portion of votes needed to take a seat might be lower than that if there are many wasted votes caused by some parties having vote tallies lower than the effective threshold of .67 percent.)"
in 2021 in Netherlands, only 2 percent of votes across the country were not used to elect anyone
the most-popular party got 34 seats with 22 percent of the vote, which works out to .64 percent per seat. 34 seats is 23 percent of the country's 150 seats.
the least-popular party to take a seat received .84 percent of the vote, well above the electoral threshold.
no party got between .34 and .84 percent so impact of Netherland's electoral threshold is hard to see.
But we can see that the effect of Netherland's electoral threshold is that a party due one seat needs at least as many votes for the one seat as the vote-per-seat of a more-popular party.
Having an electoral threshold means a small party with say .64 per cent of the vote is not awarded a seat while a more-popular party is awarded a seat for each .64 percent of the vote that it received, as we saw in 2021.
So it gives a slight advantage to more-popular parties over less-popular parties, but not as many votes are wasted as when the electoral threshold is set at 3 or 4 percent.
And fewer votes are wasted, where a lower electoral threshold is used.
Perhaps a general rule might be that votes equivalent to two to five times the electoral threshold are wasted.
Estonia saw 20 percent of valid votes actually wasted a few years ago when its electoral threshold was set at about 4 percent.
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