How is the effective number of parties calculated, and what does it indicate?

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How is the effective number of parties calculated, and what does it indicate?

The main idea is to capture how votes are spread across parties, not just how many parties exist. The standard measure uses the formula ENP = 1 divided by the sum of the squared vote shares of each party. Each party’s vote share is squared and then all those squares are added together; the inverse of that sum gives the effective number of parties. The squaring gives more weight to large parties and downplays tiny ones, so when votes are concentrated in a few parties the sum of squares is large and the ENP is small. If votes are spread more evenly across many parties, the sum of squares is smaller and the ENP grows toward the actual number of parties. For example, if two parties hold most of the votes, ENP will be closer to 2; if many parties each hold a similar share, ENP approaches the actual count of those parties. So a higher ENP means more fragmentation in practice, while a lower ENP means the vote is concentrated in fewer parties.

The main idea is to capture how votes are spread across parties, not just how many parties exist. The standard measure uses the formula ENP = 1 divided by the sum of the squared vote shares of each party. Each party’s vote share is squared and then all those squares are added together; the inverse of that sum gives the effective number of parties.

The squaring gives more weight to large parties and downplays tiny ones, so when votes are concentrated in a few parties the sum of squares is large and the ENP is small. If votes are spread more evenly across many parties, the sum of squares is smaller and the ENP grows toward the actual number of parties. For example, if two parties hold most of the votes, ENP will be closer to 2; if many parties each hold a similar share, ENP approaches the actual count of those parties.

So a higher ENP means more fragmentation in practice, while a lower ENP means the vote is concentrated in fewer parties.

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