We tend to talk about political party support as a percentage measure of the entire contry: “My party now has 18% support!” This is a useful, but it is also useful to remember that parties different level of support from constituency to constituency.
Some parties may have a relatively evenly spread support through the country. Some may have more following in urban or rural areas, because their policies harmonize better with the people of those constituencies.
The plot explores political party constiuency bias in Iceland among four “traditional” parties. Each circle represents the proportion of valid votes in a constituency over the last 5 elections. The black line is the average.
When there is a difference in how many votes are required in a constituency to get a seat in parliament, a party with strong support in a constituency where fewer votes are behind each MP may benefit.