This story from the BBC outlines what the 2015 election results may have been by using different electoral systems.
WARNING! There should be some warnings attached to all of these figures:
- In systems where a second vote is available (like AMS where you have one vote for your constituency MP and one for your regional party) or where ordinal voting exists (like in STV where a voter puts candidates in order of preference), we can not exactly know what the outcome would have been as we cannot know exactly what the preferences would have been.
- These figures are worked out using how people did vote in 2015. If we had a proportional system, people may exercise their vote different (think about tactical voting under FPTP).
- This is only one websites interpretation. For example, I am not convinced about the ‘List PR’ figures as the SNP failed to secure 5% of the vote and therefore they could have ended up with no seats, depending on what thresholds were applied under this system and where (in Germany, a party must achieve 5% in a regional list).
Nonetheless, they are useful in a longer answer essay discussing the pros and cons of electoral reform. The pros should be easy (giving people what they voted for, less wasted votes, etc). For the cons, you could use these graphs to show the lack of majority/strong & stable governments and the power (too much power?) it would lend to third parties in being the deciding factor in a Coalition being formed.