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The Election 2015 guide to parties' policies has been put together by the ´óÏó´«Ã½'s Visual Journalism team, working with our Political Research Unit in London and specialists in Belfast, Cardiff and Glasgow.

The 16 policy areas are based on those highlighted in Ipsos Mori's Issues Index, which measures the issues that the public believe to be the most important facing the country. Members of the public are polled, with questions designed to elicit spontaneous answers, which means respondents are not prompted to choose from a list of pre-selected issues.

Where there is a crossover between some issues they have been grouped together for simplicity, eg "Inflation/prices" and "Unemployment" have been grouped under the headline issue of "Economy".

The most popular issues were then chosen based on their aggregate score over the 12 months of 2014. Some issues, such as "rural affairs" and "constitution" were added on editorial grounds to fulfil the ´óÏó´«Ã½'s public service commitments.

Issues have been ranked in the guide based on their aggregate score of importance during 2014, from most to least important.

Any party represented by at least one MP when the 2010 Parliament dissolves, is represented in the guide, at UK level. They are ordered by number of seats held (and then alphabetical order if tied), with those parties registered with the Electoral Commission to field candidates in more than one part of the UK coming above those that are registered to field candidates in one nation only.

At "nation" level, parties are included and ordered by number of seats held at Westminster, followed by parties that hold seats in the national assembly/parliament, where relevant. An exception is NI21 in Northern Ireland, which has told the ´óÏó´«Ã½ it does not intend to stand candidates in the May election. Larger political parties expected to field candidates in the nations are also included, as are parties for which there is evidence of significant political support.

This is an editorial process overseen by ´óÏó´«Ã½ journalists, with parties consulted where necessary. Although many parties have not unveiled their election manifestos by the start of the official election campaign, on 30 March, frequently they have made clear public statements about their policies.

Because of devolution, the UK parliament cannot rule on, or has limited powers over, some of the issues highlighted in the guide. For example, "health" is devolved to Northern Ireland, Scotland and Wales, so voters in these nations will not see the result of parties' Westminster health policies. Yet, some parties have been campaigning locally on these devolved issues in the run up to the election, and voters may still be influenced by these parties' views. In such cases, the guide makes a clear distinction between policies and "campaign points".