The Economist has 60 indicators each of which is scored on a three point scale (0,0.5 or 1) the total is then calculated. For the participation section the following indicators are used:
Voter participation/turn-out for national elections.
Do ethnic, religious and other minorities have a reasonable degree of autonomy and voice in the
political process?
Women in parliament.
Extent of political participation. Membership of political parties and political non-governmental
organisations.
Citizens’ engagement with politics, from World Values Survey
The preparedness of population to take part in lawful demonstrations, from World Values Survey
Adult literacy.
Extent to which adult population shows an interest in and follows politics in the news, from World Values Survey
The authorities make a serious effort to promote political participation.
(full details at Democracy index 2017
The USA scores well for literacy and membership of political parties, but is still at 19% for women in the House, and doesn't score full points on indices such as percentage interest in news media and turn-out at elections.