Calculate the item score of a question. Function takes a single vector, with information on the question type and the twin type ('single' or 'pair') and calculates the zygocity item score.
zygo_weighted(x, type, n = "single")vector or recoded zygocity data (-1, 0, 1)
string. one of 'drop', 'stranger', 'dexterity', 'voice', 'eye' or 'belief.'
string, 'pair' if both twins have answered, 'single' if not.
numeric vector of weighted data