Handling of missing data on measures All the validated scores will be pro-rated, providing that the percentage of missing items does not exceed 25%. When more than 25% of the items are missing, the score will be treated as missing. The SF-12 score involves more complex scoring, a simple imputation algorithm will be applied in cases of missing data. When the percentage of missing values of a categorical variable does not exceed 1%, imputation by modal values will be performed. For a categorical variable for which the percentage of missing values exceed 1% , 'missing' will be defined as an additional level of the variable.