Laddering is that step in the appraisal process when the line manager ranks his appraisees (at the same designation). The ranking is confidential & not made known to the employee. Following the laddering is the moderation, where all line managers within the business unit / account group meet & ratings and promotion decisions are awarded (within the prescribed percentages). Quite naturally, the higher you are on your line manager's ladder, the safer you are from being moderated out from a particular grade to a lower one.
