When downsizing employee the most effective method I feel will be to based the decision on facts or documented evidence that may be difficult to dispute. The source of data can be used maybe the last few performance appraisal results, absenteeism , productivity rate and other soft skills to measure suitability to the job. Create a matrix identifying the criteria and measure the grade of each employee based on the criteria. In a way this is a measured evaluation.
The least effective i would think are those decisions based on emotional considerations.