Equivalent units are estimated separately for material, labour and overheads because

1. Work in progress is rarely at the same stage for every cost element. 2. Material is often issued in full at the start while conversion continues afterwards. 3. So the percentage completion is estimated with regard to the different elements of cost. 4. That is why equivalent production is computed element by element rather than in one figure. 5. A single blended percentage would misstate both material and conversion cost. _Source: ICAI Cost and Management Accounting Ch10 'Process and Operation Costing', section 10.4.1_