Scientists and Engineers (LWR)
Scientists
XCOM starts with 10 scientists and may acquire more throughout the campaign.
Each scientist increases the speed of research by 2% (stacks additively).
The speed to research each technology can be found here: Research
Scientists are also required in order to build laboratories (15 scientists per laboratory).
Engineers
XCOM starts with 10 engineers and may acquire more throughout the campaign.
The speed to produce an item is reduced with more engineers. An exact formula can be found here: Engineering - Item Production Speed
Engineers are also required in order to build workshops and satellite uplinks/nexus' (15 engineers per workshop and 20 engineers per satellite uplink/nexus).