Final answer:
The primary purpose of a control chart in this scenario is to determine whether the machines are in control regarding a specific quality variable.
Step-by-step explanation:
The primary purpose of a control chart in this scenario is to determine whether the three machines producing the same product are in control regarding a specific quality variable. A control chart helps the quality analyst monitor the variation in the quality variable over time and detect any abnormalities or trends that may indicate the machines are not operating within acceptable limits. This allows the analyst to take necessary corrective actions and ensure consistent quality in the production process.