A few of the possible answers are summarized in the following table. Other causes are also possible.
| Cause of variation | Speed of effect |
| 1. Feed properties such as feed composition, enthalpy, and flow rate. These could have larges effect on the distillation composition. | 1. These effects would enter at the feed tray and must influence many trays to reach the distillate product stream. This would take many minutes. |
| 2. Reflux flow changes would influence the distillate composition. This change would normally be due to the changes in the valve opening which would be known; however, changes in the pump pressure might be the cause. | 2. These changes could be faster than the effects of feed but would not be the cause of the highest frequency variation. (see below) |
| 3. Sensor noise due to sampling and analysis errors. | 3. These errors would occur every measurement result, so that the noise could be very high frequency. This might be the cause for the very fast changes in the measurement. |