Search through the data. Analyze all the data and observations together to determine the problem source. Troubleshooting a process plant involves using a number of different approaches together. Data are real observations and measurements of the unit. Data comes from more than the control room instrument panel board.
Data comes from the unit in the field as well.
Data from the unit in the field helps force observation. If the troubleshooter won't look at the unit even though it's right in front of the eyes, there's no helping that. But, even the best mind won't work to the correct conclusion with missing information. First, not everything you need to know is on the process control computer. Second, listen and look.