Measuring compliance with the analytical plan in an online system requires implementing a set of monitoring and evaluation measures. Here are some steps that can be taken:

1. Define the analytical plan: The first step is to define the analytical plan and break it down into individual tasks or steps. This should include identifying the data sources, variables of interest, and the analytical methods to be used.

2. Set up the online system: Implement the analytical plan in an online system. This system should be designed in a way that captures the data needed to execute your analytical plan.

3. Develop guidelines: Develop detailed guidelines for data collection, preparation, and analysis, which specify the specific steps that should be followed.

4. Assign roles and responsibilities: Assign roles and responsibilities for data capture, data preparation and data analysis to team members.

5. Regular reports: Develop and implement a monitoring and evaluation framework that includes regular reports, weekly or monthly, to track compliance and progress towards the analytical plan objectives.

6. Feedback system: Establish a feedback system to enable team members to provide feedback on the implementation of the analytical plan.

7. Continuous improvement: Monitor the feedback and performance of the team members to identify areas where additional training or process improvement is needed.

By following these steps, it becomes easy to measure compliance with the analytical plan in an online system and ensure that the results accurately reflect the research question and objectives.