Which must be measured regularly to identify variances from the plan in project?

basically, everything in the project.
Progress, milestones, deliverables acceptance, amount of re-work, resource availability, costs

- Actual to plan.

- Cost.

- Availability of resources required as per the project's plan.

- Agreed project's performance measures.

Project performance must be measured regularly to identify variances from the plan.
Variances are fed into the control processes in the various knowledge areas.
To the extent that significant variances are observed (i.e., those that jeopardize the project objectives), adjustments to the plan are made by repeating the appropriate project planning processes.
For example, a missed activity finish date may require adjustments to the current staffing plan, reliance on overtime, or trade-offs between budget and schedule objectives.
Controlling also includes taking preventive action in anticipation of possible problems.
Regards

project gateways are review that are planned at cirtical points in project ,it is a real time assesment carried out by independent person it allows the project to change course or address issue that have potential to undermine project objectives and benefits it can also provide evidence to stop projects that are severely oocourse or badly misaligned with buisness objectives

Khalid be careful when specialists use terms like "frequently" and "everything should be measured".
In practice - we are all time poor and dont really have time to measure EVERYTHING and FREQUENTLY.
Look at the nature of the projects and select several key indicators that you will monitor.
Ask yourself - for each indicator what is the cost of failing to meet that target in a given timeframe.
Where the cost of failure is higher - then you have just found your KPIs.

in project management there are2 pivotal indicators time and resources.
try to find the differences between scheduled and real time in which the tasks are done.
then trace the planned budget and used resources in tasks.
the important point is to balance the time and budget.
more ever you increase the budget you can cut the time and vise verse.